High-Temperature Structural Evolution in the Ba3Mo(1–x)WxNbO8.5

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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High-Temperature Structural Evolution in the Ba3Mo(1−x)WxNbO8.5 System and Correlation with Ionic Transport Properties Andrea Bernasconi,* Cristina Tealdi, and Lorenzo Malavasi Department of Chemistry, University of Pavia, Pavia, Italy S Supporting Information *

ABSTRACT: The evolution of the hybrid structure between 9R hexagonal perovskite and palmierite in the entire Ba3Mo(1−x)WxNbO8.5 solid solution (where x = 0, 0.25, 0.5, 0.75, and 1) was probed in the 100−900 K range by synchrotron high-resolution powder diffraction. Each sample exhibits a chemical-dependent structural model in the low-temperature regime (from 100 to 500 K) in which 9R and palmierite structures compete each other, the former being progressively favored as tungsten replaces molybdenum. Above 500 K, unit cell parameters and metal site occupancies start to converge toward a similar structural arrangement that is completely reached at 900 K. In fact, at this temperature, the entire solid solution discloses comparable unit cell and an almost enterely occupied M1 site, with a structure that is much closer to palmierite rather than 9R polytype. The present crystallographic results well explain the behavior of the material’s bulk ionic conductivity, whose temperature evolution for different compositions depends from the contribution of tetrahedral units proper of the palmierite structure.

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INTRODUCTION Ba3MoNbO8.5 has been recently pointed out as an oxide ion conductor,1 showing a bulk conductivity that is directly comparable with other common oxide ion conductors.2−6 Ba3MoNbO8.5 has a structure that is a hybrid between the 9R hexagonal perovskite and the palmierite structure.7,8 The structure has been described by R3̅m space group with a unit cell where a and c axes are equal to 5.92744 (3) Å and 21.0995 (2) Å, respectively. Barium atoms fully occupy the dedicated 3a and 6c Wyckoff sites in both 9R polytype and palmierite structures (Ba1 and Ba2 sites). The pentavalent (i.e., Nb) and the hexavalent (i.e., Mo) cations are distributed over a 3b and a 6c Wyckoff sites. The former, known as M2 site, is exclusive of the 9R polytype; it is weakly occupied, and it is coordinated by six oxygens. The latter, known as M1 site, is present in both 9R polytype and palmierite structures; it is mostly occupied, and it is coordinated by six oxygen atoms in the case of 9R polytype, while it is coordinated by four oxygen atoms in the case of palmierite. The presence of these two different coordination polyhedra (e.g., octahedra and tetrahedra) is related to the presence of oxygens that partially occupy a 9e Wyckoff position (this feature is exclusive for 9R polytype, known as O2 site) and to the presence of oxygens that partially occupy a 36i Wyckoff position (this feature is exclusive for palmierite, known as O3 site). Both structures include an additional 18h Wyckoff position that is entirely occupied by oxygen atoms (known as O1 site). Figure 1 displays the described 9R polytype and palmierite structures. Very recently, the possibility to partially and fully replace molydbenum with tungsten was demonstrated by our group.9 In fact, single-phase samples were synthesized in the entire Ba3Mo(1−x)WxNbO8.5 solid solution (x = 0, 0.25, 0.5, 0.75, 1). A significative difference, as molybdenum is replaced by tungsten, © XXXX American Chemical Society

Figure 1. Ba3MoNbO8.5 crystal structure representation in terms of 9R polytype (left) and palmierite (right). Dark green, light green, blue, cyan, red, orange, and yellow represent the Ba1, Ba2, M1, M2, O1, O2, and O3 sites, respectively.

relies in the larger contribution of 9R polytype, as demonstrated by fewer number of metal-centered tetrahedra per unit cell, as observed from Rietveld refinement of neutron diffraction data. Another very recent study on the Ba3WNbO8.5 composition has confirmed this evidence.10 As far as the hightemperature behavior of the material, only Ba3MoNbO8.5 composition has been characterized by neutron diffraction,11 showing a structural rearrangement above 300 °C that consists in larger metal-centered tetrahedra fractions, leading to a structure that has a bigger contribution of palmierite at the expense of the one of 9R polytype. Above 500 °C this structural rearrangement is completed and has been correlated by the Received: April 20, 2018

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DOI: 10.1021/acs.inorgchem.8b01093 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Comparison of multiple peaks measured at ID22 High-Resolution beamline at room temperature. For each hkl reflection the corresponding Miller index is displayed in the top part of each graph. the latter, O2 site occupancy was refined first, and, after that, O3 site occupancy was consequently fixed according to stoichiometry. This approach is supported by our previous findings by neutron diffraction showing the same content of oxygen per unit cell in the entire solid solution. In the case of M1 and M2 sites, niobioum, molybdenum, and tungsten were randomly distributed. This careful refinement strategy was applied to achieve consistent structural models of the complex hybrid structure at the expense of some graphical goodness of the fit that could drive the structural model to unphysical results. Electrochemical impedance spectroscopy (EIS) measurements were performed using a Solartron 1260 + 1287 system in the frequency range from 0.1 Hz to 1 MHz and applying an alternated voltage of 30 mV. Measurements were performed under static air upon cooling from 800 °C with a fixed scan rate of 3 °C/min on disc-shaped Pt-sputtered pellets. Data were treated using the Zview software. In particular, a separation between the bulk and grain boundary contributions of the conductivity was obtained by fitting the data to an equivalent circuit model shown in Figure 1 in the Supporting Information as well as an example of curve fitting (Figure 2).

authors to the ionic conductivity, addressing to these metalcentered tetrahedra a more favorable path of charge transport. Currently, temperature-dependent studies related to partial or entire molybdenum replacement with tungsten are not present in the scientific literature concerning this interesting family of materials. To fill this gap, the Ba3Mo(1−x)WxNbO8.5 solid solution (x = 0, 0.25, 0.5, 0.75, 1) was investigated by synchrotron high-resolution powder diffraction (SHRPD) from 100 to 900 K, with the aim of a deeper comprehension of the structural modifications that occur in this temperature range, which pertains the material’s application as solid electrolyte in the field of solid electrolyte. This solid solution is highly promising, being the first hexagonal perovskite with relevant transport properties at reasonable low temperature.

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EXPERIMENTAL SECTION

Five samples of the Ba3Mo(1−x)WxNbO8.5 solid solution (x = 0, 0.25, 0.5, 0.75, and 1) were synthesized by solid-state reaction. In particular, for each composition, after stoichiometric mixing of BaCO3 (Aldrich, 99.98%), MoO3 (Aldrich, 99.97%), Nb2O5 (Aldrich, 99.99%), and WO3 (Aldrich, 99.99%) reagents, a pellet was prepared by isostatic press and heated at 900 °C for 10 h. Afterward, pellets were prepared again in the same manner and heated at 1250 °C for 10 h. This procedure was performed three times to achieve pure single-phase samples. Purity check was performed by means of a laboratory X-ray diffractometer (D8 instrument from Bruker). The samples were measured at the ID22 High-Resolution Powder Diffraction beamline at ESRF (Grenoble). For each composition of the solid solution, powders were loaded into 0.4 mm quartz capillary and measured using a wavelength of 0.354 349 Å with Debye−Scherrer geometry. The diffracted signal was detected by a system of multianalyzer crystals, ensuring an optimal peak width resolution.12,13 For each sample, the diffraction signal was measured in the 0−24° 2θ range from 100 to 900 K, with steps every 100°. Rietveld refinements were then performed using the GSAS program.14 For each pattern, scale factor, instrument background (a sixth degree Chebychev polynomial was used), goniometric shift, unit cell parameters, pseudo-Voigt profile function parameters, atomic coordinates, and isotropic thermal displacement parameters were refined. Careful refinement criteria were followed for crystallographic sites occupancies related to (i) niobium, molybdenum, and tungsten and (ii) oxygen. For the former, M1 site occupancy was refined first, and, after that, M2 site occupancy was consequently fixed according to stoichiometry. For

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RESULTS AND DISCUSSION The shortening of the unit cell as tungsten replaces molybdenum we observed in our previous work9 is confirmed by the progressive shift toward higher 2θ values of all the diffracted peaks shown in Figure 2. Moreover, in Figure 2, the broadening of the diffraction pattern as tungsten replaces molybdenum is also confirmed. This effect was positively handled by Lorentzian microstrain parameter (LY in GSAS software14) and converted to a percentage of isotropic microstrain by using Equation: π × LY S(%) = 100 × (1) 18 000 whose results at 300 K are displayed for the entire compositional range in Figure 3. As it can be noticed, the introduction of tungsten in the structure at the expense of molybdenum generates strain. No evidence of peak splitting was observed, even with the excellent resolution of the instrument, as displayed in Figure 2. This result points out how, for all five samples, the R3̅m space group foreseen by literature is a good model to describe the average structural evolution over composition and temperature. B

DOI: 10.1021/acs.inorgchem.8b01093 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Isotropic strain as calculated from eq 1 as a function of tungsten content at 300 K. Dotted line represents the linear fit of the data.

For the entire solid solution, a good Rietveld fit was achieved in the whole temperature range, whose results are summarized in the Supporting Information. Some examples of the achieved fit are displayed in Figure 4. All five samples present a linear thermal expansion in the 100−500 K as well as in the 600−900 K temperature ranges, with the latter range characterized by a steeper behavior, especially for a and b lattice parameters. This effect becomes more marked as tungsten replaces molybdenum, as displayed in the top part of Figure 5. Clearly, from a metrical point of view, a structural rearrangement is occurring with the largest effect in the ab plane. Moreover, at 900 K, one can observe that the volume of all the samples is very similar, if compared, for example, to differences observed at room temperature. This effect suggests a sort of reconciliation of the hybrid structure in the high-temperature regime despite the different sizes of the two hexavalent cations (i.e., Mo and W) and the significatively different structures observed at room temperature. A corroboration of this structural rearrangement is also clear by looking at the temperature evolution of metals and oxygen site partial occupancies. In fact, even if each sample exhibits its own configuration in terms of 9R polytype and palmierite contributions, these values slightly change in the 100−500 K range compared to sizable variations above 500 K, as can be noticed from Rietveld results in the Supporting Information. In detail, for all samples, M1 and O3 site occupancies increase with temperature, while M2 and O2 site occupancies reduce in a direction compatible with a rearrangement of the structure toward the palmierite structure. To properly describe the different levels of contribution of 9R polytype and palmierite structures over temperature, we quantified the number of M2centered octahedra (OctM2), the number of M1-centered octahedra (OctM1), and the number of M1-centered tetrahedra (TetM1) by using the following Equations: OctM2 = 3 × Occ(M2)

Figure 4. (top) Rietveld refinements of Ba3MoNbO8.5 sample at 900 K. (middle) Rietveld refinements of Ba3Mo0.5W0.5NbO8.5 sample at 900 K. (bottom) Rietveld refinements of Ba3WNbO8.5 sample at 900 K. Black lines represent the experimental data, red lines represent the fit, blue lines represent the difference between experiemental data and the fit, and green bars represent the positions of the different hkl planes as from the refined model.

OctM1 =

6 × 9 × Occ(O2) 9

(3)

where Occ(O2) is the occupancy of O2 site, 9 represents its multiplicity, and 6 corresponds to the ratio between the 9

maximum possible number of M1-centered octahedra and the maximum possible number of O2 oxygens in the 9R polytype.

(2)

where Occ(M2) is the total occupancy of M2 site, and 3 represents its multiplicity.

TetM1 = 6 × Occ(M1) − OM1 C

(4) DOI: 10.1021/acs.inorgchem.8b01093 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. (top) Unit cell parameters Rietveld refinements results. (middle) Unit cell volume and M2 site metal-centered octahedra, as calculated from eq 2. (bottom) M1 site metal-centered octahedra and tetrahedra, as calculated from eqs 3 and 4, respectively. Lines were added as guides.

873 K, respectively. Such values are in good agreement with our values of 49.0% and 61.6% at 500 and 900 K, respectively, for the same composition. This suggests that our SHRPD data coupled with the adopted refined strategy are reliable. Again, a careful analysis of the multiple metal−metal refined distances provided further details of this structural rearrangement. In all the samples, the distance along c axis between two M1-centered polyhedra separated by the M2-centered octahedron does not expand from 100 to 900 K, but, unexpectedly, it becomes shorter, favored by the progressive decrease of M2centered octahedra abundancy, as displayed in the left side of Figure 6. Accordingly, along the unit cell diagonal, M1-centered

where Occ(M1) is the total occupancy of M1 site, and 6 represents its multiplicity. OctM2, OctM1, and TetM1 values were reported in Figure 5. For all samples, the number of OctM2 approaches zero at 900 K, while the number of OctM1 reduces up to values ranging from 2.5 to 2.25. Consistently, the number of TetM1 increases up to the value of 3.5, independently by tungsten content, with a very large variation, in all cases, for the W-rich samples. The values reported in the present work are in well agreement with tetrahedra percentages shown by Fop et al. in their Ba3MoNbO8.5 neutron diffraction study.11 In fact, the authors found a percentage of tetrahedra of 49.9 and 64.8 at 573 and D

DOI: 10.1021/acs.inorgchem.8b01093 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. (left) M1−M1 site distance evolution along c axis. These values regard M1-centered polyhedra separated by M2-centered octahedron. (right) M1−M1 site distances along the unit cell diagonal. Lines were added as guides.

polyhedra distances become closer (right side of Figure 6). The presence of these M2-centered octahedra, that share their edges with M1-centered polyhedra, seems to be at the basis of the largest distances between M1 sites along c axis, being a clear trend present as tungsten content progressively increases. This is an important evidence that tungsten, at least in the 100−500 K temperature range, is more prone to stay in M2 site rather than in M1 site, due to its larger size compared to molybdenum. For higher temperatures, favored by the larger available motion energy, Nb, Mo, and W appear randomly distributed over M1-centered octahedra and tetrahedra, while M2 sites become progressively empty. These larger M1−M1 site distances along c axis for samples with higher tungsten content might seem counterintuitive, in view of the reduced size of c axis described in Figure 5. An explanation of that can be found in terms of different contributions from O2 and O3 species to the network between polyhedra. In fact, while O2 species are shared by multiple polyhedra, this is not the case of O3 species, as they define the upper and lower apex of tetrahedra, without being connected to any other unit (see Figure 1). This implies a general shrinkage of the 9R polytype unit cell, which is not present in the case of palmierite. To correlate these interesting structural results to the ionic transport properties of the material, an investigation of the ionic conductivity of the Ba3MoNbO8.5 and Ba3WNbO8.5 samples was performed as a function of temperature. Figure 7 shows a comparison between the total conductivity of Ba3MoNbO8.5 and Ba3WNbO8.5 samples. The Arrhenius plot of total conductivity shows, in agreement with our9 and previous reports,1,10 that there is no significant deviation from the linear trend, and it suggests a slight reduction of total conductivity for the W-rich sample compared to the Ba3MoNbO8.5 compound. In the inset of Figure 7, the comparison between the Arrhenius plot of bulk conductivity for the Ba 3 MoNbO 8.5 and Ba3WNbO8.5 samples is reported for the low-temperature range (up to 460 °C). The separation of the bulk and grain boundary contributions is not straightforward in the whole temperature range considered as, for example, in the hightemperature regime only the mass diffusion contribution at the

Figure 7. Total conductivities of Ba3MoNbO8.5 and Ba3WNbO8.5 samples up to 800 °C. (inset) Bulk conductivities of Ba3MoNbO8.5 and Ba3WNbO8.5 samples are displayed up to 460 °C.

electrode and a small portion of the grain boundary semicircle are visible in the Nyquist plots. A considerable difference in bulk conductivity is visible between the two compositions, with the low-temperature conductivity of the W-rich phase being ∼1 order of magnitude lower. The bulk conductivities approach the same value at higher temperature, due to the different activation energies pertaining to the Mo-and W-rich phases. In particular, the calculated activation energy for bulk conductivity is 1.8 eV for the Ba3WNbO8.5 sample and is roughly half of that value for the Ba3MoNbO8.5 compound. The trend in activation energy for bulk conductivity with increasing W content is in agreement with previous reports, where activation barrier values of 1.94 eV for Ba3WNbO8.510 and 1.21 for Ba3MoNbO8.51,15 were calculated. Note that the absolute values are highly affected by the temperature range considered, as changes in the slope of the Arrhenius plots of bulk conductivity for the Ba3MoNbO8.51 E

DOI: 10.1021/acs.inorgchem.8b01093 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and Ba3WNbO8.5 compounds10 are reported as a function of temperature and considered to appear at different temperatures, dependending on composition. The observed changes in bulk conductivities as a function of composition and temperature can be nicely correlated to the structural features observed in this study. In particular, in the low-temperature region (i.e., up to 700 K), Ba3MoNbO8.5 displays better bulk conductivity than the Ba3WNbO8.5 sample, being the former characterized by a much larger number of TetM1 units (see lower right side of Figure 5) and a higher occupation of the O3 crystallographic site, which were described to be at the origin of the highly conductive behavior of the material.11 The bulk conductivity of Ba3WNbO8.5 increases with a much steeper slope than the one observed for the Ba3MoNbO8.5 sample, in line with the fact that the structural rearrangement taking place along with temperature in the range 650−750 K is more prominent in the former than in the latter (see Figure 5). Extrapolation at the highest investigated temperature (i.e., 900 K) suggests that both samples are basically displaying very similar bulk properties, the number of TetM1 being very close in the two samples. These impedance spectroscopy evidences are also in agreement with McCombie and coauthors,10 whose results can be nicely explained by the proposed SHRPD structural investigation. The fact that the total conductivities of the samples show, instead, similar behavior supports the fact that the transport properties of these oxide ion conductors are governed by microstructural features rather than the subtle structural changes observed and rationalized as a function of composition and temperature. Therefore, in view of the actual application of these compounds within a solid oxide fuel cell (SOFC) device, a careful optimization of the sintering conditions and of the samples’ morphology is of paramount importance to fully exploit the high bulk conductivity at low temperature, on the one end. On the other end, the structural features observed in this study, and their correlation to the bulk transport properties of the compounds, provide a very clear framework for the comprehension of the structural−transport relations in this family of ionic conductors, which can guide the design of novel compositions with optimized low-temperature transport properties in this system. As a final remark, we want to mention that a further comprehension of the revealed structural evolution of the entire solid solution demands a short-range investigation, like total scattering approach, to fully point out the local environment of the material. A traceable evidence of that is provided by the peak asymmetry that can be noticed in all samples (see Figure 2), and that is in our opinion a proof of the presence of some kind of defect that goes beyond the average structure described in the present paper.

solution. In all samples, the structural rearrangement observed from 500 K has been well-described by the descrease of M1and M2-centered octahedra (OctM1 and OctM2, respectively), in comparison to the larger abundancy of M1-centered tetrahedra (TetM1), which in turn implies an evolution toward a greater contribution of palmierite structure. Together with these results, the metric of the unit cell converges toward similar sizes, at 900 K, in spite of the different compositions. This metric behavior of the entire solid solutions goes in parallel with the metal site occupancies trend, in which M2 sites are fully empty and Nb, Mo, and W are randomly distributed over M1-centered octahedra (typical of 9R structure) and over M1centered tetrahedra (typical of palmierite structure). All these results are responsible of tangible effects on the transport properties of the entire solid solution: in particular, this progressively larger contribution of the palmierite structure generates low-energy transition path to transport O2− ions across O3 crystallographic sites rather than across O2 sites, as corroborated by our impedance spectroscopy evidence and by literature findings.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01093. Rietveld refinement results, impedance spectroscopy circuit scheme, impedance spectroscopy data fit for Ba3MoNbO8.5 sample at 464 °C (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Andrea Bernasconi: 0000-0002-9667-6798 Cristina Tealdi: 0000-0003-1700-1723 Lorenzo Malavasi: 0000-0003-4724-2376 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The European Synchrotron Radiation Facility (ESRF) is thanked for beamtime provision (experiment CH5150). REFERENCES

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CONCLUSIONS The average structure of the entire Ba3Mo(1−x)WxNbO8.5 solid solution has been investigated by Rietveld refinement of SHRPD data. According with literature, the hybrid structural model is one in which 9R polytype and palmierite structures compete with each other, both crystallizing in the R3̅m space group. The presented results have, first of all, clarified some aspects related to the room-temperature structure of the five samples, correlating to some strain effect and to a larger contribution of 9R polytype the progressive peak broadening and unit cell reduction, respectively, as tungsten is introduced in the solid F

DOI: 10.1021/acs.inorgchem.8b01093 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (9) Bernasconi, A.; Tealdi, C.; Muhlbauer, M.; Malavasi, L. J. Solid State Chem. 2018, 258, 628−633. (10) McCombie, K. S.; Wildman, E. J.; Fop, S.; Smith, R. I.; Skakle, J. M. S.; Mclaughlin, A. C. J. Mater. Chem. A 2018, 6, 5290. (11) Fop, S.; Wildman, E. J.; Irvine, J. T. S.; Connor, P. A.; Skakle, J. M. S.; Ritter, C.; Mclaughlin, A. C. Chem. Mater. 2017, 29, 4146− 4152. (12) Hodeau, J. L.; Bordet, P.; Anne, M.; Prat, A.; Fitch, A. N.; Dooryhee, E.; Vaughan, G.; Freund, A. K. Proc. SPIE 1998, 3448, 353−361. (13) Fitch, A. N. J. Res. Natl. Inst. Stand. Technol. 2004, 109, 133− 142. (14) Larson, A. C.; Dreele, R. B. V. General Structure Analysis System; Report LAUR; Los Alamos National Laboratory: New Mexico, 2004. (15) Fop, S.; Wildman, E. J.; Skakle, J. M. S.; Ritter, C.; Mclaughlin, A. C. Inorg. Chem. 2017, 56, 10505−10512.

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DOI: 10.1021/acs.inorgchem.8b01093 Inorg. Chem. XXXX, XXX, XXX−XXX