He Huanga,
Pengfei Jiang*ab,
Wenliang Gaoa,
Rihong Conga and
Tao Yang*a
aCollege of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 401331, P. R. China. E-mail: pengfeijiang@cqu.edu.cn; taoyang@cqu.edu.cn
bCollege of Physics, Chongqing University, Chongqing, 401331, P. R. China
First published on 8th January 2020
Oxygen-deficient perovskites are a family of important materials that may exhibit oxide ionic conductivities. We attempted to introduce oxygen-vacancy disordering in perovskite Ca4GaNbO8 (Ca4-type) by substituting Ca2+ with larger Sr2+. Sr2+-to-Ca2+ substitution did not lead to oxygen-vacancy ordering–disordering transition but an interesting Ca4-to-Sr4 type structure transition. Rietveld refinements revealed that the two-type structures exhibit similar oxygen-vacancy ordering and identical 1:1:1 triple-cation B-site ordering. Close inspection of the two-type structures revealed the subtle structure difference lies in the orientations of GaO4 tetrahedra, which is the origin of the formation of the narrow two-phase region (0.3 ≤ x < 0.65) in Ca4−xSrxGaNbO8. More importantly, the A- and B-site cavities with large differences in size for both structures resulted in a site-selective doping behaviour for Sr2+ in Ca4−xSrxGaNbO8. These structural changes found in Ca4−xSrxGaNbO8 will provide a broad route approaching new oxygen-deficient phases with oxide ionic conductivities.
Among the various perovskites, 1:1 double-cation B-site ordering perovskites usually exhibit physical properties different from the disordered ones distinctively. For example, B-site ordered perovskites Sr2FeMoO6 and Sr2CrOsO6 exhibit colossal magnetoresistance and high temperature ferrimagnetism, respectively.10,11 In perovskites, B-site rock-salt ordering is commonly observed, because such an arrangement manner of B-site cations is benefit for keeping local charge neutrality and release of structure strain. In contrast, the layered ordered and columnar ordered perovskites are relatively rare, and only a few specific phases have been reported up to now.12,13
Owing to the structural flexibility, perovskite can accommodate various defects including A-site and oxygen vacancies, which can result in ionic diffusion at high temperature. A-site ionic diffusion was observed indeed in Li3xLa2/3−xTiO3, in which the pre-existing A-site vacancies allow for easy Li+ diffusion.14–16 In contrast to A-site ionic conductivity, oxygen ionic conductivity or mixed ionic and electronic conductivities for oxygen deficient perovskites are much more widely investigated due to their potential applications in solid oxide fuel cells (SOFCs) as electrolyte or cathode. For example, Sr2+ and Mg2+ co-doped LaGaO3 (LSGM) is the one of the best oxygen ionic conducting electrolytes.17–19 Much efforts have been devoted to improve the oxygen ionic conductivities of various perovskites by increasing the number of oxygen vacancies. However, increasing the number of oxygen vacancies may result in completely ordering of oxygen vacancies, which in turn lead to a significant decrease of oxide ionic conductivity. For example, oxygen-vacancy ordered perovskite Ba2In2O5 exhibits poor ionic conductivity in low temperature range (<900 °C), though there exists a large number of oxygen vacancies in the structure.20–22 Oxygen-vacancy ordering/disordering in perovskite is closely related to the A-site cationic size (or tolerance factor, t). For example, with an increase of the Sr2+-content (increase of tolerance factor) in Ca2−xSrxFeCoO6−δ resulted in an oxygen-vacancy ordering to disordering transition.23 Therefore, the strategy of increasing the A-site cationic size of anion-ordered perovskites might be utilized to improve the oxide ionic conductivity.
Herein this contribution, our attention is turned to the newly discovered oxygen-deficient perovskite Ca4GaNbO8, where the complex 1:1:1 triple-cation B-site ordering is coupled with the oxygen-deficient ordering.24 We attempted to incorporate larger Sr2+ cations into Ca4GaNbO8 to bring in oxygen-vacancy disordering, so as to obtain new oxide ionic conductors. Substitution of Ca2+ with Sr2+ led to a Ca4-type to the Sr4-type structure transition as we expected. Unfortunately, Rietveld refinements manifested that the oxygen vacancies are also ordered in the Sr4-type structure, which was further confirmed by AC impedance spectroscopy measurements on selected compositions due to the absence of oxide ionic conductivity. The subtle structural differences between the Ca4-and Sr4-type structures lie in the orientations of GaO4 tetrahedra, which is the origin of the coexistence of two phases in a narrow region 0.3 ≤ x < 0.65. Moreover, a site-selective doping behaviour was observed for Sr2+ in Ca4−xSrxGaNbO8, however the A-site cationic ordering is only in short-range for all compositions.
Previous study on Ca4GaNbO8 revealed that this compound adopts a triple-cation B-site ordered perovskite-type structure and crystallizes in P21/c with lattice parameters a ≈ 11.18 Å, b ≈ 5.59 Å, c ≈ 14.07 Å, and β ≈ 121.55°.24 Our preliminary Le-bail fitting performed on Ca4−xSrxGaNbO8 manifested that the XRD data for compositions with x < 0.65 could be indexed by this monoclinic cell. However, a group of weak reflections for compositions with x ≥ 0.65, which is not ascribe to impurity phases, could not be fitted with this monoclinic cell any more, indicating Sr2+-doping induced a change of lattice parameters. Indexing the XRD data of Ca3SrGaNbO8 yield a monoclinic cell with lattice parameters a ≈ c1/2, b ≈ b1, c ≈ √3a1, β ≈ 97°, where a1, b1, and c1 represent the cell dimension of Ca4GaNbO8 (denoted as Ca4-type phase). Similar unit cell was also observed for Sr4AlNbO8 (P21/c), which also adopt a triple-cation B-site ordered perovskite-type structure with anionic vacancy ordering, suggesting Ca4−xSrxGaNbO8 (x ≥ 0.65) (denoted as Sr4-type phase) is isostructural to Sr4AlNbO8.27
Preliminary Rietveld refinements performed on Ca4−xSrxGaNbO8 (x = 0.5) using Ca4GaNbO8 as the initial structure model resulted in unreasonable structural parameters, i.e. too short interatomic distances. Close inspection of the XRD data revealed that some reflections from the Sr4-type phase were observed for compositions x = 0.3, 0.4 and 0.5. To clarify this clearly, the diffraction components from Cu Kα2 were striped for comparison. As shown Fig. 1b, small shoulders, corresponding to contribution of the Sr4-type phases, of some characteristic reflections were visually observed. Moreover, a representative Le-bail fitting pattern for x = 0.5 presented in Fig. S1† demonstrated that all the reflections could only be well reproduced by a two-phase model fitting. These results indicate that the compositions for x = 0.3, 0.4 and 0.5 comprise two phases. More importantly, single-phase for compositions x = 0.3, 0.4 and 0.5 was not attainable by neither calcination at elevate temperature nor prolongation of calcination time (Fig. S2†), suggesting the two phases are thermodynamically favorable. Therefore, the compositions for Ca4−xSrxGaNbO8 (x = 0–4) can be divided into three regions: (i) a single Ca4-type phase region (x < 0.3), (ii) a narrow two-phase region contains both Ca4-type and Sr4-type phases (0.3 ≤ x < 0.65), and (iii) a single Sr4-type phase region (x ≥ 0.65). We should note that the change of the relative content for the two phases within the two-phase region was slight (Fig. 1b), which is distinctly differ from commonly observed two-phase regions for perovskites e.g. the two-phase region observed in Ba3−xSrxZnSb2O9 (0.3 ≤ x ≤ 1.0).28 Such a difference is attribute to the subtle distinction in crystal structures between Ca4-type and Sr4-type structures, which will be discussed later.
The evolution of normalized lattice parameters against Sr2+-content in Ca4−xSrxGaNbO8 is presented in Fig. 2, where the linear expansion of the cell volume for single-phase compositions is in good agreement with the PXRD patterns. Interestingly, the lattice parameters for both Ca4-type and Sr4-type phases also showed a linear increase within the two-phase region, which opposite to commonly observed constant lattice parameters for the two phases in the two-phase region. This uncommon phenomenon further corroborated that the co-existence of two phases in Ca4−xSrxGaNbO8 is thermodynamically favorable.
Careful Rietveld refinements against laboratory XRD data were further performed on selected compositions Ca4−xSrxGaNbO8 (x = 0, 1, 1.5, 2, 2.5, 3, 3.5 and 4) to get an insight picture of the site occupancy preference for Sr2+. The refined structure parameters for Ca4GaNbO8 is in good agreement with that reported by Yang et al. using combined refinements against neutron and synchrotron data (Table S1†). For compositions with x ≥ 1.0, Sr4AlNbO8 was used as the starting structure model for Rietveld refinements. At first, an A-site ordered structure model was constructed for preliminary refinements, for instance, Sr2+ occupies A1-site exclusively and the remaining two A-sites were occupied by Ca2+ in Ca3SrGaNbO8. The refinement proceeded smoothly with this completely ordered model, resulting in reliable agreement factors (Rwp = 7.625%, Rp = 5.592%) and structural parameters. However, some peaks with large discrepancies between the observed and calculated were observed, suggesting the A-site cations are not completely ordering in Ca3SrGaNbO8. Consequently, the occupancy factor for Ca2+ and Sr2+ cations at B1-site and three A-sites were refined freely during the subsequent refinements. It turned out that A1-site was dominantly by Sr2+, and A2, A3, and B1 sites were mainly occupied by Ca2+. Despite the occupancies for Ca2+ and Sr2+ at four independent sites were refined freely, the refined composition Ca2.92Sr1.08GaNbO8 agrees well with the nominal formula Ca3SrGaNbO8. Moreover, the reliable factors were improved significantly to Rwp = 5.543% and Rp = 4.074%. These results manifest that the Ca2+ and Sr2+ are partially ordered in Ca3SrGaNbO8. Rietveld refinements performed on other compositions further demonstrated that all the compositions with mixed A-site cations were partially ordered. We should note that the inversion between A- and B-sites, which is commonly observed in spines,29 was not considered in Ca4−xSrxGaNbO8 because of the large differences in cationic size and coordination environment preference between (Ca, Sr) and (Ga, Nb). The final crystallographic parameters and selected bond lengths for Ca4−xSrxGaNbO8 are summarized in Table S1 and S2.† The Rietveld refinement patterns are presented in Fig. 4 and S3.†
Fig. 5 Comparison of crystal structures for Ca4GaNbO8 and Sr4GaNbO8. The open blue circles represent the oxygen-column defects. |
Given the same cationic ordering and oxygen deficient manner, the crystal structures for Ca4GaNbO8 and Sr4GaNbO8 seem identical at first glance. Close inspecting the structures revealed that the structural difference between Ca4GaNbO8 and Sr4GaNbO8 stems from the distinct orientations of GaO4 tetrahedra along-side of (Ca/Sr)O6 octahedra. As highlighted in Fig. 5, the GaO4 tetrahedra in Ca4GaNbO8 point to the same direction, however, the GaO4 tetrahedra in Sr4GaNbO8 point to opposite directions. Such a difference resulted in a doubled cell dimension along [111]p for Ca4GaNbO8 in comparison with Sr4GaNbO8 (see Fig. 5). The structural transformation between two-type structures requires the rearrangement of the orientations of GaO4 tetrahedra along [110]p, which is much more difficult than collective octahedra-tilting observed universally in cubic-type perovskites. Consequently, a narrow two-phase region is observed in Ca4−xSrxGaNbO8, and continuous structural transition is usually observed for simple cubic-type perovskites. We thus can speculate that this subtle structural difference in GaO4 orientations is the origin of the formation of the two-phase region in Ca4−xSrxGaNbO8.
The evolution of occupancy factors for Sr2+ cations at both A and B sites in Ca4−xSrxGaNbO8 is elucidated in Fig. 6a, where a site-selective doping behaviour is observed clearly. In detail, Sr2+ prefers to occupy the A1-site, which can be deduced from the sharp increase of occupancy factor to 0.926(6) for Sr2+ at A1-site when x ≤ 1.5, whereas the increase of occupancy factors at A2 and A3 sites are relatively slow. The occupancy factors for Sr2+ at A2 and A3 sites show a synchronized increase behaviour in the range of 1.5 ≤ x ≤ 3.0, where the Sr2+-occupancy at A1 site manifests a slight increase. Further incorporation of Sr2+ into Ca4−xSrxGaNbO8 lead to a sharp increase of occupancy factor for Sr2+ at B1 site when x ≥ 3.0, whereas a slight increase is observed for x ≤ 3.0. Such a sharp increase of occupancy of Sr2+ at B1-site lead to a significant deviation of the lattice parameters, especially for a, from the Vegard's law (see Fig. 2). Given the layered structure nature of Ca4−xSrxGaNbO8, when viewed along [110]p, the sharp increase of Sr2+-content in B1-site would unambiguously result in a sharp expansion along [111]p, namely a-axis of Sr4-type structure (see Fig. 5).
Fig. 6 (a) Plots of occupancy factors for Sr2+, (b) average 〈A–O〉 bond lengths, and (c) average 〈B–O〉 bond length along with Sr2+-content in Ca4−xSrxGaNbO8. |
As described above, Sr2+ showed a site-selective doping behaviour due to the distinctly large differences in size for A1, A2/A3, and B1 sites. The evolutions of the average 〈A–O〉 and 〈B–O〉 bond lengths are elucidated in Fig. 6b and c, where the change trends for both 〈A–O〉 and 〈B1–O〉 bonds are in good agreement with that of occupancy factors. The 〈Ga–O〉 and 〈Nb–O〉 bond lengths are almost kept in constant at ∼1.85 Å and ∼2.0 Å, respectively, for all compositions (Fig. 6c). The Ga–O bond lengths in all compounds are in the range of 1.76–1.90 Å, which are comparable with four-coordinated Ga3+ in LaAGa3O7 (A = Ca2+, Sr2+, Ba2+).31–33 Detailed inspection of the Nb–O bonds revealed that the Nb5+ exhibits a distorted coordination environment with three Nb–O atomic distances shorter than 2.0 Å and the remaining three bond lengths longer than 2.0 Å (Table S2†), indicating Nb5+ displaced from the centre of the octahedral cavity due to the second-order Jahn–Teller (SOJT) effect.34,35 Though both Ga3+ and Nb5+ cations can adopt four- and six-fold coordinations, incorporation of Sr2+ into A- and B-sites did not lead to anti-site disordering between Ga- and Nb-sites for all compositions, which should unambiguous attribute to their large differences in charge and cationic size.
It is well known that Raman scattering is sensitive to local structural changes, including cationic ordering, structure symmetry change, and John–Teller distortion.36 To gain an insight of structural change induced by Sr2+-doping, Raman spectra for Ca4GaNbO8 and Sr4GaNbO8 were measured. Raman spectra of Ca4GaNbO8 and Sr4GaNbO8 show similar features (Fig. S4†), which is consistent with their similar crystal structures. The intensive peaks with wavelength numbers higher than 700 cm−1, especially for the strongest peaks with frequency near 800 cm−1, are characteristic features for the complex perovskites with B-site ordering.30 Moreover, the broad band in the middle frequency range of 530–610 cm−1 is assigned to be the stretching vibration of Nb–O bonds due to the displacement of Nb5+ from the centre of NbO6 octahedra.37 All these observations from the Raman spectra are coherent with the crystal structures for Ca4GaNbO8 and Sr4GaNbO8 obtained by Rietveld refinements.
Fig. 7 (a) Typical ac impedance spectra for Sr4GaNbO8 at different temperatures. (b) Arrhenius plots of bulk conductivities for Ca4−xSrxGaNbO8 (x = 0, 2, and 4). |
Footnote |
† Electronic supplementary information (ESI) available: Rietveld refinement patterns, crystallographic parameters, selected interatomic distances for Ca4−xSrxGaNbO8 (x = 0, 1, 1.5, 2, 2.5, 3, 3.5, and 4). See DOI: 10.1039/c9ra09970k |
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