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[Masataka Tansho](https://orcid.org/0000-0001-7986-3199), [Atsushi Goto](https://orcid.org/0000-0002-9472-4098), [Shinobu Ohki](https://orcid.org/0000-0002-7357-3833), [Yuuki Mogami](https://orcid.org/0000-0002-9807-3165), Yuta Yasui, Yuichi Sakuda, [Kotaro Fujii](https://orcid.org/0000-0003-3309-9118), Takahiro Iijima, [Masatomo Yashima](https://orcid.org/0000-0001-5406-9183)

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[Solid-State <sup>95</sup>Mo and <sup>93</sup>Nb NMR Study of Ba<sub>7</sub>Nb<sub>4</sub>MoO<sub>20</sub>-Based Materials and Ba<sub>7</sub>Ta<sub>3.7</sub>Mo<sub>1.3</sub>O<sub>20.15</sub>](https://mdr.nims.go.jp/datasets/492f43fd-e7ab-4a9b-971e-fee2f5fa4a37)

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Solid-State 95Mo and 93Nb NMR Study of Ba7Nb4MoO20-Based Materials and Ba7Ta3.7Mo1.3O20.15Solid-State 95Mo and 93Nb NMR Study of Ba7Nb4MoO20-BasedMaterials and Ba7Ta3.7Mo1.3O20.15Masataka Tansho,* Atsushi Goto, Shinobu Ohki, Yuuki Mogami, Yuta Yasui, Yuichi Sakuda,Kotaro Fujii, Takahiro Iijima, and Masatomo YashimaCite This: https://doi.org/10.1021/acs.jpcc.4c02645 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Hexagonal perovskite-related oxides, Ba7Nb4MoO20,Ba7Nb4−xMo1+xO20+(1/2)x (x = 0.1), Ba7Nb4−yWyMoO20+(1/2)y (y = 0.15), andBa7Ta4−zMo1+zO20+(1/2)z (z = 0.3), have recently been reported to exhibit highoxide-ion and proton conductivity. These materials are of great interest inindustrial applications, such as solid oxide fuel cells (SOFCs) and proton ceramicfuel cells (PCFCs) and are known for their unusual structures. Although thestructures of Ba7Nb4MoO20 and their related materials were primarily analyzedby assuming an even distribution of Mo and Nb at each M (=Mo/Nb) site, solid-state nuclear magnetic resonance (NMR) spectra have revealed that Mo and Nbare unevenly distributed in Ba7Nb4MoO20. As it is crucial to determine whetherthe contributions to oxide-ion and proton conduction are the same for Mo andNb, we focused on the signal differences among these as-prepared materials,namely, Ba7Nb4MoO20, Ba7Nb3.9Mo1.1O20.05, Ba7Nb3.85W0.15MoO20.075, andBa7Ta3.7Mo1.3O20.15, using solid-state 95Mo and 93Nb NMR analysis. The 95Mo NMR similar predominant peaks revealed inBa7Nb3.9Mo1.1O20.05, Ba7Nb3.85W0.15MoO20.075, and Ba7Ta3.7Mo1.3O20.15 are also attributed to the MoO4 tetrahedron near the oxide-ion conducting layer owing to the small quadrupolar coupling constant, |CQ|. Furthermore, a minor peak of 95Mo has been observedin Ba7Ta3.7Mo1.3O20.15, which is presumed to be a MoO5 polyhedron, MoO5 monomer, or (Mo/Ta)2O9 dimer, formed by thebinding of the excess oxygen, represented by (1/2)z (z = 0.3) in the chemical formula. One shoulder peak in the 93Nb NMRspectrum of Ba7Nb4MoO20 could be attributed to the NbO4 tetrahedron near the ion conducting layer from its small quadrupolarcoupling product, |PQ|, but its intensity is smaller than that considered from the occupancy factors. The small signal intensity isplausible because many are not regular NbO4 tetrahedrons in Ba7Nb4MoO20. In Ba7Nb4−xMo1+xO20+(1/2)x (x = 0.1), the intensity ofNbO4 tetrahedron has been further reduced, indicating that the decrease is caused by the transformation of the residual NbO4tetrahedron to NbO5 polyhedron, NbO5 monomer, or (Mo/Nb)2O9 dimer, by the binding of excess oxygen, represented by (1/2)x(x = 0.1) in the chemical formula. Thus, the solid-state NMR analysis of the local structure of Mo and Nb oxide polyhedra is a vitaltool in analyzing nonstoichiometric ion conductors because it provides information on individual Mo and Nb local structures nearthe conducting layers of the disordered materials. Therefore, it will potentially contribute to further developing applications using ionconductors.■ INTRODUCTIONOxide-ion and proton conductors are of great interest forindustrial applications such as solid oxide fuel cells (SOFCs)and proton ceramic fuel cells (PCFCs).1−5 Although zirconia-based materials are widely utilized above 700 °C, there remainsa strong motivation to find electrolyte materials with higherconductivity that will lower fuel cell operating temperatures to400−600 °C and reduce costs.6,7 Improving the conductivityof oxide ions and protons in the bulk requires the detection ofthe difference in local structures due to each sample.Among various high ionic conductors, Ba−M (=Mo/Nb(Ta))−O hexagonal perovskites and related materials arepromising, and one such case is an oxide-ion conductor,Ba3MoNbO8.5, which exhibits 2.2 × 10−3 S cm−1 at 600 °C.8Recently, hexagonal perovskite materials such asBa7Nb4MoO209−12 and related materials that are mixedconductors of oxide ions and protons have also been reportedto exhibit high conductivity, for example, 4.0 × 10−3 S cm−1 at510 °C of Ba7Nb4MoO20 (Table 1).9 The high conductivitycan be attributed to the complex structures of these materials.In Ba3MoNbO8.5, the averaged structure is represented as ahybrid of overlapping 9R polytype and palmierite subunits,with one of the M sites in two equivalent positions13 (FigureReceived: April 23, 2024Revised: October 11, 2024Accepted: October 15, 2024Articlepubs.acs.org/JPCC© XXXX The Authors. Published byAmerican Chemical SocietyAhttps://doi.org/10.1021/acs.jpcc.4c02645J. Phys. Chem. C XXXX, XXX, XXX−XXXThis article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on November 11, 2024 at 08:21:30 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Masataka+Tansho"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Atsushi+Goto"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shinobu+Ohki"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuuki+Mogami"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuta+Yasui"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuichi+Sakuda"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kotaro+Fujii"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kotaro+Fujii"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takahiro+Iijima"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Masatomo+Yashima"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.jpcc.4c02645&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=agr1&ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c02645?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/JPCC?ref=pdfhttps://pubs.acs.org/JPCC?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/S1). In Ba7Nb4MoO20 and related materials, the hexagonal (h)and cubic (c) close-packed BaO3 layers and the oxygen-deficient cubic (c′) BaO2 layers are stacked in a (c′hhcchh)sequence14 (Figure S2). In the first report, it was concludedthat Mo/Nb did not exist at the M(4) site at a hightemperature of 800 °C in as-prepared Ba7Nb4MoO20(FigureS2), but several recent reports have shown that a small fractionof Nb exists in the M(4) site at a room temperature,9,11 whichis migrated from the M(2) site (Figures 1 and S3).The structures of Ba3MoNbO8.5, Ba7Nb4MoO20, and theirrelated materials were studied mainly using X-ray diffraction,neutron diffraction, and electron diffraction by assuming aneven distribution of Mo and Nb at each Mo/Nb site andanalyzed as having averaged Mo and Nb atoms.9−11,15−18 X-rays, neutrons, and electrons have close scattering capabilities,making it difficult to distinguish between Nb and Mo. One ofthe most significant advantages of solid-state nuclear magneticresonance (NMR) is the ability to independently observeadjacent elements in the periodic table. In Ba3MoNbO8.5, two93Nb and four 95Mo NMR peaks were detected at the two Mo/Nb sites, indicating an uneven distribution of Mo and Nbwithin the Mo/Nb sites.19 In as-prepared Ba7Nb4MoO20, onlyone 95Mo NMR signal was observed. Resonant X-raydiffraction (RXRD) and density-functional theory (DFT)calculations determined that all the Mo was present at theM(2)O4 site near the oxide-ion conducting layer.20 Finally, theneutron diffraction data were used to refine the occupancy ofM(1) to 100% Nb, M(2) to 42% Nb, 50% Mo, and 8%vacancy, M(3) to 100% Nb, and M(4) to 8% Nb and 92%vacancy. Here, the ratio of the number of equivalent positionsin a unit cell, M(1):M(2):M(3):M(4), is 1:2:2:2 (Figures 1and S3).The difference in the local structure of the oxygen-deficientc′ layer is significant because the oxide-ion conductivity ofBa7Nb3.9Mo1.1O20.05 is several times higher than that ofB a 7 N b 4 M o O 2 0 .1 0 H e r e , f o r e x a m p l e , i nBa7Nb4−xMo1+xO20+(1/2)x (x = 0.1), it is considered that theincrease in interstitial O(5) enhances the oxygen-ionconduction (Figure S2). Moreover, neutron scattering lengthdensity analyses of Ba7Nb3.8Mo1.2O20.1 indicate that the excessoxygen atoms are incorporated by the formation of both 5-fold-coordinated (Mo/Nb)O5 monomer and its (Mo/Nb)2O9dimer with a corner-sharing oxygen atom and that the breakingand reforming of the dimers lead to the high oxide-ionconduction in the oxygen-deficient BaO2.1 c′ layer.21 In aprevious study of Ba7Nb4MoO20, suitable attribution wasobtained for the 95Mo NMR data, but the 93Nb NMR datawere poorly attributed by DFT calculations.20 However, it ismore challenging to perform DFT calculations for non-stoichiometric compounds such as Ba7Nb3.9Mo1.1O20.05,although the supercell method can be used to resolve problemsrelated to site mixing and fractional site occupancy,22−24potentially overcoming the highlighted challenges and providevaluable insights into the structure and properties of thematerial. On the contrary, solid-state NMR has the advantagethat each nucleus can be measured independently,19 making itpossible to analyze the local structure of the relatednonstoichiometric materials by comparing differences amongrelated materials.Therefore, in this study, as a relatively quantifiable method,we performed solid-state 95Mo and 93Nb one-dimensionalmagic angle spinning (MAS) NMR measurements ofB a 7 N b 4 − x Mo 1 + x O 2 0 + ( 1 / 2 ) x ( x = 0 . 1 ) 1 0 a n dBa7Nb4−yWyMoO20+(1/2)y (y = 0.15)11 to detect the effect ofexcess oxygen. One-dimensional 95Mo NMR measurement ofBa7Ta4−zMo1+zO20+(1/2)z (z = 0.3)12 was also performed forcomparison because Ba7Ta4−zMo1+zO20+(1/2)z (z = 0.3) hasmore excess oxygen than the other samples of similarcrystalline structure. As an accurate two-dimensional (2D)analysis method, we also performed a 93Nb three-quantumTable 1. Total Conductivities of Ba7Nb4MoO20,Ba7Nb3.9Mo1.1O20.05, Ba7Nb3.85W0.15MoO20.075, andBa7Ta3.7Mo1.3O20.15materials σtot/S cm−1 temperature (°C) referenceBa7Nb4MoO20 4.0 × 10−3 510 9Ba7Nb3.9Mo1.1O20.05 5.8 × 10−4 310 10Ba7Nb3.85W0.15MoO20.075 2.2 × 10−2 660 11Ba7Ta3.7Mo1.3O20.15 1.08 × 10−3 377 12Figure 1. Refined crystal structure and site occupancies of Mo and Nb atoms at each M site of Ba7Nb4MoO20·δ H2O at 300 K (δ = 0.151).20 Theratio of the number of equivalent positions in a unit cell, M(1):M(2):M(3):M(4), is 1:2:2:2. The position of H and O derived from δ H2O near theconducting layer is omitted here for simplicity (Figure S3). Initially, the Mo and Nb atoms were assumed to be completely disordered; however,they are considered nearly site-selective.The Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c02645J. Phys. Chem. C XXXX, XXX, XXX−XXXBhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig1&ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c02645?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-as(3Q)MAS NMR measurement,25 one of the multiple-quantum(MQ) MAS measurements,26,27 on Ba7Nb3.9Mo1.1O20.05 inaddition to the previously measured Ba7Nb4MoO20.20 More-over, the attribution of the 93Nb signal, which was ambiguouslyattributed in the previous report, is rediscussed, including theeffect of H2O in Ba7Nb4MoO20. The differences in NMRmeasurements by composition may help design high oxide-ionand proton conductors with hexagonal perovskite-relatedstructures.■ EXPERIMENTAL SECTIONMaterials. Polycrystalline samples of Ba7Nb4MoO20,20Ba7Nb4−xMo1+xO20+(1/2)x (x = 0.1),10 Ba7Nb4−yWyMoO20+(1/2)y(y = 0.15),11 and Ba7Ta4−zMo1+zO20+(1/2)z (z = 0.3)12 weresynthesized by the following conventional high-temperaturesolid-state reactions as already reported.As previously reported, Ba7Nb4MoO20 with 0.15 H2O insidewas prepared using the solid-state reaction method.20 High-purity (>99.9%) BaCO3, Nb2O5, and MoO3 were mixed asethanol slurries and ground as dry powders using an agatemortar and pestle. The obtained powder was calcined at 900°C for 12 h for decarbonation. The obtained material wascrushed and ground into a fine powder in an agate mortar andpestle for 1 h as a dried powder and ethanol slurry. Theresultant powder was uniaxially pressed at 150 MPa and thensintered in air at 1100 °C for 24 h. The sintered pellet wascrushed and ground into a fine powder. The inductivelycoupled plasma atomic emission spectroscopy (ICP-AES)result indicated that the cation molar ratio of Ba7Nb4MoO20was Ba:Nb:Mo = 6.89(12):4.078(18):1.034(10), which isconsistent with the nominal composition. The bulk watercontent of δ = 0.151(5) was calculated by using the refinedoccupancy factors in the Rietveld analysis of the neutrondiffraction data at 30 K.Ba7Nb4−xMo1+xO20+(1/2)x (x = 0.1) was prepared by thesolid-state reactions.10 High-purity (>99.9%) BaCO3, Nb2O5,and MoO3 were mixed and ground using an agate mortar andpestle as an ethanol slurry and dry powder repeatedly for 1 h.The obtained mixture was calcined at 900 °C for 12 h in staticair. The calcined sample was repeatedly crushed and groundusing an agate mortar and pestle as an ethanol slurry and drypowder for 1 h. The powder thus obtained was uniaxiallypressed into a pellet at 150 MPa and subsequently sintered instatic air at 1100 °C for 24 h. The cation atomic ratio ofB a :Nb :Mo = 7 . 1 1 ( 1 4 ) : 3 . 8 1 ( 3 ) : 1 . 1 2 6 ( 14 ) f o rBa7Nb3.9Mo1.1O20.05 determined through X-ray fluorescence(XRF) analyses agreed with that of the nominal compositionwhere the number in parentheses is the standard deviation inthe last digit.Ba7Nb4−yWyMoO20+(1/2)y (y = 0.15) was synthesized viaconventional high-temperature solid-state reactions.11 BaCO3(99.95%, Kojundo Chemical Laboratory Co.), Nb2O5 (99.9%,Kojundo Chemical Laboratory Co.), MoO3 (99.98%, KojundoChemical Laboratory Co.), and WO3 (99.9%, KojundoChemical Laboratory Co.) were weighed according to theratio and mixed in an agate mortar and pestle as a dry powderand ethanol slurry. The obtained mixture was heated at 900 °Cfor 12 h in air. The calcined powder was ground, pelletized atapproximately 150 MPa, and heated again at 1100 °C for 24 hin static air. Parts of the sintered pellets were crushed andground into white powders for further measurements. Thechemical composition of Ba7Nb3.85W0.15MoO20.075 was inves-tigated through ICP-AES using a Shimadzu ICPS-8100spectrometer. It showed that the cation ratio of the y = 0.15sample was Ba:Nb:W:Mo = 7.00(4):3.74(5):0.13(4):1.03(1),w h i c h a g r e e s w i t h t h e n om i n a l r a t i o f o rBa7Nb3.85W0.15MoO20.075.Ba7Ta4−zMo1+zO20+z/2 (z = 0.3) was prepared by solid-statereactions using high-purity (>99.9%) BaCO3, Ta2O5, andMoO3 produced by Kojundo Chemical Laboratory Co. Ltd.12The starting material in appropriate molar ratios (Ba:Ta:Mo =7:3.7:1.3) was mixed and ground in the ball-milling processusing a yttria-stabilized zirconia ball. The milled mixture wascalcined using Al2O3 crucibles at 1000 °C for z = 0.3.Ba7Nb3.9Mo1.1O20.05, Ba7Nb3.85W0.15MoO20.075, andBa7Ta3.7Mo1.3O20.15 are considered to be solid solutions.They are confirmed as single phases using X-ray and neutrondiffraction experiments, and the lattice parameters are changedwith the composition. The as-prepared samples contain smallamounts of water, just as Ba7Nb4MoO20 contains 0.15 H2O.All NMR experiments were performed on the as-preparedsamples.95Mo NMR Measurements. All measurements wereperformed with a fabricated 3.2 mm single resonance MASprobe for low-resonance-frequency nuclei without temperaturecontrol, where the resonance frequency for 95Mo was 52.16MHz at 18.79 T. The natural abundance and spin number, I,are 15.7% and 5/2, respectively. All samples were filled insample tubes designed with the same volume and rotated at 20kHz. A 2.0 M Na2MoO4 solution was used as a 0 ppmreference for the chemical shifts of 95Mo. A JEOL ECA 800NMR spectrometer was used to perform the one-dimensional(1D) measurement of 95Mo. 95Mo spectra were acquired with13,000−22,000 scans using a single pulse of 1.2−1.6 μs,corresponding to a liquid standard π/6 pulse with a relaxationdelay of 20 s, which is sufficient for the recovery of theprominent peak. In particular, the same experimentalconditions, including the accumulation number, are used forBa7Nb3.9Mo1.1O20.05 as well as Ba7Nb4MoO20, and measure-ments of these two samples were obtained continuously. Dueto the large background signals detected at shift lower than−1000 ppm, signals were recorded within the range of−1000−1000 ppm.93Nb NMR Measurements. All measurements wereperformed with a fabricated 3.2 mm single resonance MASprobe for high-resonance-frequency nuclei without temper-ature control, where the resonance frequency for 93Nb was195.84 MHz at 18.79 T. The natural abundance and spinnumber, I, are 100% and 9/2, respectively. All samples werefilled in sample tubes designed with the same volume androtated at 20 kHz. One-dimensional measurements at MAS =15 kHz for Ba7Nb4MoO20 confirmed which peaks werespinning sidebands. A peak of noncubic solid phase, Pbcm, ofNaNbO3 (Sanwa Chemical Industry, 99.9%) was set at −1093ppm as a convenient secondary chemical shift reference of the93Nb chemical shift at 18.79 T,28,29 instead of a saturatedsolution of NbCl5 in acetonitrile at 0 ppm. Here, thequadrupole coupling constant, CQ, of the Pbcm phase ofNaNbO3 is reported as 19.5 MHz.29 JEOL ECZR and ECA800 NMR spectrometers were used for the 1D and 2Dmeasurements, respectively.Considering the baseline distortion in the single-pulse 93NbMAS NMR measurements (Figure S4), which are usuallyapplied for a rigorous quantitative discussion, one-dimensional93Nb spectra were acquired with the spin-echo sequence,30which are usually nonquantitative. Here, a radiofrequencyThe Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c02645J. Phys. Chem. C XXXX, XXX, XXX−XXXChttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c02645?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aspulse with a width of 3.0 μs, which is the π/2 pulse of the solidPbcm phase of NaNbO3, was applied for the single-pulsemeasurements. In the case of quadrupole nuclei, π/2 pulselengths are often distributed. Therefore, for quantifiability,pulse lengh dependece of each spectra was checked (FigureS5) before the spin-echo experiments and the reduced pulselengths of 2.0 μs as the π/3 pulse and 4.0 μs as the 2π/3 pulsewere applied instead of the π/2 pulse and π pulse of solidNaNbO3, respectively. The echo delay times were set to 50 μsunder rotor-synchronized spinning conditions at 20 kHz. Theeffective transverse relaxation time constant, T2*, of 93Nb wasapproximately 200 μs for both Nb(1)O6 and Nb(3)O6octahedra in Ba7Nb4MoO20. Placing the center frequency ofthe measurement at −850 ppm, which is similarly far from thepeaks, preserves the quantitativeness among the peaks in theecho experiment. Both single-pulse and spin-echo measure-ments were performed with a radiofrequency field amplitude ofapproximately 20 kHz. The spectra were obtained with thesame 1024 scans, and the relaxation delay was set to 1 s, whichis sufficient for the recovery of the prominent peaks. All of theone-dimensional measurements were performed continuously.We employed 3QMAS NMR measurement with a three-pulse (2.0, 0.9, and 15 μs) sequence using a zero-quantumfilter, as proposed by Amoureux et al.26,27 The RF fieldstrength was 100 kHz for the first two pulses and 8 kHz for thethird pulse. Here, the spectrum was recorded with 264transients averaged for each of the 1024 t1 increments of 3.3 μsand a relaxation delay of 0.2 s, which was sufficient to obtain anadequate signal-to-noise ratio. After that, using the sameshearing transformation as already reported, the center of theF1 axis is set to the center of the F2 axis.19,31,32 Here, in thecase that the signals are broadened as in the previouslyreported disordered materials, Ba3MoNbO8.5 (Figure S1)19and Ba7Nb4MoO20·0.15 H2O,20 the analysis of the positions ofthe experimental resonances (δF1 and δF2) in the F1 and F2dimensions, the isotropic shift, δiso, and the quadrupolarcoupling product, |PQ|, are calculated as17 1027isoF1 F2= +(1)and in the case of I = 9/2 as 93NbP 12241027( ) 10Q F1 F21/2L3| | = · ·ikjjj y{zzz (2)Here, νL denotes the Larmor frequency. |PQ| is related to |CQ|byP C (1 /3)Q Q2 1/2| | = | | + (3)The asymmetry parameter of the electric field gradient, η, takesvalues between 0 and 1.■ RESULTS AND DISCUSSION95Mo NMR. 95Mo one-dimensional MAS NMR results areshown in Figure 2, where only one prominent peak due toMo(2)O4 tetrahedron is observed at approximately −45 ppmin Ba7Nb3.9Mo1.1O20.05 and Ba7Nb3.85W0.15MoO20.075, as well asfor Ba7Nb4MoO20.20 This is in contrast to some 95Mo signalsobserved in Ba3MoNbO8.5, which are presumed to bepolyhedra of MoO5 or MoO6 as well as MoO4.19 The centralpeak of Ba7Nb3.9Mo1.1O20.05 was approximately 1.3 timeshigher than that of Ba7Nb4MoO20 measured under thesame experimental conditions. The observed peak positionsδMAS, |CQ|, and attributed polyhedron are presented in Table 2,and the DFT results of Ba7Nb4MoO20 are presented in Table3. |CQ| ≤ 2 MHz of the peaks are also confirmed by a home-written program33 (Figure S6) in addition to the DMFIT34used for Ba3MoNbO8.5. Here, the relationship among δMAS,isotropic shift, δiso, and a second-order quadrupolar-inducedshift, δqis, is expressed using the following equationMAS iso qis= + (4)It has already been reported that the signal of Ba7Nb4MoO20is assigned to the MoO4 tetrahedron, Mo(2)O(1)1O(2)3, nearthe conducting layer.20 The similar peaks in the as-preparedBa7Nb3.9Mo1.1O20.05 and Ba7Nb3.85W0.15MoO20.075 are alsodefinitely attributed to the MoO4 tetrahedron, Mo(2)O(1)1O-(2)3, because the shifts and line shapes of the NMR spectra aredirectly dependent on the local structure and state of theelectrons. There is no cubic-centered M site in Ba7Nb4MoO20,but it is a nearly spherical site because it has the smallest |CQ|value in the calculated model (Table 3). The 95Mo peak in as-prepared Ba7Nb3.9Mo1.1O20.05 is larger than the 95Mo peak inBa7Nb4MoO20 because the Mo ratio in Ba7Nb3.9Mo1.1O20.05 is1.1 times larger, and the Mo(2)O4 tetrahedron content ratio isincreased (Supporting Information, Note S1).For Ba7Ta3.7Mo1.3O20.15, in addition to a similar Lorentzian-like peak at −47 ppm, there exists a minor peak atapproximately +165 ppm also observed in Ba3MoNbO8.5 at+160 ppm (Figure S1), about one-tenth the height of themaximum peak. The DFT results for Ba7Ta4MoO20 arepresented in Table 4. Ba7Ta3.7Mo1.3O20.15 contains Ta insteadFigure 2. 95Mo MAS NMR spectra of as-prepared (a) Ba7Nb4MoO20,(b) Ba7Nb3.9Mo1.1O20.05, (c) Ba7Nb3.85W0.15MoO20.075, and (d)Ba7Ta3.7Mo1.3O20.15 measured by the single-pulse method. Panels(a) and (b) were measured under the same conditions, including thenumber of accumulations. The dashed line is a guide for the eye.Asterisks, *, denote spinning sidebands.The Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c02645J. Phys. Chem. C XXXX, XXX, XXX−XXXDhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig2&ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c02645?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asof Nb at the M sites, but the Lorentzian-like narrow peak muststill be owing to the similar Mo(2)O4 tetrahedron because theNMR parameters are strongly dependent on the localstructure, and the peak position is hardly affected when |CQ|is small. The minor peak is considered as follows. As reportedearlier, all the Mo is at the M(2) site.12 In addition, the +165ppm shift is close to neither +283 to +289 ppm calculated forthe Mo(1)O6 octahedron nor −66 to +2.5 ppm calculated forthe Mo(3)O6 octahedron in Ba7Ta4MoO20. Therefore, bothMo(1)O6 and Mo(3)O6 octahedra are unsuitable forattribution of the minor peak. Then, this peak is possibly asignal of the Mo(2)O5 monomer near the conducting layerowing to the extra oxygen involved in the chemical formula (1/2)z (z = 0.3). Here, the possibility of the M2O9 dimer, (Mo/Ta)(2)2O9 dimer, should also be considered because the (Mo/Nb)(2)2O9 dimer with a corner-sharing oxygen atom isindicated for Ba7Nb3.8Mo1.2O20.1.21 The shifts of Mo(2)2O9and Mo(2)Ta(2)O9 dimers could be different from that of theMo(2)O5 monomer because the electronic state of the Mo ofMo(2)2O9 and Mo(2)Ta(2)O9 dimers must not be the sameas that of the Mo(2)O5 monomer. Therefore, the minor peakof Ba7Ta3.7Mo1.3O20.15 could be a Mo(2)O5 monomer,Mo(2)2O9 homodimer, or Mo(2)Ta(2)O9 heterodimer.Unlike Ba7Ta3.7Mo1.3O20.15, no minor peak was observed inas-prepared Ba7Nb3.9Mo1.1O20.05 and Ba7Nb3.85W0.15MoO20.075.Here, we consider the following two reasons: Mo(2)O5polyhedra, Mo(2)O5 monomer, or (Mo/Nb)(2)2O9 dimerhave not been observed, although Mo(2)O5 polyhedra shouldexist owing to excess oxygen. First, the excess amounts ofoxygen atoms (1/2)x (x = 0.1) of Ba7Nb3.9Mo1.1O20.05 and (1/2)y (y = 0.15) of Ba7Nb3.85W0.15MoO20.075 are only 1/3 and 1/2 of (1/2)z (z = 0.3) of Ba7Ta3.7Mo1.3O20.15, respectively.Second, the line width of the quadrupole second-orderperturbation is known to depend on |CQ|2,36 and the peakheight depends on |CQ|−2. Because the calculated |CQ| of theMo(2)O5 monomer is larger than that of the Mo(2)O4tetrahedron (Table 3), the signal height of the Mo(2)O5monomer must become much smaller than that of theMo(2)O4 tetrahedron (Figure S10). Similarly, the signalheight of the (Mo/Nb)(2)2O9 dimer is possibly much smallerthan that of the Mo(2)O4 tetrahedron because |CQ(95Mo)| ofthe (Mo/Nb)(2)2O9 dimer is likely larger than that ofMo(2)O4, which is the smallest in Table 3.93Nb NMR. Figure 3 compares one-dimensional MAS NMRmeasurements of 93Nb under the same experimental conditionsTable 2. Experimental 95Mo Peak Position, Quadrupolar Coupling Constant, and Attributed Polyhedronas-prepared samples δMAS/ppma |CQ|/MHzb polyhedronBa7Nb4MoO20 −47c ≤2c Mo(2)O4cBa7Nb3.9Mo1.1O20.05 −44 ≤2 Mo(2)O4Ba7Nb3.85W0.15MoO20.075 −44 ≤2 Mo(2)O4Ba7Ta3.7Mo1.3O20.15 −47 ≤2 Mo(2)O4Ba7Ta3.7Mo1.3O20.15 +165 unclear Mo(2)O5, Mo(2)2O9, or Mo(2)Ta(2)O9aδMAS is the magnetic field-dependent value. Here, the magnetic field is 18.79 T. bSimulated by the home-written program33 (Figure S6) inaddition to the DMFIT.34 cExperimental data and attribution for Ba7Nb4MoO20.20Table 3. Calculated 95Mo Isotropic Chemical Shift,Quadrupolar Coupling Constant, and Peak Position forMo−O Polyhedron of Ba7Nb4MoO2020modelsa polyhedron δiso /ppm |CQ|/MHz δMAS /ppmb(4), (5) Mo(1)O6 +275 to +277 1.47, 2.00 +269 to +270(1)−(3), (5) Mo(2)O4 −36.5 to−28.40.36−0.90 −36.8 to−29.1(6) Mo(3)O6 +164 2.87 +145(7)−(10) Mo(2)O5 +27 to +221 5.1−10.7 −57 to +80(7), (8) Mo(4)O6 +109, +109 5.17, 5.23 +47, +49aEach model is shown in Figures S7 and S8.20 The values of Mo(2)O5were calculated by constructing models (7)−(10) combining M(4)O6and M(2)O5 (Figure S8). bδMAS is a magnetic field-dependent valuethat was corrected by the author.35 Here, the magnetic field is 18.79T.Table 4. Calculated 95Mo Isotropic Chemical Shift,Quadrupolar Coupling Constant, and Peak Position forMo−O Polyhedron of Ba7Ta4MoO20modelsa polyhedron δiso /ppm |CQ|/MHz δMAS /ppm(4), (5) Mo(1)O6 +289 to +294 0.50, 2.3 +283 to +289(1)−(3), (5) Mo(2)O4 −28.1 to−18.60.52−0.83 −29.7 to−19.3(6)b Mo(3)O6 +148, +158 7.4, 9.5 −66 to +2.5aEach model is shown in Figure S9. The M(4)O6 octahedron used toconstruct the model for the M(2)O5 polyhedron in Ba7Nb4MoO20does not exist in Ba7Ta3.7Mo1.3O20.15, so calculations similar to thosein Table 3 for the Mo(2)O5 polyhedron were not performed forBa7Ta4MoO20.12 bTwo sets of values were obtained for the samemodel because there were two minima instead of one.Figure 3. 93Nb MAS NMR spectra of (a) Ba7Nb4MoO20, (b)Ba7Nb3.9Mo1.1O20.05, and (c) Ba7Nb3.85W0.15MoO20.075 measured bythe spin-echo method under the same conditions, including thenumber of accumulations. The dashed lines are eye guides. Asterisks,*, denote spinning sidebands. Here, 1D measurements at MAS = 15kHz for Ba7Nb4MoO20 confirmed which peaks were spinningsidebands.The Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c02645J. Phys. Chem. C XXXX, XXX, XXX−XXXEhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig3&ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c02645?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asas the spin-echo method. The peak positions in this figure andTable 5, δMAS, were determined from the spin-echo30 results(Figure 3) as previously reported20 for Ba7Nb4MoO20 to avoidthe baseline distortion often seen with single-pulse methods(Figure S4). The δMAS and the attributed polyhedroncompared with DFT calculations (Table 6) are listed inTable 5, along with the results of two-dimensional experimentsshown later. The previous attribution of the two peaks, −762ppm for Nb(1)O6 octahedron and −950 ppm for Nb(2)O4tetrahedron, was the opposite of the usual octahedron−tetrahedron attribution,28,37 so this attribution remained anestimate.20 Similar prominent peaks at −761 and −944 ppmwere observed in both Ba7Nb3 . 9Mo1 . 1O20 . 05 andBa7Nb3.85W0.15MoO20.075. Both Ba7Nb3.9Mo1.1O20.05 andBa7Nb3.85W0.15MoO20.075 have increased peak heights at−944 ppm. Conversely, the height of the −950 ppm peak,which appeared close to the −944 ppm peak in Ba7Nb4MoO20,dec r e a s ed sha rp l y in Ba 7Nb3 . 9Mo1 . 1O2 0 . 0 5 andBa7Nb3.85W0.15MoO20.075. Moreover, the small peak at −990ppm in Ba7Nb4MoO20, which was suggested to be Nb(2)O5polyhedron or Nb(4)O6 octahedron in a previous work20 butwhose shift could not be well explained, is also observed inBa7Nb3.9Mo1.1O20.05 and Ba7Nb3.85W0.15MoO20.075.Figure 4 shows the 93Nb two-dimensional 3QMAS NMRresult for Ba7Nb3.9Mo1.1O20.05 and the previously reportedresult for Ba7Nb4MoO20. Here, the peak on −761 ppm isspread to the quadrupole-induced shift axis direction, and thepeak on −944 ppm is spread to both quadrupole-induced shiftand chemical shift axis direction19,32 in the same manner.Therefore, the δiso and |PQ| were obtained from this figureusing eqs 1 and 2 for disordered materials’ analysis and arelisted in Table 5. Here, in Ba7Nb3.9Mo1.1O20.05, there is nosignificant change in the two spots farther from the diagonalline corresponding to the larger |PQ| peaks, whereas the spotnearest the diagonal line that corresponds to the smallest |PQ|peak has almost disappeared. In other words, the correspond-ing component for the smallest |PQ| peak in Figure 3a is sharplydecreased in Ba7Nb3.9Mo1.1O20.05.Here, the −952 ppm spot in Ba7Nb4MoO20 was suggested tobe a NbO4 tetrahedron at the M(2) site in the previous reportbecause the 6 MHz of |PQ (93Nb)| could only be explained bythe smallest calculated case (Table 6).20,35 This estimation isreasonable because the |CQ (95Mo)| value of the similar MO4tetrahedron at the same M(2) site also corresponds to thesmallest calculated values (Table 3).For the two sites at −762 and −944 ppm of Ba7Nb4MoO20,when both peaks are all Nb and attributed to M(1) and M(3),Table 5. 93Nb Peak Position, δMAS, Measured by 93Nb 1D MAS NMR Using the Spin-Echo Method, 93Nb δiso and |PQ|Determined by 93Nb 3QMAS NMR, and Attributed Polyhedronas-prepared samples δMAS/ppma δiso /ppmb |PQ|/MHzb polyhedronBa7Nb4MoO20 −762c −748c 15c Nb(1)O6Ba7Nb4MoO20 −944c −928c 19c Nb(3)O6Ba7Nb4MoO20 −950c −952c 6c Nb(2)O4Ba7Nb3.9Mo1.1O20.05 −761 −746 20 Nb(1)O6Ba7Nb3.9Mo1.1O20.05 −944 −934 18 Nb(3)O6Ba7Nb3.85W0.15MoO20.075 −761 Nb(1)O6Ba7Nb3.85W0.15MoO20.075 −944 Nb(3)O6aδMAS is the magnetic field-dependent value. Here, the magnetic field is 18.79 T. bδiso and |PQ| calculated by eqs 1 and 2 are magnetic field-independent values. cExperimental data and attribution for Ba7Nb4MoO20.20Table 6. Calculated 93Nb Isotropic Chemical Shift,Quadrupolar Coupling Product, and Peak Position for Nb−O Polyhedron of Ba7Nb4MoO2020modelsa polyhedron δiso /ppm |PQ|/MHz δMAS/ppmb(1), (2), (3) Nb(1)O6 −800 to−81018−20 −812 to −825(1), (2), (3) Nb(2)O4 −848 to−8626.8−18 −858 to −864(1), (2), (3) Nb(3)O6 −867 to−89741−50 −929 to −988(9), (10) Nb(2)O5 −803, −812 43, 75 −884, −1044(9), (10) Nb(4)O6 −845, −858 71, 73 −1028, −1052aEach model is shown in Figures S7 and S8.20 Here, the models withMo atoms at the M(2) site were used. bδMAS is a magnetic field-dependent value that was corrected by the author.35 Here, themagnetic field is 18.79 T.Figure 4. 93Nb 3QMAS NMR spectra of as-prepared (a)Ba7Nb4MoO20 and (b) Ba7Nb3.9Mo1.1O20.05 measured under thesame condition. The diagonal line in the 2D diagram is called thechemical shift axis. The spread of the signal along the diagonal lineindicates the distribution of the chemical shift when an appropriateshearing transformation is used. The color squares are eye guides.Asterisks, *, denote spinning sidebands.The Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c02645J. Phys. Chem. C XXXX, XXX, XXX−XXXFhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645?fig=fig4&ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c02645?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asrespectively, the peak at −944 ppm should be approximately1.2 times higher than the peak at −762 ppm related to the ratioof the number of equivalent positions in a unit cell and theresult of the |PQ| difference. Therefore, Figure 3a reflects theequivalent positions’ ratio well (Supporting Information, NoteS2). Similarly, for the two sites at −761 and −944 ppm ofBa7Nb3.9Mo1.1O20.05, when both peaks are all Nb and areattributed to M(1) and M(3), respectively, the peak at −944ppm should be approximately 2.5 times higher than the peak at−761 ppm. Therefore, Figure 3b also reflects the quantity ratiowell (Supporting Information, Note S3). Thus, the previousestimation that −762 ppm corresponds to the Nb(1)O6octahedron and −944 ppm corresponds to the Nb(3)O6octahedron is correct.20 Since the DFT results support theseunusual shifts, the anomalous Nb shifts, which differ frommany materials, are probably the result of the coexistence withMo. The difference between the experimental and thecalculated values of |PQ| in the Nb(3)O6 octahedron may becaused by the effect of molecular motion, which is usuallyignored in DFT calculations, or the effect from H2O, which isnot considered in this calculation.Here, if all of the Nb at the M(2) site were formed as aNb(2)O4 tetrahedron, the area intensity of the Nb(2)O4tetrahedron in Figure 3a should be 0.84 times that of theNb(1)O6 octahedron (Supporting Information, Note S4).Since the signal of the Nb(2)O4 tetrahedron must be moreintense than Nb(1)O6 and Nb(3)O6 owing to the narrowestline width related to the smallest |PQ| in Table 5, the peakheight is likely the highest among three peaks as shown inFigure S11(a-1). However, the peak height of the Nb(2)O4tetrahedron obtained in the 1D measurement is smaller thanthat of the Nb(3)O6 octahedron (Figure 3a), and the areaintensity is not 0.84 but 0.50 times that of the Nb(1)O6octahedron, as shown in Figure S11(a-2). This is presumablybecause in Ba7Nb4MoO20, some of the Nb oxide polyhedra atthe M(2) site are not regular Nb(2)O4 tetrahedrons due to thebonding of H or O derived from H2O inside the as-preparedsample.When Nb is 8% at the M(4) site and all Mo is at the M(2)site also for Ba7Nb3.9Mo1.1O20.05 and Ba7Nb3.85W0.15MoO20.075as Ba7Nb4MoO20 (Figure 1), the Nb content ratio of the M(2)sites of Ba7Nb4MoO20, Ba7Nb3 .9Mo1 .1O20 . 05 , andBa7Nb3.85W0.15MoO20.075 is 0.84:0.74:0.8085 (SupportingInformation, Note S5). Therefore, the significant decrease inthe 93Nb signal of the Nb(2)O4 tetrahedra inBa7Nb3.9Mo1.1O20.05 (Figure 3b) and Ba7Nb3.85W0.15MoO20.075(Figure 3c) cannot be explained as a difference in the ratio oftotal Nb contained. A reasonable explanation is that the excessamounts of oxygen atoms (1/2)x (x = 0.1) or (1/2)y (y =0.15) changes Nb(2)O4 tetrahedron to Nb(2)O5 polyhedron,NbO5 monomer or (Mo/Nb)(2)2O9 dimer, and is no longerNb(2)O4 tetrahedron.The −990 ppm peak, suggested as a possible Nb(2)O5 orNb(4)O6 peak in the earlier report,20 can be considered asfollows. In Ba7Nb4MoO20, the small signal intensity ofNb(2)O4 tetrahedron is possibly caused by the effect of Hand O derived from 0.15 H2O. Therefore, as the thirdcandidate, for example, Nb(2)On (n = 4, 5, and 6) polyhedraaccompanied by H should be considered for Ba7Nb4MoO20,Ba7Nb3.9Mo1.1O20.05, and Ba7Nb3.85W0.15MoO20.075. Therefore,the attribution of the −990 ppm peak will be investigated infuture research.■ CONCLUSIONSBa7Nb4MoO20 and related materials are of great interest forindustrial applications such as solid oxide fuel cells, protonceramic fuel cells, gas sensors, and oxygen-separationmembranes because they exhibit high oxide-ion and protonconductivity. These materials are considered with respect tothe mixed oxide coordination number on M(2) sites near theoxide-ion conducting layer. Initially, the structures ofBa7Nb4MoO20 and their related materials were mostlyanalyzed by assuming an even distribution of Mo and Nb ateach site. However, in the previous report, a combination ofseveral techniques, including solid-state NMR, showed that inBa7Nb4MoO20, most Mo, unlike Nb, is located at the M(2) sitenear the ion conducting layer, since in 95Mo NMR only onesignal was observed. Because of the peak position and the small|CQ| value, the signal was attributed to the MoO4 tetrahedron.To obtain more information, solid-state NMR is promisingbecause the signal intensity, line width, and shift are directlyaffected by differences in the local structure, and the intensityratio of each peak reflects the amount ratio. This methodcircumvents the difficulty of distinguishing Nb and Mo nearthe oxide-ion and proton conducting layers using manyanalytical methods (Figure S2). This is the first experimentalstudy on hexagonal perovskite Ba7Nb4MoO20-related materials,with a focus on the differences in NMR signals near the oxide-ion conducting layer in different materials. As wellBa7Nb4MoO20, the sharp Lorentzian-like 95Mo NMR peaksrevealed in Ba7Nb3.9Mo1.1O20.05, Ba7Nb3.85W0.15MoO20.075, andBa7Ta3.7Mo1.3O20.15 are also definitely attributed to theMo(2)O4 tetrahedron near the oxide-ion conducting layerowing to the shifts and the small |CQ| values. Moreover, inBa7Ta4−zMo1+zO20+(1/2)z, an extra 95Mo peak of Mo(2)O5,Mo(2)2O9, or Mo(2)Ta(2)O9 polyhedra was observed, whichis possibly related to the excess amounts of oxygen atoms (1/2)z (z = 0.3). The results for 93Nb differ significantly fromthose for 95Mo. The 93Nb NMR peak associated with theNb(2)O4 tetrahedron of Ba7Nb4MoO20 has a small |PQ|, so itshould be quite sensitive to its amount, as should the 95Mopeak of the Mo(2)O4 tetrahedron, which also has a small |CQ|.However, 0.50 of the 93Nb NMR signal intensity associatedwith the Nb(2)O4 tetrahedron of Ba7Nb4MoO20 is consid-erably smaller than 0.84 of the Nb(1)O6 signal intensitydespite 0.84 of the composition ratio at the M(2) site.Therefore, some of the Nb in the M(2) site is no longer aregular NbO4 tetrahedron, possibly owing to H or O derivedfrom 0.15 H2O in as-prepared Ba7Nb4MoO20. Furthermore,the 93Nb NMR peak intensities associated with the Nb(2)O4tetrahedra of Ba7Nb3.9Mo1.1O20.05 and Ba7Nb3.85W0.15MoO20.075were reduced compared to Ba7Nb4MoO20. This reductionmust be caused by the bonding of excess amounts of oxygenatoms on the Nb(2)O4 tetrahedron related with (1/2)x (x =0.1) or (1/2)y (y = 0.15). That is, in Ba7Nb4MoO20, whileobserving a prominent MoO4 tetrahedral signal, some of theNb(2)−O polyhedra are not Nb(2)O4 tetrahedron, possiblydue to the bonding of H or O derived from H2O inside the as-prepared sample. Moreover, the effects of excess oxygen relatedto (1/2)x, (1/2)y, or (1/2)z are found in both Mo(2)O4 andNb(2)O4 tetrahedra near the oxide-ion conducting layer forBa7Nb4−xMo1+xO20+(1/2)x (x = 0.1), Ba7Nb4−yWyMoO20+(1/2)y(y = 0.15), and Ba7Ta4−zMo1+zO20+(1/2)z (z = 0.3). Thus, solid-state NMR enables local structural analysis of individual Moand Nb atoms in solid solutions and offers immense potentialThe Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c02645J. Phys. Chem. C XXXX, XXX, XXX−XXXGhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c02645/suppl_file/jp4c02645_si_001.pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c02645?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asfor the electrolyte design of many perovskite derivatives,especially related systems containing Mo/Nb atoms.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c02645.Eleven figures, five notes on the comparison of Mo(2)O4peak intensity, −762 and −944 ppm 93Nb peak heightratios of Ba7Nb4MoO20, −761 and −944 ppm 93Nb peakheight ratios of Ba7Nb3.9Mo1.1O20.05, the ratio of thenumbers of equivalent crystallographic sites in a unit cell,and Nb amount ratio of the M(2) site (PDF)■ AUTHOR INFORMATIONCorresponding AuthorMasataka Tansho − National Institute for Materials Science(NIMS), Tsukuba 305-0003, Japan; orcid.org/0000-0001-7986-3199; Email: TANSHO.Masataka@nims.go.jpAuthorsAtsushi Goto − National Institute for Materials Science(NIMS), Tsukuba 305-0003, JapanShinobu Ohki − National Institute for Materials Science(NIMS), Tsukuba 305-0003, JapanYuuki Mogami − National Institute for Materials Science(NIMS), Tsukuba 305-0003, JapanYuta Yasui − Department of Chemistry, School of Science,Institute of Science Tokyo, Tokyo 152-8551, JapanYuichi Sakuda − Department of Chemistry, School of Science,Institute of Science Tokyo, Tokyo 152-8551, JapanKotaro Fujii − Department of Chemistry, School of Science,Institute of Science Tokyo, Tokyo 152-8551, Japan;orcid.org/0000-0003-3309-9118Takahiro Iijima − Institute of Arts and Sciences, YamagataUniversity, Yamagata 990-8560, JapanMasatomo Yashima − Department of Chemistry, School ofScience, Institute of Science Tokyo, Tokyo 152-8551, Japan;orcid.org/0000-0001-5406-9183Complete contact information is available at:https://pubs.acs.org/10.1021/acs.jpcc.4c02645Author ContributionsThe manuscript was written with contributions from allauthors. All authors have approved the final version.FundingThis work was partly supported by Grants-in-Aid for ScientificResearch (KAKENHI, JP21K18182, and JP19H00821) fromMEXT.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThe authors thank Dr. T. Shimizu and K. Deguchi of NIMS fortheir help with NMR measurements, and this work wassupported by the Research Network and Facility ServicesDivision in NIMS. The authors also thank Y. Shimoikeda ofJeol Resonance Inc. for his help with the MQMAS analysis andY. Suzuki for the helpful discussion and assistance in theexperiments/analyses. The authors acknowledge T. Shibata ofKojundo Chemical Laboratory Co. for the ICP measurementsand for supplying chemical materials.■ ABBREVIATIONSNMR, nuclear magnetic resonance; SOFCs, solid oxide fuelcells; PCFCs, proton ceramic fuel cells; RXRD, resonant X-raydiffraction; DFT, density-functional theory; ICP-AES, in-ductively coupled plasma atomic emission spectroscopy;XRF, X-ray fluorescence; MAS, magic angle spinning; 3Q,three-quantum; MQ, multiquantum■ REFERENCES(1) Staffell, I.; Scamman, D.; Velazquez Abad, A.; Balcombe, P.;Dodds, P. E.; Ekins, P.; Shah, N.; Ward, K. R. The Role of Hydrogenand Fuel Cells in the Global Energy System. Energy Environ. Sci. 2019,12 (2), 463−491.(2) Coduri, M.; Karlsson, M.; Malavasi, L. Structure-PropertyCorrelation in Oxide-Ion and Proton Conductors for Clean EnergyApplications: Recent Experimental and Computational Advance-ments. J. Mater. Chem. A 2022, 10 (10), 5052−5110.(3) Zhang, W.; Yashima, M. Recent Developments in Oxide IonConductors: Focusing on Dion-Jacobson Phases. 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