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[Maria Rita Cicconi](https://orcid.org/0000-0002-4106-4057), [Koji Kimura](https://orcid.org/0000-0001-5485-3672), [Henrik Bradtmüller](https://orcid.org/0000-0001-7971-2163), [Hongyi Deng](https://orcid.org/0009-0000-8216-433X), [Shinji Kohara](https://orcid.org/0000-0001-9596-2680), [Yohei Onodera](https://orcid.org/0000-0002-3080-6991), [Tomokatsu Hayakawa](https://orcid.org/0000-0003-1817-8854), [Seiya Shimono](https://orcid.org/0000-0003-1608-3296), [Koichi Hayashi](https://orcid.org/0000-0002-8782-4293), [Dominique de Ligny](https://orcid.org/0000-0001-9621-4609)

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[Unraveling the structural complexity of niobate units in aluminosilicate glasses and glass–ceramics](https://mdr.nims.go.jp/datasets/92b47dd8-177d-45ec-b343-be441dadb8bc)

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Unraveling the structural complexity of niobate units in aluminosilicate glasses and glass–ceramics© 2025 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2025, 6, 3863–3874 |  3863Cite this: Mater. Adv., 2025,6, 3863Unraveling the structural complexity ofniobate units in aluminosilicate glasses andglass–ceramics†Maria Rita Cicconi, *a Koji Kimura, bcd Henrik Bradtmüller, e Hongyi Deng, aShinji Kohara, cd Yohei Onodera, c Tomokatsu Hayakawa, fSeiya Shimono, d Koichi Hayashi bd and Dominique de Ligny aNiobium-containing glasses and glass–ceramics play an important role in several technological applica-tions, but our understanding of the structure–property relationships of many Nb-containingcompositions is still rudimentary. To address the current limitations, the present contribution reportsdata from synchrotron high-energy X-ray diffraction data to unravel the structural evolution of niobateentities in alkali–aluminosilicate glasses. The data obtained are compared with complementary Ramanand solid-state NMR spectroscopy data to provide a better interpretation of the macroscopic propertiesof the glasses and their crystallization behavior in terms of the glass structure. The data show that theincorporation of niobium into the glass network (from 0.2 mol% to 10 mol%) causes a rearrangement ofthe units and induces large modifications, particularly in the medium-range order. Nb5+ is present in allglasses predominantly in the form of 6-coordinated [NbO6] units with rather invariant hNb–Oi bond dis-tances around 2.0 Å. This observation correlates well with the 93Nb NMR data showing similarly smallchanges in the chemical shift values. A contrasting scenario is presented when looking beyond the firstcoordination sphere, with a particular focus on the A–Nb (A = alkali) and Nb–Nb correlations. Both arestrongly dependent on bulk chemistry, which, in turn, is influenced by the availability and nature ofcharge-compensating alkali ions. The addition of Nb has a relatively minimal effect on Si and Al units,promoting the association of Nb with other Nb species, thereby initiating the formation of a subnetworkof [NbO6] units in a corner shared environment. Both alkali species and Nb5+ ions in the amorphousstate tend to favor a structural arrangement very similar to that of the stable crystalline phase.1. IntroductionNiobium-containing glasses have applications in many fieldsand have been studied for their enhanced optical and dielectricproperties, radiation shielding capabilities, and energy storageapplications.1–5 Nb ions can be found in various coordinationenvironments in glasses and crystalline materials. In glasses,Nb2O5 is considered an intermediate oxide: it can play a networkmodifying role, inducing a greater number of non-bridgingoxygens, or it can help to crosslink the structural units, improv-ing the overall glass connectivity, e.g., ref. 6–8. Based on previousstudies, it seems that this dual role depends on the bulkcomposition but also on the total Nb2O5 content.8–10 Indeed,there is a fair consensus in the literature that, regardless of thenetwork former cation selected, Nb2O5 tends to form 3D corner-shared [NbO6] clusters1,8–11 at high Nb-contents, independentlyof the present network former. These clusters provide specificglass properties, such as the enhancement of the electro-opticalKerr coefficient.12,13 However, it has been observed that thetransition between the different Nb environments/structuralroles does not purely depend on a specific niobium contentbut also strongly on the chemistry of the glass.9Recently, in order to develop niobate perovskite-like crystalsin stable glass matrices, the Nb2O5 solubility and the propertiesa Department Werkstoffwissenschaften, Institut für Glas und Keramik, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Martenstrasse 5, 91058 Erlangen,Germany. E-mail: maria.rita.cicconi@fau.de; Tel: +49 9131 85-27555b Department of Physical Science and Engineering, Nagoya Institute of Technology,Gokiso-cho, Showa-ku, Nagoya 466-8555, Japanc Center for Basic Research on Materials, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japand Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Sayo-cho, Sayo-gun,Hyogo 679-5198, Japane Department of Materials Engineering, Vitreous Materials Laboratory, FederalUniversity of São Carlos, CP 676, 13565-905, São Carlos, SP, Brazilf Department of Life Science and Applied Chemistry, Nagoya Institute of Technology,Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ma00082cReceived 31st January 2025,Accepted 25th April 2025DOI: 10.1039/d5ma00082crsc.li/materials-advancesMaterialsAdvancesPAPEROpen Access Article. Published on 28 April 2025. Downloaded on 6/23/2025 3:23:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttps://orcid.org/0000-0002-4106-4057https://orcid.org/0000-0001-5485-3672https://orcid.org/0000-0001-7971-2163https://orcid.org/0009-0000-8216-433Xhttps://orcid.org/0000-0001-9596-2680https://orcid.org/0000-0002-3080-6991https://orcid.org/0000-0003-1817-8854https://orcid.org/0000-0003-1608-3296https://orcid.org/0000-0002-8782-4293https://orcid.org/0000-0001-9621-4609http://crossmark.crossref.org/dialog/?doi=10.1039/d5ma00082c&domain=pdf&date_stamp=2025-05-07https://doi.org/10.1039/d5ma00082chttps://doi.org/10.1039/d5ma00082chttps://rsc.li/materials-advanceshttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ma00082chttps://pubs.rsc.org/en/journals/journal/MAhttps://pubs.rsc.org/en/journals/journal/MA?issueid=MA0060123864 |  Mater. Adv., 2025, 6, 3863–3874 © 2025 The Author(s). Published by the Royal Society of Chemistryof three aluminosilicate glass series have been investigated,starting from very low doping contents (0.2 mol% Nb2O5) up to15 mol%.9,14 It was observed that the Nb2O5 incorporation limitvaried quite a lot depending on the degree of glass polymerization(alkali/Al ratio), being three times lower in more polymerizedglasses. Most importantly, it was observed that the properties ofthe developed glasses showed different trends and even oppositebehavior in a few cases.9 Raman spectroscopy was used to identifysome fingerprint vibrations that were correlated to the differentconnectivity between [NbO6] entities and to the availability ofcharge compensator ions as the driving forces in determiningthe solubility and properties evolution.9 The differences in theglass topology also give rise to quite dissimilar behavior in terms ofoptical properties and, in particular, regarding the use of theseglasses as self-activated lanthanides for photoluminescenceapplications.14 Another distinctive feature of Nb-containing glassesconcerns the crystallization kinetics and crystallization mechan-isms, which are strongly influenced by the bulk chemistry. Indeed,the glass–ceramics obtained from peralkaline compositions (Na2O/Al2O3 molar ratio = 2.3) show preferential surface crystallizationwith alkali–niobate crystals propagating from the edge towards thecenter. A polymerized glass (Na2O/Al2O3 molar ratio = 1), on theother hand, shows a volume crystallization of sub-micron crystals,resulting in a glass–ceramic with a high degree of transparencyeven after prolonged thermal treatments.9,14 Similar crystallizationbehavior has been observed in a glass in the SiO2–Li2O–Nb2O5ternary system with a Li2O/Nb2O5 molar ratio of 1.4.15While Raman9,13 and NMR15–17 spectroscopy have providedsome insight into the compositional dependence of the Nbstructural units, more specific information on the local environ-ment and connectivity of the Nb units, which is critical for thedesign of Nb-based multifunctional materials remains elusive.A major challenge, especially when considering cation arrange-ments beyond the first coordination sphere, is to unravel the‘‘order’’ in disordered materials. In this work, we have addressedthis issue through a combination of synchrotron X-ray scattering,Raman, and NMR data. The collected data documents the struc-tural evolution of the niobate entities in aluminosilicate glasses inmore detail, helping to shed some light on the structural behaviorof this technologically important element.2. Experimental methodsDetails of the glass synthesis, bulk chemistries, and physical,thermal, and optical properties, along with the study of the glassstructure via Raman of the present glasses and glass–ceramics,were previously described in ref. 9, 14 and 16. The glasses belongto the ternary system SiO2–Na2O–Al2O3, and the pristine materialshave a constant SiO2 molar content (B66.6 mol%) and Na2O/Al2O3molar ratios of 2.3 and 1, respectively corresponding to a peralka-line composition (Na2O 4 Al2O3, sample label NA66.10), and apolymerized glass at the metaluminous joint (Na2OQAl2O3, labelNA66.17). Portions of the base glass were finely ground, dopedwith commercially available K0.5Na0.5NbO3 ceramic powder (KNN,d50 = 0.95 mm, CerPoTech, Norway) from 0.4 up to 20 mol%, thenremelted in air with a dwell time of 1 h at 1500 1C and 1650 1C,respectively for the NA66.10 and the NA66.17 series (the Nb2O5content is about half that of KNN. The chemical compositions ofthe glasses obtained by ICP analysis9 are reported in Table S0,ESI†). In the polymerized series NA66.17, the sample containing10 mol% Nb2O5 crystallized during quenching (label xx17-20);therefore, the maximum amount of Nb2O5 in this glass series isB4.8 mol% (label NA66.17-10KNN). Glass–ceramic (GC) sampleswere prepared from polished glass specimens (containing10 mol% KNN) through heating at the respective exothermic eventwith a 10 K min�1 heating rate and a holding time of 2 to 4 hours(see ESI,† S0). The samples are labeled GC10-10 and GC17-10,respectively, for the peralkaline and metaluminous series. BothGCs show the presence of a single crystalline phase KxNa(1�x)NbO3(XRD patterns and unit cell parameters are reported in ref. 14).The variation in the local structures as a function of KNNmolar content was investigated by high energy X-ray total scattering.The experiments were carried out at the high-energy X-ray diffrac-tion beamline BL04B2 at SPring-8 (Japan) by using a horizontal two-axis diffractometer.18–21 Bulk glass samples and crushed glass–ceramics samples were measured, with the latter inserted into silicaglass capillaries. Additionally, the crystalline KNN material wasmeasured as a reference. Only sample with significant amound ofNb were studied to obtain a good enough signal. Samples wereirradiated with a monochromatic X-ray beam with an incidentenergy of 112.826 keV (l = 0.1099 Å), and the scattered X-rays weredetected by seven-point-type detectors. The highest scattering vectorQ (Q = 4psiny/l) was 25 Å�1. The raw data were corrected forpolarization, absorption, and background, and the contribution ofCompton scattering was subtracted using the data analysis softwaredeveloped by the beamline scientists (see ref. 20). The fully correcteddata were normalized to give a Faber–Ziman total structure factorS(Q). The reduced pair distribution function (PDF), G(r), and totalcorrelation function T(r) were obtained by the inverse-Fourier trans-form of S(Q) with a Lorch function.20,22 The data for the Fouriertransform were terminated at Qmax = 23.5 Å�1 beyond which thesignal-to-noise ratio of the unsmoothed data was considerably low.The concept of pair-function developed by ref. 23 and 24 to analyzethe average distances and coordination numbers of vitreous SiO2and B2O3 was used here to estimate the interatomic distances andthe cation–oxygen coordination numbers of the glasses. The proce-dure is described in detail in ref. 21. The parameters for Si–O, Na–O,O–O, and K–O correlations were extracted from the RMC-MD modelof alkali–silicate glasses reported in ref. 25, while the parameter forAl–O was extracted from MD simulations of a sodium aluminosili-cate glass.26Multinuclear 23Na, 27Al, 29Si, and 93Nb solid-state NMRexperiments were carried out at 5.7, 7.05, and 14.1 T on anAgilent DD2, a Varian INOVA 300 MHz, and a Bruker Avance600 Neo spectrometer. 29Si MAS NMR experiments wererecorded at a Larmor frequency of 59.2 MHz in a 7 mm doubleresonance probe operating at uMAS = 5 kHz. 64–128 transients of4096 data points were recorded using 901 pulses of 6 msduration and recycle delays of 500–1000 s. 29Si chemical shiftsare reported w.r.t. tetramethylsilane (TMS – 0 ppm) usingkaolinite (�91.2 ppm) as a secondary standard. 23Na centralPaper Materials AdvancesOpen Access Article. Published on 28 April 2025. Downloaded on 6/23/2025 3:23:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ma00082c© 2025 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2025, 6, 3863–3874 |  3865transition (CT) MAS NMR experiments were performed at14.1 T (Larmor frequency of 158.8 MHz) in a 2.5 mm triple-resonance probe at uMAS = 20 kHz. Short 1 ms pulses corres-ponding to a 301 tip angle were used for excitation and 256–1024 transients were collected at recycle delays of 0.5–5 s. 23Nachemical shifts are reported w.r.t. a 0.1 M NaCl solution(0 ppm). 27Al CT MAS NMR experiments were performed at14.1 T (Larmor frequency of 156.4 MHz) in a 2.5 mm triple-resonance probe at uMAS = 20 kHz. Short 0.8 ms pulses corres-ponding to a 251 tip angle were used for excitation and 256transients were collected at recycle delays of 0.5–1 s. 27Alchemical shifts are reported w.r.t. a 0.1 M Al(NO3)3 solution(0 ppm) using AlF3 as a secondary standard (�16 ppm). 93Nb CTMAS NMR experiments were conducted at 14.1 T (Larmorfrequency of 146.80 MHz) in a 1.3 mm double-resonance probeat uMAS = 60.0 kHz and a nutation frequency of urf = 150 kHz. Forthe rotor-synchronous Hahn-Echo spectra, excitation pulselengths of 0.2 ms and 0.6 ms were used at a recycle delay of0.1 s. To minimize coil ringing effects, spectra were acquiredafter 2 rotor cycles. Chemical shifts were referenced against asaturated solution of NbCl5 in acetonitrile (MeCN 0 ppm), usingLiNbO3 (diso = �1004 ppm) as a secondary standard.3. Results3.1 High energy X-ray total scatteringFig. 1 shows the total structure factor S(Q) of the samples understudy. The signals show distinct oscillations up to high Qvalues, indicating the presence of well-defined coordinationshells in the glass network. By adding KNN, there is a generaldamping of the oscillations at high scattering vector values(Q 4 10 Å�1), even though there is a large similarity in theoscillation width and position. At low Q vectors, the mostprominent features are the first two peaks at 1.64 Å�1 and2.17 Å�1, with the latter becoming dominant as KNN is added.In pure silica glass, the first peak at B1.6–1.7 Å�1, also referredto as first sharp diffraction peak (FSDP), is associated with themedium-range order of amorphous materials.19,27,28 The absoluteposition of these two peaks does not seem to change as a functionof the KNN content, and only the relative intensities vary. On thecontrary, the local signal maxima in the higher Q range showrather strong changes with the increase of alkali–niobate species,i.e., the peaks at 5 Å�1 and 9 Å�1 become suppressed, while thepeak at B4.5 Å�1 shifts toward smaller Q vectors.The total correlation function T(r) for an aluminosilicateglass is presented in Fig. 2, along with data of crystalline(K0.5Na0.5)NbO3 (KNN), used as a reference for typical Nb-related distances. The assignment of the different peaks isdone by comparing the interatomic distances given by theradial distribution curve with the interatomic distances ofcrystalline materials and previous reports on aluminosilicateglasses. The T(r) of the glass having only 0.2 mol% Nb2O5(NA66.10-0.4KNN) shows the expected peaks of aluminosilicateglass, with no strong contributions from Nb. The intense firstpeak at 1.65 Å, related to nearest neighbor intra tetrahedralbonds (T–O, T = Si, Al) derives from the partial contributions ofSi–O (B1.6 Å) and Al–O pairs (B1.75 Å).29,30 The two followingsmaller correlations arise from the alkali cations–oxygen (A–O,B2.30 Å) and oxygen–oxygen (O–O, B2.63 Å) distances. Thestrong contribution peaking around 3.10 Å derives mainly fromtetrahedra–tetrahedra (T–T) distances.31 The reference alkali–nio-bate crystalline compound (KNN) shows well-defined, strongpeaks. The experimental PDF data analysis of the crystallinematerial was refined by PDFgui32 and the results are reported inthe ESI† (Fig. S1 and Table S1). The refined structure of the KNNcrystalline compound results in an average bond length of 2.00 Åfor the Nb–O octahedron, and a distortion index of the bondlength of 0.06 (Baur’s polyhedra distortion index33) compatiblewith the broad Nb–O correlation observed in Fig. 2 (see Table S1for the refined correlations, ESI†). Furthermore, mathematicalfunctions (Gaussians) were used to decompose the real-space databoth for the KNN crystalline material and the glass NA66.10-0.4(KNN). The latter shows four main contributions that wellagree with the expected distances in a sodium aluminosilicateglass. Regarding the KNN crystal compound, the first broadFig. 1 Measured X-ray total structure factors for the two aluminosilicate glass series having different KNN contents (from 0.4 mol% to 20 mol%).Materials Advances PaperOpen Access Article. Published on 28 April 2025. Downloaded on 6/23/2025 3:23:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ma00082c3866 |  Mater. Adv., 2025, 6, 3863–3874 © 2025 The Author(s). Published by the Royal Society of Chemistrycontribution peaking around 2.00 Å is split and can be deconvo-luted into two Gaussian functions centered at 1.90 and 2.14 Å(Fig. 2). The position of both functions stands in agreement withthe PDF refined structure, and the displacement of the Nb cationsobserved by Levin et al.,34 reported to be predominantly along thepolar c-axis of the orthorhombic unit cell. In a similar fashion, thesecond broad peak around 2.8 Å derives from two sets of distancesassociated with the alkali cations–oxygen pairs (A–O). The A–Nband Nb–Nb correlations are very prominent, and the Gaussianfunctions peak, respectively, around 3.44 Å and 3.98 Å. Thesevalues are consistent with the average distances obtained from therefinement of the crystalline compound (respectively, at 3.40 Åand 4.00 Å; see Table S1.2, ESI†).The distances obtained from these two materials will beused as a guideline for the assignment of the correlations of theglass samples under study.An overview of the real-space transforms of the data for thetwo glass series is presented in Fig. 3 (and Fig. S2, ESI†). Theexperimental reduced pair distribution function G(r) obtainedfrom the Fourier transformation of the total scattering struc-ture factor S(Q) (in Fig. S2, ESI†) and the total correlationfunction T(r) show the presence of well-defined peaks, even atlonger distances (i.e., 44–5 Å). In Fig. 3, the correlations relatedto the aluminosilicate matrix are the most prominent ones, andgradually, their intensities decrease as KNN is added. Only forthe richest composition (20KNN) do the Nb-related correlationsdominate the functions. Despite the significant changes in therelative intensity, the distances related to the aluminosilicatematrix appear to be rather constant. Indeed, the strong firstpeak at 1.65 Å, related to T–O bonds, and the T–T contributionpeaking at B3.10 Å, show just lower relative intensities but nostrong changes in their positions. Regarding the Nb-relatedcorrelations, it is clear that the addition of Nb2O5 results in apeak around 2 Å, whose intensity increases as the fraction of Nbin the glass increases. Therefore, this peak can be attributed toNb–O bonds. Based on the previous structural characterizationof these glasses,9 we assume those distances to be related to six-fold coordinated Nb5+ units. Still, a more detailed discussionon the coordination will be done in the Discussion section. Theother two features that are clearly emerging with increasingKNN content are centered at B3.45 Å and B3.85 Å. Worth to benoted is also the occurrence of an isosbestic point around 3.21–3.24 Å in the peralkaline NA66.10 glasses. Usually, the occur-rence of an invariant point represents the presence of twopredominant species whose ratio changes depending on exter-nal factors (in our case, the increase in KNN and a matrixrelatively depleted in the aluminosilicate fraction).The T(r) of the glass–ceramics (GC) are compared to the glassseries in Fig. 3. The two partially crystallized samples contain(Kx, Na1�x)NbO3 crystals with a perovskite-like structure.14 TheGC10-10 signal resembles well the one of the parent glass(NA66.10-10KNN, blue line) with the exception of a strongercorrelation centered close to 4 Å. The same considerations canbe applied to the GC17-10 signal, which has a very prominentcontribution peaking at 3.96 Å.To better evaluate the evolution of the interatomic distancesdepending on the niobate content and to highlight how thesedistances differ depending on the glass polymerization, the T(r)and their second derivatives are shown on the lower panels ofFig. 3. The direct comparison highlights the significant differ-ences in the atom–atom correlations of the two glass series, andespecially notable are the more pronounced features at 2.0–2.05 Åand around 3.45–3.48 Å in the NA66.10 series. Therefore, the bulkchemistry influences the local environment of the Nb species notonly with respect to the nearest neighbors but predominantlybeyond the first coordination shell. Another aspect to highlight isthe lower intensity of the Nb–O correlation in the NA66.17 seriesfor the same niobate content: it suggests that the probability offinding Nb in a particular distance is much lower than in theperalkaline bulk composition. This difference could be explainedby a larger distribution of distances stemming from a potentiallygreater distortion of the polyhedra when Nb is incorporated into aperaluminous system.3.2 Nuclear magnetic resonance spectroscopy – NMRFig. 4a shows the 29Si MAS NMR spectra of selected specimensunder study. The spectra of the aluminosilicate glasses withoutKNN are centered around �93 and �95 ppm, respectively, forthe NA66.10 and NA66.17 systems, indicating the presence ofmostly Q3 and Q4 units (see also Table 1). In the present case,we refrained from fitting the 29Si MAS NMR spectra because ofthe potentially great number of possible SinmAl and SinmNbspecies, which cannot be resolved due to the intrinsicallylimited resolution.35 The addition of KNN produces a subtlepositive shift of the center of gravities of both glass systems.Typically, positive shifts are interpreted in terms of depolymer-ization of the Si network, which can be expected since theintroduction of each mole of KNN introduces a mole equivalentof (K/Na2O), increasing the alkali/silica ratio. However, Sispecies bonded to Al35 and Nb10 also resonate at higherfrequencies than their Si(OSi)4 counterparts, all of which areFig. 2 Total correlation function T(r) for the aluminosilicate glassNA66.10-0.4(KNN) and the crystalline (K0.5Na0.5)NbO3 (KNN), used respec-tively as references for T–O, A–O and T–T distances in the glasses andNb-related distances. T = tetrahedra (Si, Al); A = alkali ions (Na, K). Greydotted and dashed curves are the Gaussian functions used to decompose,respectively, the signals of the crystalline and glass materials.Paper Materials AdvancesOpen Access Article. Published on 28 April 2025. Downloaded on 6/23/2025 3:23:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ma00082c© 2025 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2025, 6, 3863–3874 |  3867possible outcomes in the present glass systems. It should benoted that the width of the 29Si line decreases slightly as afunction of KNN content. Fig. 4b shows the 27Al MAS NMRspectra of selected specimens under study. The spectra of thesamples corresponding to the NA66.10 and NA66.17 seriesshow a single line centered at about 58 ppm, independentlyof KNN content, corresponding to four-coordinated [AlO4]units. All spectra were successfully simulated using a singlecomponent according to the Czjzek distribution model ofquadrupolar parameters36 (see Fig. S3, ESI†). No significantamounts of higher coordinated Al species can be detected, andcrystallization has no apparent effect on the NMR spectra.Thus, it can be established that Al ions do not partition intothe B site of the crystallized perovskite in the GC samples. The23Na NMR spectra in Fig. 4c show more pronounced variations,both in terms of isotropic chemical shift and line width. The23Na center of gravity of the NA66.17 series is located near�15 ppm compared to �13 ppm for the NA66.10 series, inagreement with previous data on aluminosilicate glasses.37 Inparticular, the lower 23Na average isotropic chemical shift in theNA66.17 metaluminous glass compared to the peralkaline one isexpected because of the lower number of non-bridging oxygens(NBO) in the composition.37 The addition of KNN inducessimilar variations in the two glass series in terms of chemicalshift and line broadening: the spectra of the metaluminousNA66.17 series show a shift toward even lower frequencies witha peak maximum around �16 ppm, while no significant effectis seen for the peralkaline glasses. In the crystallized sampleFig. 3 The upper panels display the experimental total correlation functions T(r) for the two aluminosilicate glass series having increasing KNN contents(from 0.4 mol% to 20 mol%). The T(r)s of the glass–ceramics (GC) for each respective composition are reported as well. In the lower panels, thecorrelation functions of the two glass systems (red lines: NA66.10; black lines: NA66.17) and of the two GCs are compared at equal KNN content. Tohighlight the changes, their second derivative signals are shown.Materials Advances PaperOpen Access Article. Published on 28 April 2025. Downloaded on 6/23/2025 3:23:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ma00082c3868 |  Mater. Adv., 2025, 6, 3863–3874 © 2025 The Author(s). Published by the Royal Society of Chemistryxx17-20, the 23Na signal clearly shows a shoulder correspondingto at least a second contribution, with maxima peaking around�9 ppm and �16 ppm. Interestingly, in the GC sample for thesame metaluminous composition (GC17-10), both contributionsare visible, albeit the much stronger intensity of the one athigher frequencies (�9 ppm). According to previous studies onsilicate glasses and minerals, e.g., ref. 37 and 38, the 23Naaverage isotropic chemical shift generally decreases for anincrease in the ion coordination and/or the average hNa–Oi bonddistances. In ANbO3 (A = Na, K) perovskites, the positionaldisorder of the alkali atoms is mirrored by the broadening ofthe NMR signals. Nevertheless, two main peaks have beenassigned to the Na–O distances in hydrothermally synthesizedNaNbO3 and KNN, respectively, at �13 ppm and �21 ppm.39 Inaluminosilicate glasses, the hNa–Oi distances reported arebetween 2.30 and 2.43 Å, whereas, in perovskites, these distancesare in the range of 2.4–2.8 Å, in agreement with the low-frequency contribution observed in the GCs. The line width inthe peralkaline series is very large and likely reflects the manystructural environments of Na in this composition as (i) networkmodifier, introducing NBOs, (ii) charge compensator for 4-foldcoordinated Al3+ ions (Na–BO), and (iii) charge compensator ofthe [NbO6] units. In the metaluminous composition NA66.17, allNa+ ions are charge-compensators for 4-fold coordinated Al3+ions. For each mole of KNN added there is one atom of Nb and0.5 atoms of both alkaline elements (Na/K) inserted into thestructure: the newly added alkaline elements can disrupt thealuminosilicate network and create NBO or favour the chargebalance of 6-fold coordinated Nb5+ species, enhancing theformation of an alkali compensated-[NbO6] sub-network. Bothscenarios are likely; however, the very limited shift observed forthe 29Si NMR of the NA66.17 series, tends to favor the chargecompensating role as the main one.The 93Nb MAS NMR spectra in Fig. 4d (and Fig. S3, ESI†)show an interesting compositional effect: while all spectra arerather large and asymmetrically broadened due to the presenceof distributions of electric field gradient (EFG) tensor andchemical shift anysotropy (CSA) tensor components, the 93Nblinewidth of the NA66.10 glasses stays rather invariant as afunction of KNN content. On the other hand, its center ofgravity moves systematically from ca. �1197 ppm to around�1168 ppm when adding KNN. Different is the situation in theNA66.17 series: the lines become significantly larger for KNNcontents above 1.5 mol%, and, on average, the center of gravityshifts to about �1220 ppm, indicating the formation of multi-ple unresolved Nb local environments or significant differencesFig. 4 29Si, 27Al, 23Na and 93Nb MAS NMR spectra for selected samples in the two glass series (in panels a to d, respectively).Paper Materials AdvancesOpen Access Article. Published on 28 April 2025. Downloaded on 6/23/2025 3:23:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ma00082c© 2025 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2025, 6, 3863–3874 |  3869in the Nb second-coordination sphere. The crystallized xx17-20and the glass ceramic sample GC17-10 (Fig. 4d) exhibit nar-rower linewidths of about 160 and 60 ppm that peak, respec-tively, around �1135 ppm and �1095 ppm. These values arevery similar to those reported for nanofibers Pbcm NaNbO3(�1094 ppm) by ref. 39, but smaller than the isotropic chemicalshift found in various NaNbO3 (�1078 ppm40) or KNbO3 (�1056ppm39) polymorphs. According to the authors, the smaller shiftin the nanofibers is due to the occurrence of a higher degree ofstructural disorder, and in particular, a mixture of coexistingorthorhombic and monoclinic phases having different localenvironments of the alkali ions.Noteworthy is the difference observed in the 93Nb MAS NMRspectrum of the sample NA66.17-3 (see Table 1), which is thebroadest of all the glasses with a FWHM of 578 � 1 ppm, twicethat of the 0.8 mol% KNN sample, and has a lower center ofgravity dCSCG �1245 � 1 ppm. This out-of-trend spectrumindicates a different Nb structural environment that cannotbe easily resolved by NMR. Additional considerations will bepresented in the Discussion section.4. Discussion – the Nb structuralenvironment’s evolutionTo provide an overview of the structural evolution of the Nbspecies in aluminosilicate glasses, newly acquired structuralinformation by nuclear magnetic resonance (NMR) and high-energy X-ray total scattering can be coupled with the polariza-tion information of the vibrations from Raman spectroscopyand the physical, thermal and optical properties previouslyreported.9,14The high-energy X-ray total scattering data show well-defined coordination shells in the glass network and thedistances associated with the aluminosilicate network agreewell with previous reports. The addition of niobium (as KNN)into the glass network results in a rearrangement of the units,leading to significant modifications, particularly extendingbeyond the first coordination spheres. Prior to examining theatomic ordering in the niobo-aluminosilicate glasses, it isimportant to recall the interatomic distances of Nb5+ in crystal-line materials to facilitate an assessment of the probable localenvironments of the Nb species.In crystalline materials, there are many possible arrange-ments for the Nb5+ cations, since they can be found in differentcoordination environments, with different connectivity (edge,face, and corner shared) and with polyhedra having differentdegrees of distortion. For instance, in CeNbO4, Nb5+ is coordi-nated by four oxygens arranged in a distorted tetrahedron,with an average hNb–Oi bond length of 1.88 Å.41 Its high-temperature tetragonal polymorph is more regular and hasshorter Nb–O distances (1.85 Å).42 [NbO5] trigonal bipyramidshave been reported in Na5NbO543,44 with Nb–O bond lengths of1.99 (�2) Å, 1.90 (�2) Å and 1.88 Å. The six-fold coordination isthe most common in Nb-crystalline compounds: for example,KNbSi2O7 (P4bm) has [SiO4] tetrahedral linked to chains ofcorner-shared [NbO6] units.45 These octahedra are highly dis-torted with alternating short (1.76/1.78 Å) and long (2.28/2.32 Å)Nb–O bonds and Nb–Nb distances of B4.16 Å. The closest Nb–Si correlation is around 3.59 Å. CaNb2O6 exhibits chains ofedge-sharing [NbO6] octahedrons linked to each other alongtheir shortest edge and an average bond length of 2.02 Å(quadratic elongation = 1.04). The closest Nb–Nb interatomicdistance is 3.66 Å, while the longest one is at 3.83 Å.46 ANbO3(A = Na, K) compounds have a perovskite-type structure andatom arrangements similar to the KNN reference materialreported in this study. All octahedral units present a slightlydistorted environment due to the displacement of the centralcations, and the hNb–Oi bond lengths are B2 Å, while the Nb–Nb distances range between 3.91 and 4.04 Å.47–49 In NaNbO3(Pmc21), K0.5Na0.5NbO3 (Amm2), and KNbO3 (Bmm2), the A–Nbdistances range between B3.30 and 3.55 Å. HigherTable 1 29Si, 23Na, 27Al, and 93Nb MAS NMR parameters for selected samples under studySampledCSCG/ppm (� 1) FWHM/ppm (� 1) CQ�� ��ðsÞ.MHz �0:3ð Þ29Si 23Na 27Ala 93Nbb 29Si 23Na 27Alc 93Nbd 27Al66.10 �93 — — — 18 23 — — —66.10-0.8 — — — �1204 — — — 210 —66.10-1.5 — �13 62 �1194 — — 9 230 4.0 (2.0)66.10-3 — — — �1201 — — — 210 —66.10-5 �91 — — �1208 17 — — 240 —66.10-10 �90 �13 63 �1194 17 22 9 240 4.1 (2.1)66.10-20 — — — �1180 — — — 250 —GC10-10 — �15 62 — — 19 9 — 4.5 (2.2)66.17 �95 — — — 17 — — — —66.17-0.8 — �16 62 �1209 — 18 9 260 4.5 (2.2)66.17-3 — — — �1245 — — — 578 —66.17-5 �93 — — �1228 16 — — 450 —66.17-10 �93 �15 62 �1242 16 19 9 370 4.6 (2.4)xx17-20 — �12 63 �1135 — 12 9 160 4.2 (2.1)GC17-10 — �15 62 �1095 — 20 9 60 4.5 (2.3)�980 340a Isotropic chemical shift, dCSiso. b (�5). c FWHM of dCSiso distribution. d (�10).Materials Advances PaperOpen Access Article. Published on 28 April 2025. Downloaded on 6/23/2025 3:23:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ma00082c3870 |  Mater. Adv., 2025, 6, 3863–3874 © 2025 The Author(s). Published by the Royal Society of Chemistrycoordination environments for Nb5+ ions have been reported inNaNb3O850 where there are chains of two Nb5+ sites: a dodeca-hedral one (hNb–Oi = 2.075 Å) and a distorted pentagonalbipyramid with an average hNb–Oi bond length of 2.09 Å.As observed in Fig. 3, in glasses, the interatomic distancesassociated with KNN incorporation are centered around 2 Å,B3.40/3.45 Å and B3.85 Å. Additionally, the NA66.17 composi-tion presents a correlation around 3.25–3.30 Å. The crystallineKNN material (Fig. 2 and Fig. S1 and related text in the ESI†)can be used as a reference for the 6-fold coordinated Nb5+-related distances: the hNb–Oi correlation at 2.00 Å, the hA–Oiat 2.83 Å, the hA–Nbi at 3.40 Å and the hNb–Nbi correlationat 4.00 Å.The incorporation of KNN in the aluminosilicate glassesresults in an enhancement of the correlation associated withNb–O bonds and a shift toward shorter distances: from 2.07 to2.0 Å in the peralkaline series and from 2.02 to 1.98 Å in thepolymerized glasses. However, given the difficulties in estimat-ing the exact position of the correlation at low Nb2O5 contents,the Nb–O distances should be considered almost unaffected bythe KNN content. Nevertheless, the correlations are visibly at ashorter distance in the polymerized samples. The real spacefunctions, the NMR data, and the observed invariance in thehNb–Oi distance suggest that the octahedral environment is thepredominant one. Indeed, if species with lower coordinationwere present, the Nb–O correlation should be expected at muchshorter distances, considering the average hNb–Oi of 1.88 Å inthe 4-fold coordinated CeNbO4, and the hNb–Oi of 1.93 Å in the5-fold coordinated Na5NbO5. In turn, the first correlationrelated to T–O should have a larger width. However, this isnot the case. It is, therefore, reasonable to conclude that aprevailing 6-fold coordination environment for Nb is present inall glasses. In order to verify these considerations, the cation–oxygen coordination numbers were estimated on the basis ofthe pair-function concept developed by ref. 23 for SiO2 glass.The pair-functions obtained are compared to the experimentaldata in Fig. 5 for the structure factors and the total correlationfunctions. The average coordination numbers and distances forNA66.10 and NA66.17 glass series obtained by the pair-functionmethod are reported in Table S2 (ESI†), along with the resultsof the two glass–ceramics. Samples with low Nb2O5 contents(0.4 and 0.8 mol% KNN) could not be accurately analyzed dueto the very weak Nb–O correlations, leading to large errors inboth coordination numbers and distances. The pair-functionmethod confirms the near invariability of the two networkformer cations (Al and Si) and, most importantly, confirms thatthe Nb coordination number is predominantly 6, regardless ofthe bulk chemistry and Nb content. Another important aspectis obtained for sample GC17-10, where the Nb–O correlationpeak shows an asymmetric shape. The Nb coordination num-ber was studied by considering two Nb–O functions located at1.94 Å and 2.21 Å, as shown in Fig. 5, and an average Nbcoordination number of 5.9 (5.2 and 0.7; Table S2, ESI†). ThisNb structural environment is very similar to that of the refer-ence KNN material, where the distortion is mainly caused bythe displacement of the Nb cations along the polar c-axis.It is worth highlighting the results obtained for the sampleNA66.17-3KNN (see Fig. 3, Table 1 and Fig. S3, Table S2, ESI†).The first observation concerns the lower intensity of the Nb–Ocorrelation around 2 Å compared to the peralkaline glass,suggesting that the probability of finding Nb at a given distanceis much lower in a metaluminosilicate composition (Fig. 3).Furthermore, the Nb–O pair-function gives a lower bond dis-tance and coordination number (respectively, 1.86 Å and 4.1), aunique result among all compositions. However, in the realspace function, the contribution around 2 Å is visible, and thepair-function method suggests the presence of a correlation at2.17 Å, albeit very low (0.4 coordination) (all data in Table S2,ESI†). Therefore, similarly to the glass–ceramic, the polyhedraunits in NA66.17 show a distribution of both shorter and longerNb–O distances. These observations, together with the verybroad 93Nb NMR signal observed in Fig. 4, lead to the conclu-sion that there is a greater distortion of the NbO6 polyhedra inthe metaluminous composition than in the peralkaline one.The reason for this distortion could be the preferential use ofhigh-field strength Na+ ions to balance the negative charge ofthe [AlO4/2]� units compared to the larger K+ ions, leaving thelatter to preferentially associate with the [NbO6/2]� units. Thishypothesis also explains the preferential partitioning of K+ ionsinto the perovskite crystal phase observed in the resultingglass–ceramics.14Although the Nb coordination remains predominantly thesame in both glass series, it is evident that the bulk chemistryplays a key role in effecting the distortion, with the mostsignificant differences occurring between 3.2 Å and 4.0 Å:correlations related to Nb – 2nd nearest neighbor distances.As previously observed in the series NA66.10, the contributionat B3.15 Å, associated with T–T distances, decreases steadilywith the inclusion of KNN, while at the same time, the B3.42–3.45 Å distance increases in intensity. The latter distance isconsistent with the A–Nb correlation reported in crystallinematerials, and the isosbestic point marks the equilibrium pointbetween the two arrangements. Therefore, the invariant point isbetween distances mainly related to T–T and A–Nb pairs, whichin turn suggests a progressive increase of [NbO6] units charge-balanced by alkali species, and a very limited interactionbetween the aluminosilicate network and the Nb units. Onlyabove 10 mol% of KNN, the peralkaline series shows a clearcorrelation at higher distances (B3.85 Å) related to Nb–Nb.This evolution is in agreement with the structural dataobtained by 23Na NMR, where the broad Na distributionremains almost invariant in the NA66.10 series, confirmingthat there is no drastic or sudden change at a specific Nb2O5content, but rather is a continuous adjustment of the networkconnectivity. Additionally, the structural evolution observedwell agrees with the polarized Raman spectroscopy and thephysical and thermal properties of the NA66.10 glass series.9Indeed, the occurrence of many intermediated vibrational Nb-units was observed in the polarized Raman, with their propor-tions shifting as Nb2O5 content increased, ultimately leading to[NbO6] cluster formation at the highest KNN contents. Also, theincrease of the glass transition temperature of the peralkalinePaper Materials AdvancesOpen Access Article. Published on 28 April 2025. Downloaded on 6/23/2025 3:23:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ma00082c© 2025 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2025, 6, 3863–3874 |  3871glass series (B50 1C higher than its pristine glass) explained bythe incorporation of Nb species in the modifier channels fitswell with the observed A–Nb correlations that further crosslinkthe network.The metaluminous NA66.17 series shows a quite differentevolution in the 2nd coordination shell, as observed in Fig. 3.Namely, the correlation at B3.14 Å becomes broader, andanother contribution appears at longer distances (B3.50 Å).Additionally, the Nb–Nb correlation moves from 3.80 Å toB3.94 Å by increasing the Nb2O5 content. The change in theNb–O–Nb bond angle distribution could explain the shift of theNb–Nb correlation toward longer distances; in fact, by consid-ering the average hNb–Oi distance of 2.00 Å, a 1801 corner-shared (opposite bond angle) would imply that the niobium–niobium distance is approximatively twice that of niobium–oxygen, at about 4.00 Å. If edge-shared octahedronswere present, the minimum distance would be reduced toB2.83 Å, a correlation not seen in any signal. Therefore, themost reasonable explanation is the formation of [NbO6] chainswith Nb–O–Nb interatomic angle lower than 1801 (estimated as1651). The charge-compensator K+/Na+ ions would change theiraverage bond distances accordingly, as observed from the 23Naisotropic chemical shift of the NA66.17 series. A comparison ofthe GC and their respective parent glasses reveals no significantvariations in the distances of the aluminosilicate matrix (bothT–O and T–T). Therefore, it can be concluded that both alkalinespecies and Nb5+ ions in the amorphous state already favour atopology very similar to that of the stable crystalline phase.Fig. 5 X-ray structure factors and total correlation functions of the two glass series having different KNN contents (from 0.4 mol% to 20 mol%), and GCssamples. The red solid curves represent the calculated structure factors and total correlation functions using the pair function methods for Si–O, Al–O,Nb–O, Na–O, O–O, and K–O correlations, while the blue lines are the estimated total correlation functions for the Nb–O correlation.Materials Advances PaperOpen Access Article. Published on 28 April 2025. Downloaded on 6/23/2025 3:23:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ma00082c3872 |  Mater. Adv., 2025, 6, 3863–3874 © 2025 The Author(s). Published by the Royal Society of ChemistryThe different evolution of the glass network depending onthe glass chemistry or rather, on the alkali/Al ratio explains thedifferent Nb2O5 solubility observed in the NA66.10 and NA66.17series and also explains the different macroscopic properties.Also, it might clarify why the solubility of Nb2O5 is much lowerin aluminosilicate glasses compared to other systems, such asthe borophosphate1 or aluminophosphate glasses.51 Indeed,31P and 27Al NMR measurements on aluminophosphate com-positions with Nb2O5 contents ranging from 0 to 30 mol% haveshown a drastic change in both the P and Al coordinationenvironment and the predominant formation of Nb–O–P lin-kages, while Nb5+ speciation was rather constant and indepen-dent of its content.51 In the aluminosilicate glasses studied here,there is a very limited interaction between Nb and the alumino-silicate network, since a linkage between Nb–O–Si(Al) does notprovide an efficient charge distribution, as in the case of thephosphate matrix. In the future, it would be interesting to testwhether the inclusion of P5+ ions in an aluminosilicate glasswould boost Nb solubility due to the Nb–O–P crosslinking.The good agreement between polarized Raman spectra,NMR, and synchrotron X-ray scattering is remarkable. To high-light this agreement and schematically represent the correla-tions found in this work, in Fig. 6, the radial distributionfunction RDF, the perpendicularly polarised (VH) Raman spectra,and the 93Nb MAS NMR spectra are compared. The details andanalysis of the Raman spectra have already been reported in ref. 9.Based on these data, we propose the presence of four mainfingerprint features in the Raman spectra that can be used onother systems to identify the local environment of the Nb andforecast the glass properties:(a0) Alkali compensated-[NbO6] structural units. The largeNMR peak width and the frequency of the Raman vibrationindicates a highly distorted environment. This feature dominatesonly in the compositions having low Nb2O5 contents (r5 mol%KNN) and prevails in compositions with high amounts of charge-compensating ions (i.e., peralkaline). As the Nb content increases,this contribution diminishes in intensity. The higher distortion ofthe [NbO6] structural units of the NA66.17 series, and in turn, thebroader bond distribution around these units, is mirrored by thelower intensity of the Nb–O correlation in the radial distributionfunction and the Raman vibrational modes, and probably, alsofrom the NMR component at lower shifts (as observed in sampleNA66.17-3KNN).(A) This contribution is related to the center of the NMRband and to the main band associated with Nb-vibrations in theRaman spectra. In the latter, the high polarizability of the bond9indicates that there is a directional character of the vibrations,while the lower frequency position compared to a0 indicates adecrease in [NbO6] distortion. These considerations are consis-tent with the appearance of a correlation at longer distancesrelated to Nb–Nb pairs. The formation of Si(Al)–O–Nb bondswould also be compatible with a more polarizable bond (moresensitive to light polarization because of the deformation of theelectron cloud). However, Al NMR data do not show strongchanges in the Al local environment, and the vibrational modesrelated to Si–O and O–Si–O are unchanging. Therefore, theformation of bridging bonds Nb–O–Nb is the most probable one.(B/C) The Raman vibrations between 680 and 800 cm�1 havelower polarizability9 (and constant polarization ratio), compa-tible with the formation of more ionic clusters of [NbO6] unitswith no preferential direction, charge compensated by thealkali ions. These clusters do not seem to have edge-sharedunits. Indeed, the Nb–Nb correlations in the T(r) are consistentwith the formation of [NbO6] chains with Nb–O–Nb interatomicangle of about 1651, which will evolve in the more regular 3Darrangement of corner-sharing [NbO6] units, similar to those inFig. 6 The structural information obtained from synchrotron X-ray scattering, perpendicularly polarised (VH) Raman spectroscopy,9 and NMR aresummarized here, together with a schematic representation of the different possible structural units associated with specific fingerprint features in NMRand Raman spectra (labeled a0, A, B, and C; see text for details). Upper panels: data for glasses in the NA66.10 series; lower panels: data for glasses in theNA66.17 series. The vertical axis of the Raman signals represents the raw intensity of the scattered light in counts (�103).Paper Materials AdvancesOpen Access Article. Published on 28 April 2025. Downloaded on 6/23/2025 3:23:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ma00082c© 2025 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2025, 6, 3863–3874 |  3873the perovskite structure (C). The formation of these 3D arrange-ment could be recognized also in the 93Nb NMR from thecomponent at higher chemical shift values that overlaps withthe NMR band found in perovskite-bearing glass–ceramics.The fingerprint features identified can be used to predictpreferred bonding and network connectivity in many glasssystems. This information will allow us to identify promisingcompositions with tailored functionalities, such as highly dis-torted niobate units able to enhance the Nb photolumines-cence emission,14 or higher fractions of edge-shared units,useful for electro-optical devices.135. ConclusionsThis work presents, for the first time, high-energy X-ray diffrac-tion data for two series of alkali–aluminosilicate glasses tounravel the evolution of the niobate units. In order to under-stand the structure–properties relationship, these data arecoupled with complementary solid-state NMR and polarisedRaman spectroscopy data.The results show that Nb5+ is present in all aluminosilicateglasses predominantly as 6-fold coordinated units, and with anearly constant hNb–Oi bond distance of 2.0 Å. The additionof Nb to the aluminosilicate network has a marginal effect onthe Si and Al tetrahedral units with very limited crosslinking.The unchanging T–O and T–T correlations, together with the27Al NMR data confirm that the aluminosilicate matrix pre-serves its network, while the Nb5+ ions in the amorphous statealready favour a topology very similar to that of the stablecrystalline phase. Indeed, strong changes can be seen in theA–Nb (A = alkaline) and Nb–Nb correlations. Therefore, theoverall glass connectivity is influenced by the availability andnature of charge-compensating alkali ions. The different prop-erties and structural arrangements observed in other glasssystems such as the aluminophosphate are rationalised basedon favourite connectivity due to efficient charge distribution.The structural information obtained from NMR andsynchrotron X-ray scattering is complemented by that fromperpendicularly polarised Raman spectra, and we propose specificfingerprint features in the NMR and Raman spectra that representthe different possible structural units. In particular, we show thatwe can use polarised Raman spectra to predict preferred bondingand network connectivity, and use this information to identifypromising compositions with tailored functionalities.Author contributionsMRC: conceptualization, resources, project administration, fund-ing acquisition, investigation, formal analysis, writing – originaldraft, review & editing; KK: conceptualization, investigation, for-mal analysis, writing – review & editing; HB, HD, SK, YO, TH, SS:investigation, data curation, validation, writing – review & editing;KH, DdL: resources, funding acquisition, validation, writing –review & editing. All authors each made a significant contributionto the research reported and have seen and approved the finalversion of the manuscript being submitted.Data availabilityThe data supporting this article have been included as part ofthe ESI.†Conflicts of interestThe authors declare no competing interests that could influ-ence the work in this paper.AcknowledgementsThe authors thank SPring-8 for the provision of synchrotronradiation (Proposal 2023A1202) and the staff of BeamlineBL04B2 for assistance. The authors gratefully acknowledge thefinancial support from the Deutsche Forschungsgemeinschaftunder grant numbers CI294/2-1, and GRK2495/2/G and the JapanSociety for the Promotion of Science (JSPS) Grant-in-Aid forTransformative Research Areas (A) ‘‘Hyper-Ordered StructuresScience’’ (Grant numbers 20H05878 and 20H05881). H. B. isgrateful to FAPESP for infrastructural and post-doctoral supportreceived under grant numbers 2013/07793-6 (CEPID program) and2019/26399-3, respectively. M. R. 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