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[Florian Stainer](https://orcid.org/0000-0003-2072-2687), [Junji Akimoto](https://orcid.org/0000-0001-9636-7680), [Yoshitaka Matsushita](https://orcid.org/0000-0002-4968-8905), [Kazutaka Mitsuishi](https://orcid.org/0000-0002-9361-4057), [Kazunori Takada](https://orcid.org/0000-0001-7568-1806), [H. Martin R. Wilkening](https://orcid.org/0000-0001-9706-4892)
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[From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H 5.2 Li 1.3 La 3 Zr 1.5 Ta 0.5 O 12](https://mdr.nims.go.jp/datasets/f7d11c71-ead3-4b6a-b534-c72404985dbf)
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From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12Journal ofMaterials Chemistry APAPEROpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal | View IssueFrom lithium to paGraz University of Technology, Institute of(NAWI Graz), Stremayrgasse 9, 8010 Grastainer@tugraz.atbNational Institute for Materials Science (NE-mail: akimoto.junji@nims.go.jp; matsukazutaka@nims.go.jp; takada.kazunori@nimCite this: J. Mater. Chem. A, 2026, 14,20491Received 5th December 2025Accepted 14th April 2026DOI: 10.1039/d5ta09970frsc.li/materials-aThis journal is © The Royal Society oroton mobility in garnetelectrolytes: an NMR and conductivity study ofH5.2Li1.3La3Zr1.5Ta0.5O12Florian Stainer, a Junji Akimoto, b Yoshitaka Matsushita, bKazutaka Mitsuishi, b Kazunori Takada b and H. Martin R. Wilkening *aGarnet-type oxides are among the most promising solid-state electrolytes owing to their high chemicalstability and comparatively high lithium-ion conductivity. They may also provide a basis for protonconductors through Li+/H+ exchange. In a recent 1H and 7Li NMR study, we showed that minor protonincorporation in single-crystalline garnets exerts only a weak influence on Li-ion diffusion. Here, weinvestigate the polycrystalline hydrogarnet H5.2Li1.3La3Zr1.5Ta0.5O12 (HLZTO), obtained by aqueous Li+/H+exchange from Li6.5La3Zr1.5Ta0.5O12 (LLZTO). The precursor, synthesised at the remarkably lowtemperature of 600 °C, exhibits a large surface area enabling an efficient exchange process. X-raydiffraction reveals a slight lattice expansion upon protonation, while 6Li high-resolution MAS NMR andRaman spectroscopy indicate a pronounced alteration of the local Li environment. Thermogravimetricanalysis shows a 5.5 wt% loss between 200 and 600 °C, consistent with the release of H2O. The totalionic conductivity of LLZTO is constrained by poor interparticle contact, whereas in HLZTO it dropssharply upon heating as protons are removed. Variable-temperature 1H NMR relaxation revealsa diffusion-induced maximum between 125 and 180 °C, resulting in a self-diffusion coefficient D z 1 ×10−15 m2 s−1 (ca. 150 °C). As seen by 7Li NMR, the remaining Li ions in HLZTO are almost immobile onthe NMR time scale, revealing that proton motion dominates charge transport in HLZTO. While defectsin LLZO promote localized motions, their healing appears to facilitate the establishment of long-rangeLi+ transport pathways.IntroductionProton-conducting ceramics have attracted growing attention askey materials for next-generation electrochemical energydevices. Their appeal stems from the fact that proton diffusiontypically involves a lower activation energy than, e.g., oxygen-iondiffusion, allowing high ionic conductivities to be achieved atconsiderably lower temperatures.1 Over the past decades, a widevariety of ceramic proton conductors has been developed forsolid oxide fuel cells, electrolysers, and related applications.2–5Among them, certain inorganic proton conductors such asCsHSO4 (ref. 6) and CsOH$H2O7 exhibit high proton conduc-tivities but tend to undergo deprotonation or melting attemperatures near 200 °C. Perovskite-type oxides, includingSrCeO3,8 co-doped BaZrO3,9–11 and BaCeO3 (ref. 12) displaymuch greater thermal stability, yet still require operation aboveChemistry and Technology of Materialsz, Austria. E-mail: wilkening@tugraz.at;IMS), Tskuba, Ibaraki 305-0044, Japan.shita.yoshitaka@nims.go.jp; mitsuishi.s.go.jpf Chemistry 2026400 °C to reach signicant conductivities. Consequently,considerable research efforts have focused on identifying newmaterials that support fast proton transport in theintermediate-temperature regime. A particularly promisingdirection involves lithium/proton-exchanged lithium conduc-tors, such as the lithium superionic conductor Li13.9Sr0.1-Zn(GeO4)4 (ref. 13) and the garnet-type Li7La3Zr2O12.14–16Another route toward proton incorporation in garnetsemerged from studies on their chemical instability underhumid conditions. It was shown that in ambient air, the garnetLi7La3Sn2O12 undergoes spontaneous partial protonation, whilethe released lithium reacts with CO2 to form Li2CO3.17 To avoidcarbonate formation, several researchers have insteademployed controlled Li+/H+ exchange reactions using benzoicacid,17,18 acetic acid,18,19 or even water.20 Over the past decade,a variety of techniques has been applied to probe protontransport in such systems. Howard et al. reported that in Ga-substituted Li7La3Zr2O12, the bulk ionic conductivitymeasured under wet N2 decreased, while the grain boundaryresistance was simultaneously reduced.21 Truong et al. observedan enhanced conductivity (∼10−5 S cm−1) in Li5La3Nb2O12using acid-treated Naon and Pt–C electrodes under wet N2,conrming that neither electrons nor oxide ions contributed toJ. Mater. Chem. A, 2026, 14, 20491–20503 | 20491http://crossmark.crossref.org/dialog/?doi=10.1039/d5ta09970f&domain=pdf&date_stamp=2026-05-25http://orcid.org/0000-0003-2072-2687http://orcid.org/0000-0001-9636-7680http://orcid.org/0000-0002-4968-8905http://orcid.org/0000-0002-9361-4057http://orcid.org/0000-0001-7568-1806http://orcid.org/0000-0001-9706-4892http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970fhttps://pubs.rsc.org/en/journals/journal/TAhttps://pubs.rsc.org/en/journals/journal/TA?issueid=TA014031Journal of Materials Chemistry A PaperOpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinethe total conductivity.22 However, these studies could notseparate the individual contributions of Li+ and H+, high-lighting the need for local probes such as solid-state nuclearmagnetic resonance (NMR) spectroscopy.Subsequent investigations have characterised proton diffu-sivity in garnets by various methods. Hiebl et al. determineda diffusion coefficient D of ∼10−17 m2 s−1 in Al-stabilised Li7-La3Zr2O12 single crystals using X-ray diffraction (XRD).23 Forpolycrystalline Ga-stabilised Li7La3Zr2O12, Smetaczek et al. ob-tained D z 7 × 10−17 m2 s−1 at 80 °C by laser-induced break-down spectroscopy,24 while Brugge et al. reported a similardiffusivity (∼10−16 m2 s−1 at 100 °C) using secondary ion massspectrometry.25 Recently, we demonstrated for single-crystallineLi6La3ZrTaO12 that low proton concentrations introduced viaLi+/H+ exchange only slightly reduce lithium-ion mobility, asshown by 7Li spin-lattice relaxation (SLR) NMR.19 Complemen-tary 1H NMRmeasurements revealed that these protons are alsomobile with a self-diffusion coefficient of D z 1.2 × 10−15 m2s−1 at 125 °C, albeit less so than Li+. Furthermore, we showedFig. 1 Scheme illustrating the preparation steps to obtain the hydrogarZr1.5Ta0.5O12; the structure is based on that of single-crystalline Li6.5Lframework consists of corner-sharing ZrO6/TaO6 octahedra (16a sites) andisordered substructure distributed over two crystallographically distinct s(42% occupancy), whereas Li2 corresponds to a split tetrahedral 24d sitecharacteristic of garnet-type Li conductors. Oxygen atoms are omitted20492 | J. Mater. Chem. A, 2026, 14, 20491–20503that at lower temperatures, proton spin relaxation is inuencedby the spin uctuations of the faster-diffusing lithium ions.In the present study, we investigate the diffusion propertiesof a polycrystalline lithium/proton-exchanged garnet. Theprecursor Li6.5La3Zr1.5Ta0.5O12 (LLZTO) was synthesised froma mixture of Li2O and uorite-type La3Zr1.5Ta0.5O8.75 usinga so-sintering process at only 600 °C, signicantly lower thanthe >1000 °C typically required for conventional garnetsynthesis. This low-temperature route yielded a material witha comparatively large surface area, which facilitated efficientLi+/H+ exchange in water at mildly elevated temperatures. Theextent of proton exchange was quantied by inductively coupledplasma optical emission spectroscopy (ICP-OES) and thermog-ravimetric calorimetry (TGA).Aer the ion-exchange reaction, the material retained itscubic structure but exhibited a slightly expanded latticeparameter. Fig. 1 shows the crystal structure of Li6.5La3Zr1.5-Ta0.5O12, obtained from single-crystal measurements, includingthe Li-site occupancies determined by neutron diffraction.26 Innet. Cubic crystal structure (space group Ia-3d) of H-bearing Li6.5La3-a3Zr1.5Ta0.5O12 taken from the ICSD (collection code: 22957).26 Thed La3+ cations occupying the 24c positions. Lithium ions form a highlyites, both located on 96h: Li1 arises from a split octahedral 48g position(12% occupancy). The partial occupancies reflect the intrinsic disorderfor clarity.This journal is © The Royal Society of Chemistry 2026http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970fPaper Journal of Materials Chemistry AOpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineour previous work, we demonstrated that at low protonconcentrations, protons preferentially replace Li+ ions on theless-occupied octahedral sites.19 At higher exchange levels,however, occupation of all Li sites becomes increasingly likely.Furthermore, in materials with high surface areas, protons mayalso participate in diffusion processes along grain boundaries,as reported for various oxides,27 and such behaviour can beanticipated for the present system as well.The aim of this study is to investigate the proton and lithiumdynamics in both the polycrystalline garnet precursor LLZTOand the corresponding hydrogarnet (see below, denotes asHLZTO) over long-range and local length scales. To this end, weemployed 7Li and 1H SLR NMR spectroscopy and conductivityspectroscopy to probe ion transport across different spatial anddynamic regimes. In addition, Raman spectroscopy and 6Limagic-angle spinning (MAS) NMR were used to elucidatechanges in the local environment of lithium ions betweenLLZTO and the hydrogarnet.ExperimentalPreparation of the lithium-bearing garnet samplesPolycrystalline Li6.5La3Zr1.5Ta0.5O12 (LLZTO) was prepared byreacting Li2O with uorite-type La3Zr1.5Ta0.5O8.75 at 600 °C, asrecently reported.28–30 La3Zr1.5Ta0.5O8.75 was rst synthesised bya polymerised complex method. The educts La(NO3)3$6H2O(Fujilm Wako Pure Chemical Industries, 99.9%), TaCl5 (RareMetallic Co., Ltd, 99.9%), and ZrOCl2$8H2O (Fujilm WakoPure Chemical Industries, 99.9%) were dissolved in dehydratedethanol at room temperature in a stoichiometric molar ratio ofLa : Zr : Ta = 6 : 3 : 1. Ethylene glycol (Fujilm Wako PureChemical Industries, 99.9%) and citric acid (FujilmWako PureChemical Industries, 95%) were then added, and the solutionwas stirred at 70 °C for 5 h and subsequently dried at 200 °C for12 h. The resulting mixture was heated in a mantle heater at350 °C for 2 h, followed by calcination in an electric furnace at1000 °C for 12 h in air.LLZTO powder samples were prepared from Li2O (KojundoChemical Laboratory, 99%) and the precursor La3Zr1.5Ta0.5-O8.75. The precursor oxide was mixed with a 30 mol% excess ofLi2O and ball-milled for 1 h in a zirconia vessel lled with Argas. The mixture obtained was then heated at 600 °C for 12 h inalumina crucibles under owing argon gas atmosphere.3.25 Li2O + La3Zr1.5Ta0.5O8.75 / Li6.5La3Zr1.5Ta0.5O12 (1)Preparation of protonated samples via ion exchangeThe protonated, polycrystalline garnet H6.5−xLixLa3Zr1.5Ta0.5O12was obtained by Li+/H+ ion exchange of LLZTO powder in waterat 60 °C for 5 days.Li6.5La3Zr1.5Ta0.5O12 + (6.5 − x) H2O /H6.5−xLixLa3Zr1.5Ta0.5O12 + (6.5 − x) LiOH (2)The solution was refreshed daily, and the progress of the ion-exchange reaction was monitored via the pH of the solution.This journal is © The Royal Society of Chemistry 2026Aer completion, the sample was washed with ethanol anddried under vacuum at 120 °C.Characterization of structure, composition and morphologyThe powder samples were characterised in air by powder XRDusing a Rigaku SmartLab3 diffractometer equipped with a D/teXUltra250 one-dimensional detector and operating with mono-chromatised Cu Ka radiation (l= 1.54187 Å, 40 kV, 30 mA). XRDpatterns were collected over a 2q range of 5 to 140° with a stepsize of 0.02° and a scan speed of 1.0° min−1. Lattice parametersand crystal structures of the samples were rened using theJana2020 program.31 To investigate structural changes in theprotonated garnet upon heating, X-ray diffraction was per-formed on a Rigaku Miniex 600 diffractometer equipped witha benchtop heating stage (BTS 500, Anton Paar) over thetemperature range 30 to 300 °C. The diffractograms wererecorded with Cu Ka radiation over a 2q range of 8 to 90° witha step size of 0.01° and a scan speed of 5.0° min−1. The particlesize and morphology were analysed by scanning electronmicroscopy (SEM, Hitachi FlexSEM 1000 II).The residual Li content was determined by ICP-OES (Agilent5800). To evaluate the hydrogen content and thermal stability ofthe protonated garnet sample up to 1000 °C in air, thermogra-vimetric and differential thermal analysis (TG-DTA) was per-formed using a Rigaku (TG-DTA8122) instrument with heatingand cooling rates of 10 °C min−1 and a 1 h hold at 1000 °C.Nuclear spin relaxationNMR experiments were performed on a Bruker Avance IIIspectrometer equipped with an ultrashielded 300 MHz wide-bore magnet operating at a nominal eld of 7 T. This corre-sponds to Larmor frequencies of u0/2p = 116 MHz for 7Li and300 MHz for 1H. 7Li NMR spectra were recorded using single-pulse excitation with a 90° pulse length of 2.6 ms and a radio-frequency power of 200 W; 16 scans were accumulated for eachspectrum. For 1H NMR, pulse lengths of 1.1 to 1.7 ms (depend-ing on temperature) and a power of 200 W were used, with 4scans collected per spectrum. All spectra were obtained byFourier transformation of the free induction decays (FIDs)without further processing.1H and 7Li NMR SLR rates in the laboratory frame of reference(1/T1) were measured using the saturation recovery pulsesequence (see Epp et al.32). In this sequence, a train of 10 closelyspaced 90° pulses destroys any longitudinal magnetization Mz.The recovery ofMzwas then recorded as a function of the variablewaiting time td using a 90° detection pulse. The time integral ofthe FIDs was used to construct the magnetization curves M(td),which were parameterised with stretched exponentials: Mz(td) f1 − exp(−(td/T1(T))g). The stretching factor g varied between 0.4and 1 depending on temperature. The corresponding SLR ratesin the rotating frame (1/T1r) were obtained using the spin-lockingmethod. A locking eld corresponding to 20 kHz was applied forboth 1H and 7Li NMR measurements, which locks the spinprecession in the (x − y)0-plane aer the excitation pulse. Thedecaying magnetization along the y0-axis, My(tlock), was recordedas a function of variable locking pulse times and analysed withJ. Mater. Chem. A, 2026, 14, 20491–20503 | 20493http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970fJournal of Materials Chemistry A PaperOpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinestretched exponentials: My(tlock) f exp(−(tlock/T1r(T))g). Here, weobtained g values ranging from 0.3 to 1 depending on tempera-ture. Unless stated otherwise, SLR measurements were per-formed over the temperature range from −95 to 260 °C. Prior tothe measurements, samples were re-sealed under vacuum inDuran glass tubes (4 cm length, 3 mm inner diameter) to preventany reaction with oxygen or moisture. Sample temperature wasmonitored in close proximity and controlled via a stream ofheated nitrogen gas.6Li and 1H MAS NMR6Li MAS NMR spectra were acquired on a Bruker Avance III 500spectrometer equipped with an ultrashielded 500 MHz wide-bore magnet (11.74 T), corresponding to resonance frequenciesof 73.6 MHz for 6Li and 500 MHz for 1H. Chemical shis werereferenced to 6Li in CH3COOLi$(H2O)x and to the 1H resonanceof adamantane, respectively. All measurements were performedat spinning speeds of 22 or 25 kHz and a bearing gas temper-ature of 303 K. 6Li spectra were obtained using a single p/2pulse of 2.4 ms at 100 W, accumulating 500 scans with a recycledelay of 10 s. 1H spectra were recorded with a p/2 pulse of 0.8 msat 20 W, accumulating 16 scans with a 3 s delay between scans.The powdered samples were packed under an argon atmo-sphere into 2.5 mm ZrO2 rotors tted with Vespel caps.Raman spectroscopyRaman spectra were recorded using a Thermo Fisher DXRRaman microscope equipped with a 532 nm, 5 mW laser. Thepowdered samples were loaded into glass capillaries witha 4 mm diameter, and spectra were collected over the range 40to 3500 cm−1.Conductivity measurementsPellets with a diameter of 5 mm were prepared by pressing thepowder samples at 260 MPa and subsequently coating bothsides with a Li-blocking Au layer (z50 nm) using a sputtercoater (Leica SCD050). These pellets, with an estimated densityabove 85%, were used for alternating-current conductivitymeasurements. The sample LLZTO500 was so-sintered ex situat 500 °C for 12 h inside an argon-lled glovebox prior tomeasurement.Conductivity data were recorded using a NovocontrolConcept 80 broadband dielectric spectrometer equipped withan Alpha-A analyzer and an active ZGS sample cell (Novocon-trol). The sputtered pellets were stored and measured underargon using an airtight in-house fabricated sample holder madeof Teon with brass contacts. Electrical contact between thepellet and the sample cell was ensured via secondary brasselectrodes and a spring washer.Conductivity measurements typically consisted of a heatingrun (20 °C to 260 °C) followed by a cooling run (260 °C to−100 °C)in 20 °C steps. The temperature inside the impedance cell wascontrolled by heated nitrogen gas supplied through a QUATROcryo-system (Novocontrol). For high-temperature measurementsup to 600 °C, a Novotherm-HT system (Novocontrol) equippedwith a tube furnace and a ceramic sample cell with platinum20494 | J. Mater. Chem. A, 2026, 14, 20491–20503electrodes was used. In this setup, the pellet was clamped betweenthe electrodes using multiple springs and maintained undera constant nitrogen gas ow during measurement. The frequencyrange covered was 0.05 Hz to 10 MHz.ResultsSynthesis and structural details of the proton exchangedgarnetThe precursor LLZTO (Li6.5La3Zr1.5Ta0.5O12) was successfullysynthesised from La2.4Zr1.2Ta0.4O7 and Li2O, as describedabove. The LLZTO powder exhibits submicrometer-sized parti-cles (Fig. S1), as conrmed by SEM. To achieve a high degree ofLi+/H+ exchange, the LLZTO powder was immersed in water at60 °C for 5 days, yielding the proton-exchanged garnet. Thehighly porous morphology of LLZTO facilitated the exchangeprocess, while the submicrometer particle size was preservedaer proton exchange (Fig. S1).Energy-dispersive X-ray spectroscopy (EDX) elementalmapping conrmed a homogeneous distribution of La, Zr, andTa across the hydrogarnet particles (see Fig. S2). Elementalanalysis by EDX yielded a composition of 20.6 at% La, 10.6 at%Zr, 2.4 at% Ta, and 66.3 at% O. Transmission electron micros-copy (TEM) images revealed particle sizes in the range of 100 to300 nm, consistent with the SEM observations. Nanobeamdiffraction TEM analysis conrmed that the particles are crys-talline and free of amorphous regions.An ICP-OES analysis of the hydrogarnet yielded a metal ratioof Li : La : Zr : Ta = 1.30 : 3 : 1.50 : 0.50, in good agreement withthe values obtained from EDX. The proton content was deter-mined from TG-DTA measurements. As shown in Fig. 2a, theTGA curve displays two distinct weight-loss steps: a minor lossof 0.29 wt% between 25 and 200 °C, attributed to adsorbedwater, and a more pronounced loss of 5.50 wt% between 200and 600 °C, corresponding to dehydration of the compounditself. This latter value agrees well with the theoretical weightloss (5.54 wt%) expected for 5.2 protons per formula unitreleased as H2O. Accordingly, the chemical composition of theprotonated powder sample was estimated as H5.20Li1.30La3-Zr1.5Ta0.5O12 (hereaer referred to as HLZTO), based on thecombined results from optical emission spectroscopy and TG-DTA analysis. The signal at 733 °C in DTA might be ascribedto the melting of Li2CO3 present in the sample, see below.Fig. 2b shows the XRD patterns of the LLZTO and HLZTOsamples. All reections can be indexed to the cubic garnet-typestructure. The cubic lattice parameter of the LLZTO sample,rened by the Le Bail method,31 was a= 12.95767(10) Å, in goodagreement with previously reported values.28–30 For the HLZTOsample, the cubic lattice parameter was determined to be a =13.04605(7) Å, which is larger than that reported for a partiallyproton-exchanged sample, Li3.72H2.78La3Zr1.5Ta0.5O12 sample (a= 13.0049 Å).33 This pronounced lattice expansion indicatesthat the Li+/H+ exchange reaction proceeded further in thepresent study, likely facilitated by the ne particle morphologyand large specic surface area of the powders.In addition, an extra reection at 2q = 21.5°, correspondingto the (310) plane, was observed in the XRD pattern. TheThis journal is © The Royal Society of Chemistry 2026http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970fFig. 2 (a) TG-DTA results of H5.20Li1.30La3Zr1.5Ta0.5O12. (b) XRD patterns of the parent Li6.5La3Zr1.5Ta0.5O12 and the proton-exchangedH5.20Li1.30La3Zr1.5Ta0.5O12 samples. Miller indices (hkl) are indicated for the reflections, and the corresponding space groups are also shown.Paper Journal of Materials Chemistry AOpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineappearance of this peak suggests a lowering of the crystalsymmetry from Ia-3d to I-43d, consistent with previous reportsfor H1.74Li3.59La2.93Zr1.05Ta0.95O12.34Local structures of LLZTO and HLZTOTo probe subtle local differences in the proton and lithiumsubstructures of HLZTO and LLZTO, we recorded 1H and 6LiMASNMR spectra, shown in Fig. 3. In Fig. 3c 6Li MAS spectra areshown before and aer the NMR SLR measurements at elevatedtemperatures, which will be discussed below. The 1H NMRspectra of both samples (see Fig. 3b) exhibit a strikingly broadline (>80 kHz), attributed to protons from the small Vespel capsof the MAS rotors. To conrm the contribution from the rotormaterial, a rotor was lled with dried Li1.3Al0.3Ti1.7(PO4)3,35 whichwas assumed to be proton-free. In HLZTO, the 1H signal isslightly anisotropic, with a maximum at 2 ppm (referenced to the1H NMR signal of adamantane, C10H16). As rst-order chemicalshi anisotropies vanish under sufficiently fast MAS conditions,the observed asymmetry likely reects a convolution of multiplecontributions, including the Vespel signal. Interestingly, the 1Hspectrum of LLZTO differs from that of proton-free LATP, sug-gesting that LLZTO can rapidly absorbmoisture at least involvingthe surface regions. The LLZTO signal comprises two lines: one at2.7 ppm, resembling the main line of HLZTO, and anotherat −3.2 ppm, absent in HLZTO. A similar lineshape was previ-ously reported by Larraz et al. for supposedly dry tetragonalLi7La3Zr2O12.15In contrast, the 6Li MAS spectra of LLZTO and HLZTO(Fig. 3a) each display a single resonance. The main line inLLZTO appears at approximately 1 ppm, whereas in HLZTO it isshied to around 0.3 ppm, reecting the modied chemicalenvironment induced by protonation. Additionally, the signalintensity in HLZTO is signicantly lower, consistent with thereduced lithium content, and the width is also narrower, indi-cating much weaker dipole-dipole interactions due to theincreased Li–Li interatomic distances.Fig. 3d compares the Raman spectra of LLZTO and HLZTO.The band between 80 and 130 cm−1 is dominated by vibrationsof the LaO8 units and is clearly observed in both spectra.15,36This journal is © The Royal Society of Chemistry 2026Contributions from Li–O vibrations are expected in the 200 to500 cm−1 range, which can be further divided into modes ofLiO6 (200 to 300 cm−1) and LiO4 (350 to 500 cm−1).37,38 Bands atapproximately 653 cm−1 and 732 cm−1 correspond to thestretching vibrations of the ZrO6 octahedra and Ta–O units,respectively. While the bands associated with La, Zr, and Ta arevery similar in LLZTO and HLZTO despite the difference in unitcell parameters, the Li–O vibration bands show morepronounced differences. In particular, the prominent band at300 cm−1 in HLZTO indicates modications within the lithiumsubstructure. Both samples also exhibit a relatively small bandaround 1090 cm−1, characteristic of trace amounts ofLi2CO3.38,39Conductivity measurementsTo probe macroscopic ion transport in HLZTO and LLZTO,conductivity was measured over a broad temperature range.Because the conductivity of HLZTO was signicantly lower thanthat of LLZTO, measurements focused on temperatures aboveroom temperature. The ionic DC conductivity (Fig. 4a), sDC, wasextracted from conductivity isotherms (Fig. S3), obtained byplotting the real part of the conductivity, s0, versus frequency, n.Above 120 °C, a shallow plateau emerges at low frequencies. Thecorresponding electrical capacitance C of ∼10−10 F indicatesa grain boundary process rather than electrode polarization. Athigher frequencies, a second, less pronounced plateau corre-sponds to the bulk process, with a capacitance C of approxi-mately 3 × 10−12 F.Heating HLZTO to 260 °C (1st run) slightly reducesconductivity, as seen in the Arrhenius plot (Fig. 4a) showingsDCT = f(1/T); the data referring to the cooling run showsslightly lower conductivities at temperatures lower than 100 °C.This effect is more pronounced for the grain-boundaryconductivity, suggesting that heating combined with drying,without additional compression, deteriorates the electricalcontact between grains. Activation energies are rather high,reaching values of 0.83 and 0.92 eV.To emphasise bulk properties more clearly, the corre-sponding electric modulus M was analysed (Fig. 4b). TheJ. Mater. Chem. A, 2026, 14, 20491–20503 | 20495http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970fFig. 3 (a) 6Li MAS NMR spectra (73.6 MHz) of LLZTO and HLZTO, showing a pronounced difference in chemical shift that reflects distinct local Lienvironments. (b) 1H NMR spectra (500 MHz, 22 kHz spinning speed) of HLZTO, LLZTO, and dried, proton-free LATP, included to illustrate thebackground 1H signal from the small Vespel rotor caps. Despite protection from moisture, LLZTO exhibits 1H NMR signals, indicating thepresence of protons at least on the particle surfaces and in regions slightly beneath the surface. Chemical shifts are referenced to adamantane,see text. (c) 6Li MAS NMR spectra (25 kHz) of HLZTO (a) and LLZTO (b) before and after heating in the static NMR experiment (see below). InHLZTO, the Li NMR signal remains unchanged, whereas in LLZTO, the line becomes significantly narrower, reflecting defect healing. Additionally,a smaller signal appears at −0.3 ppm, likely originating from Li2CO3. (d) Raman spectra of LLZTO and HLZTO, with bands characteristic of LiO6and LiO4 highlighted in blue.Journal of Materials Chemistry A PaperOpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineimaginary part of the electric modulus,M00, exhibits a maximumat a frequency nmax that corresponds to the characteristic elec-trical relaxation rate, ss−1 z 2pnmax.40 Because M is inverselyproportional to the electrical capacitance C, the quantity M00 isparticularly sensitive to bulk processes, which typically havecapacitances ca. 2 orders of magnitude smaller than grainboundary processes.41 Arrhenius plots derived from thesehopping rates42 show no difference between heating and cool-ing cycles in HLZTO, indicating that temperature primarilyaffects grain boundary conductivity rather than bulk iontransport, which again is to be characterized by a rather higheractivation energy of 0.88 eV, see Fig. 4b.In most oxides, conductivity is strongly limited by blockinggrain boundaries, a phenomenon particularly pronounced ingarnets.43–45 To probe macroscopic diffusion, high-temperaturesintering (∼1000 °C) is typically required. However, TGAmeasurements of HLZTO (Fig. 2a) reveal signicant mass lossstarting around 200 °C, due to the removal of incorporated20496 | J. Mater. Chem. A, 2026, 14, 20491–20503protons as water.43 Consequently, higher temperatures cannotbe applied to enhance proton conductivity in HLZTO.In contrast, an LLZTO pellet was annealed ex situ at 500 °Cfor several hours in an argon atmosphere (LLZTO500), resultingin a four-order-of-magnitude higher bulk conductivity; thecorresponding conductivity isotherms are shown in Fig. S3.This treatment also allowed clear separation of bulk (∼2 ×10−12 F) and grain boundary (∼4 × 10−11 F) contributions attemperatures below 60 °C, as illustrated in Fig. 5. The bulkconductivity of LLZTO is characterised by a much lower acti-vation energy of 0.4 eV compared with that of HLZTO.Furthermore, a high-temperature setup was employed to trackconductivity changes at elevated temperatures using a non-annealed sample, see Fig. 5. Aer a rst heating cycle to only300 °C, the overall conductivity increased markedly, eventhough sintering effects are generally not expected at such lowtemperatures. This observation is consistent with reports byAkimoto et al., who demonstrated that LLZTO pellets can besuccessfully sintered by hot pressing at 400 °C.29This journal is © The Royal Society of Chemistry 2026http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970fFig. 4 (a) Arrhenius diagram of bulk and grain boundary DC conductivities of HLZTO during heating from room temperature to 260 °C. Above120 °C, a second process appears, attributed to grain boundaries (empty symbols). The sample was held at 260 °C for two hours. During cooling,the conductivity decreases more rapidly (blue symbols), particularly the apparent grain boundary contribution. (b) Arrhenius plot of the peakfrequencies obtained from the imaginary part of the electric modulus (M00, inset) during heating and cooling. Themain peak ofM00 originates fromelectrical bulk relaxation; the peaks shown cover the temperature range from 20 °C to 260 °C.Paper Journal of Materials Chemistry AOpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineIn a second heating run up to 600 °C, sintering effectsbecome more pronounced. Between 300 °C and 480 °C, theconductivity increases more gradually than below 300 °C(Fig. 5a; see also the magnication in Fig. 5b). A simple expla-nation might be a phase transition; however, temperature-Fig. 5 (a) Arrhenius diagram showing the evolution of the DC conductisitu-annealed LLZTO500. LLZTO data were recorded using the high-temheating run (II) to 600 °C, with 40 °C heating/cooling steps. LLZTO500−100 °C and 260 °C, using 20 °C steps. Bulk conductivity for LLZTO (daAkimoto et al.29 at 20 °C and 60 °C on pellets obtained by low-tempenumbers indicate the sequential temperature profile during heating andThis journal is © The Royal Society of Chemistry 2026dependent XRD measurements in this range show no changein the cubic garnet structure. We therefore attribute the slowerincrease to the healing of defects that enhance Li-ion conduc-tivity, resulting in a reduced activation energy. Above 480 °C, theconductivity rises more rapidly. Upon cooling, the overallvities (plotted as sDCT) of (non-annealed, cold-pressed) LLZTO and experature setup in a first heating run (I) to 300 °C, followed by a seconddata were measured on the standard low-temperature setup betweenrk blue) could be extracted and is consistent with previous reports byrature sintering. (b) Magnified view of the high-temperature region;cooling, as also indicated in (b).J. Mater. Chem. A, 2026, 14, 20491–20503 | 20497http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970fJournal of Materials Chemistry A PaperOpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineconductivity remains higher by roughly half an order ofmagnitude, indicating improved macroscopic transport with anactivation energy of 0.48(2) eV. Aer in situ annealing, the totalconductivity at ambient temperature, affected by blocking grainboundaries, is comparable to that of LLZTO (10−6 S cm−1). Thecorresponding bulk conductivity of LLZTO500 (2 × 10−5 S cm−1at 20 °C, 0.40 eV), measurable only for the ex situ-annealedpellet, agrees well with previously reported values.29 The strongagreement between the present data and earlier results onsintered pellets29 demonstrates that our study successfullyprobes bulk properties in cold-pressed samples.1H nuclear spin relaxationTo probe local and long-range H+ ion transport energy barriers,we performed NMR SLR measurements (Fig. 6). This techniquemonitors the time-resolved recovery of the longitudinal spinmagnetization along the laboratory z-axis. Initially, a series of p/2pulses ips the macroscopic magnetizationM, originally alignedwith the external magnetic eld B0, into the (xy)-plane, where it israpidly dephased due to fast spin–spin relaxation. The subse-quent recovery back along the z-axis, known as nuclear spinrelaxation, is recorded via time-shied p/2 reading pulse toobtain the magnetization transient Mz (Fig. S4). The inectionpoint of this transient denes the relaxation time T1, and therelaxation rate, R1^1/T1, is temperature-dependent for thediffusing nuclei. Here, we analysed the transients with stretchedexponential functions (see above) to precisely determine theFig. 6 (a) 1H NMR R1 and R1r relaxation rates of HLZTO (blue) comparepartial Li/H exchange in glacial acetic acid (gaa), see Gombotz et al.19temperature peak arises from proton-lithium spin interactions, while theshows a broad maximum between 120 °C and 180 °C. Starting at 80 °C,(filled and open squares). The T1 relaxation rates of gaa:LLZTO display onmaximum at 150 °C, superimposed by a strong background signal, see atemperature range, comparing the first and second heating runs. Errortransients become double exponential. After heating HLZTO above 20modification of the sample.20498 | J. Mater. Chem. A, 2026, 14, 20491–20503rates. According to the Bloembergen-Purcell-Pound (BPP) model,R1 is proportional to the spectral density function J(u0), which isthe Fourier transform of the motional correlation function G thatcontains the motional correlation rate sc−1.46 A detailed discus-sion is provided elsewhere.32 The temperature dependence of 1/scis assumed to follow an Arrhenius law, sc−1= sc0−1exp[−Ea/(kBT)],and approximates the ionic jump rate, s−1, within a factor of two.In the low-temperature regime (u0s [ 1), the relaxationrates increase with temperature. At a characteristic temperatureTmax, the rate reaches a maximum when u0 approaches thejump rate s−1. At higher temperatures (u0s � 1), the ratesdecrease, forming the high-temperature ank of the diffusion-induced relaxation rate peak.In Fig. 6a, the relaxation rates of HLZTO (blue data points)are compared with those from our previous study on a single-crystalline garnet treated in glacial acetic acid (gaa:LLZTO,grey). For both the single crystal and the polycrystalline powder,the R1 data exhibit only the low-temperature ank, reectingshort-range proton dynamics possibly governed by very lowactivation energies as low as 0.13 eV. In the present case, thehigh-temperature ank, which would provide the activationenergy for long-range ion transport, could not be accessedbecause such measurements would require signicantly highertemperatures, at which the material is no longer stable (seebelow).In many three-dimensional materials, this limitation can beovercome by determining the relaxation rate in the rotatingd with those of single-crystalline gaa:LLZTO (grey), which underwentIn gaa:LLZTO, two distinct maxima appear in the T1r data: the low-second is attributed to proton diffusion. In polycrystalline HLZTO, R1rthe transients become double exponential, yielding two distinct ratesly the low-temperature flank of a peak, whereas HLZTO shows a locallso (b). (b) 1H NMR R1 and R1r rates of HLZTO probed over an extendedbars are shown for the R1r values; larger uncertainties occur where0 °C, the relaxation rates change significantly, indicating irreversibleThis journal is © The Royal Society of Chemistry 2026http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970fPaper Journal of Materials Chemistry AOpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineframe of reference using the spin-lock technique.32 In thisapproach, the external magnetic eld B0 is effectively replacedby the locking eld B1, which maintains magnetization in the(xy)0-plane. This allows the characterization of slower dynamicprocesses in the kHz regime within a suitable temperaturewindow. The condition for the relaxation-rate maximum, 1/T1r(^R1r), is given by u1s z 0.5 where u1 is the angular lockingfrequency corresponding to B1. Consequently, the maximumshis toward lower temperatures, making the high-temperatureank accessible. For systems exhibiting three-dimensionaldiffusion, this ank coincides with that of T1, as shown, forexample, for Li6La3ZrTaO12 by Stanje et al.47Although the 1H NMR R1 rates of HLZTO and gaa:LLZTOboth reveal only the low-temperature ank, subtle differencesexist. While gaa:LLZTO exhibits a gradual rise leading toa broad maximum at higher temperatures, HLZTO showsa distinct, but very shallow, local maximum around 140 °C (seealso Fig. 6b), followed by another increase in R1. This behavioursuggests the presence of multiple overlapping dynamicprocesses, with the high-temperature feature above 200 °C likelyrelated to the release of protons as water molecules. If the localmaximum at 140 °C indeed arises from diffusion, it wouldcorrespond to rapid proton jumps with rates on the order of 1.9× 109 s−1.Comparison of the R1r NMR data between HLZTO andgaa:LLZTO provides further insight. The gaa:LLZTO sampleexhibits two overlapping rate maxima: the rst, at about 60 °C,corresponds to a relatively fast process that cannot be attributedto proton diffusion. We therefore assign it to proton spin uc-tuations induced by the rapid motion of lithium ions ingaa:LLZTO, as discussed elsewhere.19 Proton diffusion becomesevident only at higher temperatures, around 125 °C, as indi-cated by the second 1H NMR rate peak.19In the strongly protonated HLZTO, this rst rate peak isabsent, clearly indicating that fast lithium diffusion does notoccur. Instead, the relaxation rates increase gradually withtemperature (0.17 eV), then more steeply above 60 °C (0.31 eV),reaching a broad maximum between 120 °C and 180 °C, whichlikely reects two overlapping relaxation processes. From themaximum condition u1s z 0.5 and the locking eld frequency(u1/2p = 20 kHz), a proton jump rate s−1 = 2.5 × 105 s−1 isobtained for the R1rmaxima. Using the Einstein–Smoluchowskirelation and assuming a jump distance of 1.7 Å between tetra-hedral and octahedral Li sites for three-dimensional diffusion,this corresponds to an NMR self-diffusion coefficient DNMR,H =1.2 × 10−15 m2 s−1. Around 150 °C, this describes a process ofsimilar magnitude to that in the single crystal, while in HLZTOthis faster motion is superimposed by a second, slower processreaching comparable diffusivity only near 180 °C.As R1r decreases again above 180 °C, both processes likelycontribute to both short-range dynamics and partly long-rangeion transport. Activation energies for short-range iondynamics are considerably lower (0.17 eV, 0.31 eV) than thoseseen in conductivity spectroscopy for bulk ion transport (ca.0.88 eV, see above). Capturing the high-temperature ank,which would provide detailed information on long-range iontransport, was hindered by a sudden, pronounced rate increase,This journal is © The Royal Society of Chemistry 2026probably caused by the decomposition of the sample. Thisdecomposition irreversibly alters proton dynamics, as is alsoevident from the second heating run (Fig. 6b). Interestingly, thisabrupt rate change reappears reproducibly in the second run,reminiscent of a phase transition. Although no distinct signalwas detected in the DTA (Fig. 2), TGA measurements revealeda marked mass loss beginning at about 200 °C. Adding to thecomplexity, the magnetization transients become double-exponential above 80 °C (Fig. S4), likely reecting the coexis-tence of two relaxation processes with distinct characteristictimescales.To compare the NMR-derived diffusion parameters withthose obtained from conductivity spectroscopy, we applied theNernst–Einstein relation.48 Owing to the extremely lowconductivities, this yielded a solid-state diffusion coefficient ofDs= 3.8× 10−18 m2 s−1 at 180 °C. For comparison, Fleig and co-workers reported a bulk interdiffusion coefficient of 7 × 10−17m2 s−1 at 80 °C,24 while Kilner and co-workers obtainedapproximately 10−16 m2 s−1 at 100 °C for proton-exchanged, Ga-substituted single-crystalline Li7La3Zr2O12.25 Both literaturevalues are in reasonable agreement with the NMR self-diffusioncoefficient (DNMR,H = 1.2 × 10−15 m2 s−1 at ca. 150 °C) deter-mined here at somewhat higher temperature. The comparisonsuggests, however, that the conductivities measured for HLZTOunder dry nitrogen are likely underestimated, most probablydue to reduced interparticle contact.Static 1H NMR line measurementsSimultaneously with the NMR SLR experiments, we recordedstatic 1H NMR spectra of HLZTO over the same temperaturerange (Fig. 7). Covering such a broad range allows the detectionof structural changes that may occur during the SLR measure-ments and provides additional insight into the dynamicbehaviour of the nuclei. For mobile nuclei, the width of theresonance line typically decreases with increasing temperature,an effect commonly known as motional narrowing.49Fig. 7a shows the complete 1H NMR spectrum of HLZTO,while Fig. 7b presents a magnied view of the sharper line at thecentre of gravity. Because the 1H nucleus has spin I = 1/2, noquadrupolar interactions are expected, and the observed linebroadening therefore arises solely from dipolar interactions.From Fig. 7a, it is seen that the two broad components graduallydiminish with increasing temperature and disappear entirelyabove 200 °C. These broad signals, representing the majority ofthe 1H spins in HLZTO, indicate strong homonuclear dipolarcouplings between protons, which weaken signicantly asproton mobility increases. The requirement for high tempera-tures to signicantly narrow these spectral features is consistentwith the relatively poor H+ transport properties observed inconductivity spectroscopy.As shown in Fig. 7b, the much sharper line near the spectralcentre of gravity is composed of at least three distinct compo-nents. Because these contributions strongly overlap, it is notpossible to precisely determine the temperature interval inwhich the overall motional narrowing sets in. However, the dataclearly show that a small fraction of protons accesses multipleJ. Mater. Chem. A, 2026, 14, 20491–20503 | 20499http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970fFig. 7 Static 1H (spin quantum number I = 1/2) NMR spectra of H-richHLZTO recorded at the indicated temperatures and at 300 MHz, thatis, using the spectrometer for variable-temperature NMR studies. (Left)Full spectra including the broad background contributions. (Right)Magnified view of the central region highlighting the evolution ofa narrow 1H NMR signal arising from proton mobility that is sufficientlyfast on the NMR timescale.Fig. 8 Comparison of 7Li NMR SLR rates of LLZTO (orange) andHLZTO (blue). Both R1 and R1r indicate that Li+ diffusion is stronglysuppressed in HLZTO. In H-free LLZTO, the relaxation maxima are notdirectly observable due to concurrent changes in the local defectstructure at elevated temperatures, as revealed in a second heatingrun. Activation energies, extracted from linear fits (continuous lines) ofthe flank regions, are indicated. The value of 0.34 eV observed forLLZTO agrees with the activation energy determined for bulk iondynamics via conductivity spectroscopy.Journal of Materials Chemistry A PaperOpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinediffusion pathways, leading to a stepwise change in the overall1H NMR spectrum. Most protons, however, are represented bythe two very broad spectral components shown in Fig. 7a.7Li NMR SLR measurementsProtonation of garnets slightly impedes lithium diffusion, evenwhen only a small fraction of Li ions is replaced by protons, asdemonstrated previously in our single-crystal NMR relaxationstudy.19 To assess the extent of this effect in the presentsamples, we measured 7Li NMR SLR rates in both the laboratoryand rotating frames for HLZTO and LLZTO (Fig. 8). In HLZTO,both rates R1 and R1r are shied markedly toward highertemperatures compared with LLZTO. Consequently, no distinctrate maximum is visible in R1r up to 270 °C, indicating thatlithium ions undergo only restricted, localised jumps within theobserved temperature range. These jumps are characterized byactivation energies of ca. 0.27 eV, which determine the slopes ofthe corresponding low-temperature anks (Fig. 8).In contrast, in H-free LLZTO shows a much earlier increaseof R1 and R1r, reecting faster ionic motion. Again, the anksindicate relatively low activation energies ranging from 0.30 to0.34 eV. The value of 0.34 eV is relatively close to that suggestedby bulk conductivity measurements (ca. 0.4 eV, Fig. 5a). Around−20 °C, the R1r transients become double-exponential; between20500 | J. Mater. Chem. A, 2026, 14, 20491–20503−10 and 20 °C, both components contribute equally to the totalmagnetization (see Fig. S4). A discontinuity in R1r is observednear 330 K. For comparison, in single-crystalline LLZTO, NMRrelaxation shows a maximum already at 185 K.47 This shi isconsistent with the lower conductivity of our polycrystallineLLZTO, which is inferior to that of highly sintered36,50,51 orsingle-crystalline garnets that reach conductivities in the mSrange at ambient conditions.26,52At lower temperatures, R1 of LLZTO increases only gradually,signifying ionic motion with a low activation barrier or relaxa-tion due to non-diffusive effects. Above −20 °C, the rates risemore steeply, marking the onset of a diffusion-induced relaxa-tion peak with a ank yielding to 0.34 eV, a value beingconsistent with bulk conductivity measurements (see above).Beyond roughly 60 °C, however, the increase slows, producinga kink along the low-temperature ank. Even aer heating tohigher temperatures, a distinct rate maximum did not develop.Aer cooling and re-measuring in a second heating run, therelaxation rates were signicantly reduced, revealing dimin-ished localised motion following thermal exposure to approxi-mately 200 °C. The apparent R1r maximum also shied tohigher temperatures in this run.Any discrepancies in activation energies and differences intrends upon annealing, as observed by NMR and conductivitymeasurements, can be explained by the differing time- andlength-scale sensitivities of the two techniques. The low-temperature ank primarily reects activation barriers oflocal, short-range Li+ jumps, which are strongly inuenced byThis journal is © The Royal Society of Chemistry 2026http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970fFig. 9 Static 7Li (I = 3/2) NMR lines of (a) LLZTO and (b) HLZTO recorded at the indicated temperatures. The inset in (a) shows the full spectrumincluding the 90° satellite singularities of the quadrupolar powder pattern pointing to a coupling constant of ca. 100 kHz. In (c), the line widths,FWHM (full width at half maximum), are plotted against temperature. The shift of the second run of LLZTO to higher temperatures shows that(local) Li diffusivity was irreversibly decreased after heating. 7Li NMR line narrowing in HLZTO occurs only at much higher temperatures, indi-cating sluggish Li+ mobility.Paper Journal of Materials Chemistry AOpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinedefects such as dislocations and grain boundaries, see, forexample, the defect-affected NMR rates of Li3InCl6 samplesprepared by different methods, as investigated recently.53 Uponheating, defect healing reduces the number of such extrinsicsites, suppressing localised defect-mediated motion detectableby NMR. At the same time, annealing improves intergranularcontact and may also enhance long-range transport pathwayswithin the bulk, thereby enhancing the overall (total) macro-scopic conductivity measured by impedance spectroscopy.Both NMR and conductivity spectroscopy conrm that Lidiffusion is strongly suppressed in HLZTO as a result of protonexchange. A straightforward explanation is that the Li diffusionpathways are no longer continuously occupied by Li ions but arelargely blocked by protons, which move more slowly due todifferent binding environments and stronger electrostaticinteractions with the host lattice. This behaviour is also re-ected in the 7Li NMR line shapes.7Li NMR line shapesA consistent picture of Li diffusivity in the two samples emergesfrom the static 7Li (I = 3/2) NMR spectra shown in Fig. 9. Thesame spectra, shown over the full frequency range to highlightthe quadrupolar contributions visible as singularities ankingthe central line, are shown in Fig. S5 and for −95 °C in the insetof Fig. 9a. Quadrupolar intensities are more pronounced inLLZTO due to the higher Li concentration; the 90° singularitieswith a splitting of 50 kHz point to a quadrupolar couplingconstant of ca. 100 kHz if an axially symmetric electric eldgradient at the nuclear Li site is assumed.Focusing on the central lines shown in Fig. 9a, in H-freeLLZTO, the lines begin to narrow around −60 °C, whereas inThis journal is © The Royal Society of Chemistry 2026HLZTO only minor changes in width occur between 20 °C and200 °C (Fig. 9b), followed by a more pronounced narrowing stepabove 180 °C. Such clear changes undouble reveal much slowerLi+ ions in H-rich HLZTO. In Fig. 9c, we plotted the line widths(full width at half maximum, FWHM) of the static 7Li NMRsignals as a function of temperature. It is evident that Li spins inLLZTO diffuse clearly faster on the NMR timescale compared tothose in HLZTO. This signicant difference is apparent becausemotional line narrowing in LLZTO (rst run) sets in at muchlower temperatures, while the corresponding curve for HLZTOis shied to higher temperatures. The pronounced reduction inLi conductivity inferred from NMR line narrowing is consistentwith the decreased Li+ mobility observed in SLR NMR (Fig. 8).Because NMR probes bulk ion dynamics and is largely inde-pendent of pellet densication or sintering quality, the lowerconductivity observed for HLZTO (see Fig. 4a) cannot beattributed solely to insufficient sintering of the correspondingpellets. The overall decrease in Li+ is clearly seen in NMRmeasurements and largely contributes to the much loweroverall (ionic, Li+ and H+) conductivity in HLZTO.In a second run, we observe that the 7Li NMR motional nar-rowing curve for LLZTO shis toward higher temperatures, indi-cating reduced Li+ diffusivity aer so annealing. Assuming thatNMR lines are inuenced not only by averaging of dipolar inter-actions through long-range ion dynamics but also, to some extent,by localized motions triggered by point defects, this behaviormirrors the changes seen in the R1 NMR rates (see the discussionabove and Fig. 8). We propose that defect healing suppressesshort-range (localized) hopping while enhancing through-going(long-range) ion transport in the studied compounds as isnally probed in conductivity spectroscopy (see Fig. 5b).J. Mater. Chem. A, 2026, 14, 20491–20503 | 20501http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970fJournal of Materials Chemistry A PaperOpen Access Article. Published on 15 April 2026. Downloaded on 5/27/2026 7:41:42 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineStructural changes of HLZTO as seen by NMR and XRDTo assess structural changes in HLZTO upon exposure to hightemperatures, high-temperature XRDmeasurements up to 300 °C show only a slight contraction of the lattice parameter(Fig. S6), which is consistent with proton removal in the form ofH2O. The accompanying loss of oxygen may further contract thelattice through deformation of the Zr/TaO6 octahedra. Hence,the slight change in linewidth below 180 °C (see Fig. 7) may beattributed to the structural modications observed in the XRDdata described above. In contrast, the local environment of theremaining Li ions remains largely unaffected, as evidenced bythe 6Li MAS NMR spectra of the powder used in the NMR SLRexperiments (see Fig. 3c). Similarly, 6Li MAS NMR of H-freeLLZTO aer the SLR measurements shows a signicant linenarrowing, indicating faster Li dynamics, and a small addi-tional signal at approximately −0.3 ppm, consistent with minorlithium carbonate formation.Consistent with trends observed recently,19 Li diffusivity isslowed in Li–H exchanged garnet samples. Proton mobility isobservable in these samples, with moderate diffusion coeffi-cients as discussed above. Additionally, 1H NMR SLRmeasurements indicate fast localised proton motion, which,however, does not signicantly contribute to long-range Htransport in HLZTO.ConclusionWe studied proton dynamics in Li–H exchanged garnets,selecting LLZTO as a model system to quantify jump rates,diffusion coefficients, and activation energies governing protonmobility in these oxides. Starting from lithium-rich LLZTO, thegarnet structure is retained even when most lithium ions arereplaced by protons. ICP-OES and 6Li MAS NMR measurementsshow that a small fraction of the original lithium remains aerthe Li/H exchange reaction reaches completion, as indicated bya stable pH. Raman spectra conrm the presence of lithiumwithin the garnet lattice, in addition to the characteristicvibrations of LaO8, ZrO6, and TaO6 units. The ne sub-micrometer particle morphology facilitates efficient protonexchange and contributes to the observed dynamics.Conductivity spectroscopy combined with element-specicNMR reveals that the remaining lithium ions are largelyimmobilised, making protons the dominant charge carriers.Although long-range proton transport is impeded by grainboundary resistance, relatively fast proton dynamics areobserved via 1H NMR SLR measurements. Several dynamicprocesses overlap within the studied temperature range, witha broad maximum between 120 and 180 °C, suggesting theonset of long-range bulk proton diffusion, characterised bya self-diffusion coefficient of DNMR,H = 1.2 × 10−15 m2 s−1 atapproximately 150 °C. In contrast, lithium motion remainshighly restricted under the same conditions.In LLZTO, conductivity can be enhanced bymoderate thermaltreatment (∼300 °C), which reduces grain boundary resistance.However, NMR shows that localised ionic motion, driven byextrinsic defects, diminishes aer heating above 200 °C. In20502 | J. Mater. Chem. A, 2026, 14, 20491–20503HLZTO, excessive thermal treatment leads to proton loss,impairing both intergrain conductivity and likely localised bulkdiffusion. Overall, these results highlight NMR as a powerful toolfor probing bulk diffusion processes and resolving local ionicdynamics in proton-conducting garnets, with implications for thedesign of solid electrolytes where protonmobility dominates overlithium transport.Author contributionsF. S. performed the NMR and conductivity measurements,contributed to conceptualization, and prepared the manuscript.J. A. was responsible for sample preparation, characterization,and data analysis. Y. M. carried out the TG-DTA and structuralanalyses. K. M. conducted the SEM and TEM analyses. K. T.contributed to sample preparation and scientic discussion.H. M. R. W. contributed to conceptualization, manuscriptpreparation, scientic discussion, and supervised the project.Conflicts of interestThe authors declare that they have no conicts of interest.Data availabilityAll data generated or analysed during this study are included inthis published article (and its supplementary information (SI)).Supplementary information is available. See DOI: https://doi.org/10.1039/d5ta09970f.AcknowledgementsWe thank Francesco Carraro and Paolo Falcaro for the Ramanmeasurements. We acknowledge nancial support from theDFG (WI3600 2-1 and 4-1), the Japan Society for the Promotionof Science (JSPS) KAKENHI (Grant Number JP24K08593), andthe National Institute for Materials Science (NIMS) BatteryResearch Platform, as well as from the “Advanced ResearchInfrastructure for Materials and Nanotechnology in Japan(ARIM)” of the Ministry of Education, Culture, Sports, Scienceand Technology (MEXT), Japan (Proposal NumbersJPMXP1224NM5346 and JPMXP1225NM5365).References1 W. 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A, 2026, 14, 20491–20503 | 20503http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09970f From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12 From lithium to proton mobility in garnet electrolytes: an NMR and conductivity study of H5.2Li1.3La3Zr1.5Ta0.5O12