Effect of germanium substitution on the structure and ionic conductivity of the hexagonal perovskite derivative compound Ba₃WVO_8.5

Nitin Kumar^ab, K. Sandeep Rao^cd, Anu^ab, A. K. Sahu^e, S. N. Achary*^cd and S. K. Deshpande^a
aUGC-DAE Consortium for Scientific Research, Mumbai Centre, Bhabha Atomic Research Centre, Mumbai 400085, India
^bSavitribai Phule Pune University, Ganeshkhind, Pune 411007, India
^cChemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India. E-mail: sachary@barc.gov.in
^dHomi Bhabha National Institute, Anushakti Nagar, Mumbai 400094, India
^eGlass and Advanced Materials Division, Bhabha Atomic Research Centre, Mumbai 400085, India

First published on 28th July 2025

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1. Introduction

Materials with high oxide ion conductivity are used as electrolytes in solid oxide fuel cells (SOFCs) for direct conversion of chemical energy into electrical energy.¹ In general, an electrolyte material should have high ionic conductivity with no or minimum electronic conductivity, along with other properties such as high chemical and thermal stability, low cost and high mechanical strength.^1,2 The most common commercially used electrolyte is yttria-stabilized zirconia (YSZ), which is known for its high ionic conductivity and stability at high temperatures.³ However, it shows significant ionic conductivity only at high temperatures. Thus, the operating temperature for commercially available SOFCs is usually in the range of 973–1273 K.^1,4 Devices operating at high temperatures present several issues such as limited material compatibility, poor durability and high cost.^4–7 Since most of the known oxide ion conductors show appreciable conductivity only at high temperatures, new materials with high ionic conductivity at lower temperature are required to operate SOFCs at moderate temperatures and, hence, to reduce the operational cost.^1,4

High ionic conductivities have been reported in compounds of only a few structural families, such as fluorite,⁷ apatite,⁸ melilite,⁹ and perovskite.^10–12 A number of studies are focused on normal perovskite-type compounds due to their flexible structure, which can accommodate varieties of cation and anion vacancies, and among them, Na_0.5Bi_0.5TiO₃ and LaGaO₃ are considered good solid-state electrolytes for moderate-temperature applications.^10–12 Such compositional and structural flexibilities allow convenient modes to tune carrier and transport properties in perovskite-type materials, and that makes them attractive for solar cell,¹³ optoelectronic and sensor applications.^14–16 Recently, high oxide ion conduction at moderate temperatures in certain types of hexagonal perovskites like Ba₃M′M′′O_8.5 (M′ = W and Mo; M′′ = Nb and V) have been reported in the literature.^17–20 The high chemical and thermal stability along with high ionic conductivity at moderate temperatures of such compounds attracted widespread research attention to understand and improve the ionic conductivity.^21–24 Fop et al. have reported conductivity as high as 10⁻² to 10⁻³ S cm⁻¹ at around 873 K with high stability in an atmosphere with wider partial pressure of oxygen in hexagonal Ba₃NbW/MoO_8.5 systems.^21,22 It may be mentioned here that the structure of the hexagonal perovskite derivative compounds is intermediate between that of perovskite 9R and the palmierite structure.^25–28 The 9R perovskite has a 3D structure built by linking of trimers of face-shared MO₆ octahedra through corner oxygen atoms, while the palmierite structure is made up of isolated MO₄ tetrahedra.^20,27–29 Indeed, the palmierite structure is a 9R perovskite with ordered arrangement of cations and oxygen vacancies, where one of the three closely packed [BaO₃] layers is replaced by an oxygen-deficient [BaO₂] layer.^18,20,29 This arrangement separates the face-shared octahedral trimers of 9R to two isolated octahedra separated by a vacant octahedral space. In the 9R perovskites, there are three different types of oxygen sites (O1, O2 and O3), and two of them (O2, and O3) are underoccupied. These two underoccupied anion sites form disordered coordination polyhedra, which can be approximated by the disordering of octahedra and tetrahedra. Thus, Ba₃WNbO_8.5 has a more disordered structure than the 9R perovskite and palmierite, with partial occupancies of the metal site (M1 and M2) and oxygen site (O2 and O3) layers, while Ba₃WVO_8.5 has a completely vacant M2 site and the M1 site is fully occupied.^20,30

The inherent oxygen vacancies and the disordering of coordination polyhedra have been shown as the origin for their high oxide ion conduction.^25,26,31,32 The anion site disorder has been shown to be related to occupancy and site symmetry of the O3 ion.^33–35 Additionally, it has been shown that the alteration of oxygen vacancies and ratio of tetrahedra to octahedra enhances the ionic conductivity,^28,34,35 and hence, such materials have been the subject of many recent studies. The variation in conductivity with composition in Ba₃W_1+xV_1−xO_8.5+x/2,^28,33 Ba₃MoNb_1−xTa_xO_8.5−δ,³¹ Ba₃MoNb_1−xGa_xO_8.5−δ³² and Ba₃W_1+xNb_1−xO_8.5+x/2³⁴ has been explained by these factors. Fop et al. have shown that substitution of a part of Nb⁵⁺ of Ba₃WNbO_8.5 by V⁵⁺ enhances the conductivity solely due to the increased disorder by increasing the fraction of tetrahedral MO₄ units over octahedral MO₆ units.²¹ The enhancement in conductivity of the Ba₃Mo_1+xNb_1−2xGe_xO_8.5 (x = 0, 0.1, and 0.2) system has been attributed to the increase in the concentration of tetrahedral units.³⁵

Recently, the role of oxygen ion vacancy and stoichiometry of cations in the ionic conductivity of Ba₃WVO_8.5 has been studied by investigating the structure and conductivity of Ba₃W_1−xV_1+xO_8.5−x/2.²⁸ The bulk conductivity of Ba₃W_1−xV_xO_8.5+x/2 has been tuned to about 1.2 × 10⁻⁴ S cm⁻¹ by decreasing the vanadium content, while under similar conditions, Ba₃WVO_8.5 had a conductivity of about 2.3 × 10⁻⁶ S cm⁻¹ (1023 K).²⁸ Further, it was reported that the conductivity is enhanced by both increasing and decreasing the oxygen stoichiometry in the samples, which has been attributed to the easy migration of ions and the increase in the concentration of the mobile ions. Additionally, it was concluded that the fraction of tetrahedral units increases with the increase in V⁵⁺ concentration in the samples. Cheng et al.³⁵ have reported conductivity of about 1.23 × 10⁻⁴ S cm⁻¹ for Ba₃Mo_1.2Nb_0.6Ge_0.2O_8.5 at 673 K, which is about two times higher than that of Ba₃MoNbO_8.5 (5.8 × 10⁻⁵ S cm⁻¹) and Ba₃MoNb_0.8V_0.2O_8.5 (7.6 × 10⁻⁵ S cm⁻¹) around the same temperature. This suggests that tuning of anion vacancies or introducing excess anions as well as disorders in anion sites can further enhance the ionic conductivity.

In order to substantiate the role of oxygen ion vacancies and the fraction of tetrahedral units, a systematic study was conducted on the structure and dielectric properties of Ba₃WV_1−xGe_xO_8.5−x/2 (x = 0, 0.1, 0.2, 0.3). To obtain single-phase samples, Ge⁴⁺ can be considered as an ideal substituent for V⁵⁺ due to their comparable ionic radii. It is known that Ge⁴⁺ has a preference for tetrahedral coordination due to a smaller ionic radius, and hence, can change the ratio of M1–O₄/M1–O₆ in these compounds. Additionally, a systematic increase in oxygen ion vacancies can be introduced by the substitution of V⁵⁺ with Ge⁴⁺. Moreover, the preference of Ge⁴⁺ for forming tetrahedral coordination can also alter the anion disorder in the BaO₃ layer of hexagonal perovskite system. By adopting this strategy, a conductivity of 1.80 × 10⁻⁵ S cm⁻¹ at 973 K could be achieved at Ba₃WV_0.7Ge_0.3O_8.35, while that of Ba₃WVO_8.5 was 1.03 × 10⁻⁷ S cm⁻¹ at a similar temperature. From the analyses of impedance data using equivalent circuit models, it could be revealed that the bulk conductivity of the grains systematically increases with the increase in Ge⁴⁺ concentration in the compositions, while the activation energies for conductivity remain at 0.9 to 1.6 eV. In order to understand the conductivity mechanisms, the modulus spectra were analysed, and it was concluded that the distribution of relaxation time is appreciably asymmetric with respect to the mean value of all the samples, which increases with the increase in Ge⁴⁺ concentration. This is a characteristic feature of the distribution of spatial arrangement of carriers and the energetics for carrier migration. From the in situ high-temperature XRD studies, structural and compositional stability of all the samples was concluded. The details of the structure and transport properties at high temperatures are explained herein. Such a study can provide further insights into the influence of such substitutions on the structure and conductivity, which is relevant to the possible applications of this class of materials as solid electrolytes in fuel cells.

2. Experimental

The series of Ba₃WV_1−xGe_xO_8.5−x/2 (x = 0 (BWVO), 0.1 (BWVGO1), 0.2 (BWVGO2), and 0.3 (BWVGO3)) compounds were prepared via a solid-state reaction route using stoichiometric amounts of BaCO₃ (Alfa Aesar, 99.9%), WO₃ (Sigma Aldrich, 99.95%), GeO₂ (Sigma Aldrich, 99.95%), and V₂O₅ (Alfa Aesar, 99.9%) as reactants. Before weighing for reactions, BaCO₃, V₂O₅, and GeO₂ powders were heated to 573 K overnight, while WO₃ was heated at 973 K overnight. Desired amounts of pre-dried reactants for the composition Ba₃WV_1−xGe_xO_8.5−x/2 were homogenized for about 2 h using an agate mortar and pestle. Acetone was used as a grinding medium to facilitate homogenization. The obtained uniform powder was pelletized and heated at 823 K for 12 h in a static air atmosphere. The formation of product was monitored by recording the powder XRD data for the sample after the heat treatment. The phase-pure product was obtained after multiple heating at 1373 K and repeated homogenization. After confirmation of phase purity, the finely powdered samples were pressed into pellets of 8 mm diameter and 1–2 mm thickness, and sintered at 1473 K for 8 h. Platinum boats were used as sample holders for all the treatment. The heating and cooling rate for all the heat treatment processes was 2 K min⁻¹.

In order to examine the thermal stability, structural characterization was carried out by in situ high-temperature X-ray diffraction studies. All the high-temperature XRD patterns were recorded using a PROTO AXRD theta–theta X-ray diffractometer with an Anton Paar HTK 1600 high-temperature attachment. Slurries of a finely powdered sample in acetone were smeared on a platinum strip used as a sample holder and heater. The samples were heated to the desired temperature and equilibrated for 5 minutes prior to recording the diffraction data. The XRD patterns were recorded at several temperatures from room temperature to 973 K. The XRD data were analyzed by the Rietveld refinement method using the Fullprof program.³⁶

The measurement of dielectric properties was carried out using an Alpha-AN impedance analyzer (Novocontrol Technologies, Germany) equipped with a Novotherm HT1200 high-temperature attachment. Sintered cylindrical pellets of the samples were coated with platinum paste and annealed at 973 K to ensure proper electrical contact. The platinum-coated pellets were placed in a dielectric sample cell (Probostat, NorECS, Norway) mounted in a tubular furnace, in a parallel-plate capacitor configuration with platinum electrodes. The temperature was varied from 623 K to 973 K. The complex dielectric parameters were measured in the frequency range of 10 Hz–10 MHz. All the measurements were carried out while heating in air and with an applied rms ac voltage of 0.5 V. Data at every temperature were recorded with an error of 0.5 °C in temperature.

3. Results and discussion

3.1. Structure of Ba₃WV_1−xGe_xO_8.5−x/2 (x = 0, 0.1, 0.2, and 0.3)

The phase purity and structure of the prepared Ba₃WV_1−xGe_xO_8.5−x/2 (x = 0, 0.1, 0.2, and 0.3) samples were studied by comparing their powder XRD patterns with the data reported for Ba₃WVO_8.5 (JCPDS-PDF 78-0529). All the observed peaks in the powder XRD patterns could be accounted for by the hexagonal perovskite-type lattices. All the peaks in the XRD pattern of Ba₃WVO_8.5 (BWVO), i.e., x = 0 sample marked with Miller indices (hkl), are shown in Fig. S1 of the SI. Moreover, a figure comparing the XRD patterns of all the studied Ba₃WV_1−xGe_xO_8.5−x/2 (BWVGO) samples with the pristine compound Ba₃WVO_8.5 is shown in Fig. S2 of the SI. In both figures, the peaks are indexed according to the JCPDS data (JCPDS 78-0529, Ba₃WVO_8.5).

Further, the powder XRD data of the samples recorded at 323 K were analysed by the Rietveld refinement method using a structural model based on the reported structural details of Ba₃WVO_8.5.^28,30,33,37 In this model, Ba atoms are located on two atomic sites Ba1 (3a) and Ba2 (6c), while W, V and Ge are on site M1 (6c) according to their stoichiometry. Another possible site for the octahedral cation, M2 (6c), was assumed as a fully vacant site as reported earlier for the Ba3WVO_8.5 compound (30). The oxygen ions occupy three sites, namely, O1 (18h), O2 (9e) and O3 (6c). The site O1 is fully occupied while the sites O2 and O3 are under-occupied, hence retaining the intrinsic anion vacancy arising from the composition. For the refinement of data for Ba₃WVO_8.5, the M1 site was occupied by both W and V with an occupancy of 0.5 for each, while for the Ge-doped samples, the Ge ions are distributed in the M1 site keeping the occupancy of V and Ge totaling to 0.5. Attempts to fill the cations in the M2 site were not successful in the Rietveld refinement as their occupancies turn out to be negative on refinement. Hence, similar to the earlier reports,^28,30,33,37 the M2 site is kept vacant while maintaining the cation stoichiometry as the nominal composition of the samples. For the refinement of anions, the O1 (18h) site is considered as a fully occupied site and the rest of the anions as per the stoichiometry are placed at the O2 (9e) site, i.e., occ. 0.833 and O3 site (6c) as 0.0. Subsequently, the O3 sites are occupied by liberating the O2 site anions to O3 sites with a constraint, and the total anions at O2 and O3 sites, i.e., g_O2 + g_O3 = 7.5 − x/2, are in agreement with the nominal composition. Since the anion stoichiometry decreases due to the substitution of V⁵⁺ with Ge⁴⁺, the refinement of occupancies of underoccupied oxygen sites was carried out by using a constraint of nominal anion stoichiometry. The occupation of a larger fraction of O²⁻ ions at the 6c site (i.e., O3) compared to 9e (O2) is in accordance with the larger fraction of tetrahedral group compared to the octahedral group in Ba₃WVO_8.5 and Ba₃WV_1−xGe_xO_8.5−x/2.

It may be noted here that several possible sites have been proposed for the O3 atoms, viz. 6c (0, 0, z; z ∼ 0.5), 3b (0, 0, 0. 5) or 36i (x,y,z, x ∼ 0, y ∼ 0, and z ∼ 0.5). This variation can be due to the distortion and orientational disorder of the M1O₄ tetrahedra. In the present study, the refinement was carried out using O3 atoms at 6c sites, and appreciable displacement from the 3b (special position) along the c-axis is evident.

The typical structure of the hexagonal perovskite-type Ba₃WVO_8.5 (BWVO) compound, depicting different sites is shown in Fig. 1. It can be seen from the figure that O2 atoms make up the M1O₆ octahedra, while the O3 atoms make up the M1O₄ tetrahedra. Thus, the octahedral and tetrahedral units of M1 atoms are related to the occupancy of the O2 and O3 atoms. All the XRD patterns of Ba₃WV_1−xGe_xO_8.5−x/2 (x = 0, 0.1, 0.2, and 0.3) samples could be fitted very well as single-phase rhombohedral (space group: R3m), which suggests the structural stability and formation of the single-phase sample on Ge substitutions. The refined unit cell parameters and residuals for the Rietveld refinements of the XRD data are summarized in Table 1. The Rietveld refinement plots for the XRD data of BWVO (x = 0) and BWVO3 (x = 0.3) samples recorded at 323 K are shown in Fig. 2, and those of samples with x = 0.1 and 0.2 are given in Fig. S3 of the SI. The refined position coordinates and other refinement parameters for BWVO at 323 K are given in Table S1.

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The thermal and structural stabilities of the samples are crucial factors in determining their suitability as electrolyte materials in solid oxide fuel cells. In this regard, the structural stability and evolution of structural parameters with temperature were studied by analysing the recorded high-temperature XRD data. The XRD patterns recorded at higher temperatures were analysed by the Rietveld refinement method using the observed structural details of the corresponding sample at 323 K. In all the cases, the HT-XRD patterns recorded up to 1023 K could be successfully refined with the considered rhombohedral lattice. This suggests that all the studied materials do not have any phase transition or decomposition within the studied temperature range. The XRD patterns of the BWVGO3 sample show additional peaks due to platinum (Pt), which was used in the sample holder cum heater. Hence, the refinement of the XRD patterns of the BWVGO3 sample was carried out by considering Pt as an additional phase.

Typical Rietveld refinement plots for the XRD data of Ba₃WVO_8.5 (BWVO), and Ba₃WV_0.7Ge_0.3O_8.35 (BWVGO3) samples recorded at 1023 K are shown in Fig. 2. The Rietveld refinement plots of XRD data of samples with x = 0.1 and 0.2 are given in the SI (Fig. S3). The refined structural parameters for all the samples at 1023 K are given in Table S1. The unit cell parameters of Ba₃WV_1−xGe_xO_8.5−x/2 (x = 0, 0.1, 0.2, and 0.3) samples at 1023 K are presented in Table 1. Appreciably, good residuals were achieved for the refinement of the XRD data recorded at all the temperatures, and the representative residuals for the two temperatures are also given in Table 1.

Fig. 3(a) shows the variation in unit cell parameters of the Ba₃WV_1−xGe_xO_8.5−x/2 (x = 0, 0.1, 0.2, 0.3) samples with x, i.e. the Ge content. It can be seen that the unit cell parameters (a, c) and the cell volume V increase with the increase in Ge content in the series. This is in accordance with the larger ionic radius of Ge⁴⁺ compared to that of V⁵⁺ (ionic radii of Ge⁴⁺ and V⁵⁺ in tetrahedral coordination are 0.390 Å and 0.355 Å, respectively).³⁸ Similar expansion of the unit cell was reported by Cheng et al.³⁵ for the Ba₃Mo_1+xNb_1−2xGe_xO_8.5 series. Compared to the study by Cheng et al.,³⁵ the present studied series showed an increase in unit cell parameters even though there is an increase in anion vacancies. This suggests that the expansion due to the larger ionic radii of Ge⁴⁺ ions dominates over the possible contraction due to the anion vacancies in the lattice. The variations in unit cell parameters (a, c and V) of Ba₃WV_1−xGe_xO_8.5−x/2 (x = 0, 0.1, 0.2, and 0.3) with temperature are shown in Fig. 3(b)–(d). The plots show an almost linear expansion for all the samples with temperature. It was also observed that the temperature-induced lattice expansion is more or less similar for all the studied samples. Hence, the change in the anion vacancy with the increase in Ge concentration might be the governing factor for their ionic conductivity. It is reported in the literature that ionic conductivity in the perovskite-type structure increases as the unit cell volume increases.^31,32 Thus, the expanded lattice without any change in the crystal structure of the present studied samples at higher temperatures is likely to facilitate the migration of ions.

3.2. Scanning electron microscopy and EDX analysis

The microstructure of the BWVGO1, BWVGO2, and BWVGO3 samples was studied by SEM, and the recorded images at different magnifications are shown in Fig. 4. The SEM image and EDX spectra of Ba₃WVO_8.5 (BWVO) were explained in our earlier report,²⁸ and the present observed SEM images of all the other samples closely resemble that of Ba₃WVO_8.5.

All samples exhibit irregular grains of spherical to cubical shapes having sizes in the range of 1 to 5 microns. Moreover, in all these samples, the grains are well formed and interconnected. The elemental distribution and composition were analysed using the EDX spectra collected in the area-scan mode. The EDX spectra for the samples are displayed in the SI (Fig. S4), and they exhibit almost similar spectral characteristics. Additionally, it may be mentioned here that no elements other than those expected are observed in the studied samples. The elemental compositions were determined by EDX analysis, and the details of elemental compositions are presented in Table 2. Furthermore, the EDX analysis indicates a consistent trend in the variation of Germanium (Ge) and Vanadium (V) across the series, which adheres to the stoichiometry relevant to the samples. Further, to support the grain structures, we conducted a TEM study for a representative sample (BWVGO3), and the grains and their connectivity could be visualized with more clarity (Fig. S5 of the SI). The crystalline natures of the grains were also confirmed by EDX patterns.

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3.3. Dielectric and ac conductivity studies

4. Conclusion

A new series of hexagonal perovskite derivative compounds with the formula of Ba₃WV_1−xGe_xO_8.5−x/2 (x = 0, 0.1, 0.2, and 0.3) have been successfully synthesized in single phase, and a complete analysis of their structure and dielectric properties was performed. The results show that the substitution of V with Ge in the Ba₃WVO_8.5 sample considerably increases the ionic conductivity. The dc conductivity was obtained from the analysis of the ac conductivity, and a conductivity as high as 1.80 × 10⁻⁵ S cm⁻¹ was observed for BWVGO3 (i.e., x = 0.3) at 973 K. At the same temperature, the BWVO sample showed a conductivity of 8.50 × 10⁻⁷ S cm⁻¹. In all the cases, the activation energies for ion conduction were found to be similar, while for each sample, it increases with the increase in temperature. Complex-plane impedance analysis also revealed that the grain conductivity systematically increases with the increase in Ge concentration. Shape parameters obtained by analysis of electrical modulus indicated that the distribution of relaxation times is greater with the increasing substitution of V by Ge, while the asymmetry in the relaxation time distribution about a mean remains similar for all the samples. From the values of β, it was concluded that the mechanism of ion migration in all Ba₃WV_1−xGe_x samples is similar, and they have similar energy barriers and hopping distances in their lattices. At high temperatures, an increase in β for x = 3 sample indicates a changeover of mechanism arising from the opening of alternate migration paths due to increased anion disorders and anion vacancies. The highest dc conductivity over the entire temperature range was observed in the sample with the highest Ge content. This has been attributed to the increasing oxygen ion vacancies and unit cell volume of Ba₃WVO_8.5 by substitution with Ge. This study suggests that the increasing anion deficiency and unit cell volume favor ion migration in the hexagonal perovskite derivatives and significantly increase the conductivity.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI.

The powder XRD data of Ba₃WVO_8.5 (BWVO) (Cu Kα X-ray) indexed according to the JCPDS data, comparison of the powder XRD data of all the studied composition, Rietveld refinement plots for BWVGO1 and BWVGO2 at 323 K and 1023 K, refined structural parameters of all the BWVGO samples at 323 and 1023 K, EDX spectra BWVGO1, BWVGO2 and BWVGO3 samples and TEM image and ED pattern for BWVGO3. See DOI: https://doi.org/10.1039/d5tc01785h

Acknowledgements

The authors thank Dr B. Viswanadh and Mr Sanjay Kumar, Materials Science Division, Bhabha Atomic Research Centre, Mumbai 400085, for their support in acquiring TEM and ED data for this study.

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