Site-selective doping effect, phase separation, and structure evolution in 1:1:1 triple-cation B-site ordered perovskites Ca4−xSrxGaNbO8

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.


Introduction
Perovskites with the general formula ABO 3 can accommodate cations with a wide range of ionic radii and charge, thus giving rise to rich chemical compositions accompanied with diverse physical properties such as magnetism, 1,2 superconductivity, 3,4 dielectricity, 5,6 ferroelectric. [7][8][9] Such diversity in chemical compositions as well as physical properties for perovskites have motivated an intense and enduring interesting for chemists to design new materials and modify the physical properties of a specic phase by chemical substitution.
Among the various perovskites, 1:1 double-cation B-site ordering perovskites usually exhibit physical properties different from the disordered ones distinctively. For example, Bsite ordered perovskites Sr 2 FeMoO 6 and Sr 2 CrOsO 6 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 benet 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 specic phases have been reported up to now. 12,13 Owing to the structural exibility, 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 Li 3x La 2/3Àx TiO 3 , in which the pre-existing A-site vacancies allow for easy Li + diffusion. [14][15][16] In contrast to A-site ionic conductivity, oxygen ionic conductivity or mixed ionic and electronic conductivities for oxygen decient perovskites are much more widely investigated due to their potential applications in solid oxide fuel cells (SOFCs) as electrolyte or cathode. For example, Sr 2+ and Mg 2+ co-doped LaGaO 3 (LSGM) is the one of the best oxygen ionic conducting electrolytes. [17][18][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 signicant decrease of oxide ionic conductivity. For example, oxygen-vacancy ordered perovskite Ba 2 In 2 O 5 exhibits poor ionic conductivity in low temperature range (<900 C), though there exists a large number of oxygen vacancies in the structure. [20][21][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 Sr 2+ -content (increase of tolerance factor) in Ca 2Àx Sr x FeCoO 6Àd 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-decient perovskite Ca 4 GaNbO 8 , where the complex 1:1:1 triple-cation B-site ordering is coupled with the oxygen-decient ordering. 24 We attempted to incorporate larger Sr 2+ cations into Ca 4 GaNbO 8 to bring in oxygen-vacancy disordering, so as to obtain new oxide ionic conductors. Substitution of Ca 2+ with Sr 2+ led to a Ca 4 -type to the Sr 4 -type structure transition as we expected. Unfortunately, Rietveld renements manifested that the oxygen vacancies are also ordered in the Sr 4type structure, which was further conrmed by AC impedance spectroscopy measurements on selected compositions due to the absence of oxide ionic conductivity. The subtle structural differences between the Ca 4 -and Sr 4 -type structures lie in the orientations of GaO 4 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 Sr 2+ in Ca 4Àx Sr x GaNbO 8 , however the A-site cationic ordering is only in short-range for all compositions.

Synthesis
The polycrystalline samples Ca 4Àx Sr x GaNbO 8 (x ¼ 0-4) were prepared by high temperature solid-state reactions using calcium carbonate (CaCO 3 , 99.99%), strontium carbonate (SrCO 3 , 99.99%), gallium oxide (Ga 2 O 3 , 99.99%), niobium oxide (Nb 2 O 5 , 99.99%) as raw materials. The starting materials were preheated at 500 C for 10 h to remove the absorbed moisture. Stoichiometric starting materials with a total weight of 1.2 g were weighted and mixed in an agate mortar by hands and then preheated at 950 C to decompose the carbonates. Aer this calcination, the resultant white powders were further grinded and pressed into pellet with 13 mm diameter and then calcinated between 1200 C and 1300 C for 60 hours with intermediate grinding and pressing.

X-ray diffraction
The phase purity of the samples was investigated by powder Xray diffraction (PXRD). PXRD were performed on the PANalytical Empyrean diffractometer equipped with a PXIcel 1D detector (Cu Ka radiation). The operation voltage and current were 40 kV and 40 mA, respectively. The PXRD data for phase identication were collected with the setting of 30 s/0.0262 . High quality PXRD data, which were used for Rietveld renements with the TOPAS-Academic V6 soware, 25 were collected with a setting of 200 s/0.0131 .

AC impedance spectroscopy
AC impedance spectroscopy measurements for Ca 4Àx Sr x -GaNbO 8 (x ¼ 0, 2, and 4) were carried out over temperature range from room temperature to 800 C using a Solartron 1260A impedance phase analyzer with the frequency from 10 À1 to 10 7 Hz. Before the measurements, the pellets were coated with platinum pasts and then heated at 800 C for 1 h to form electrodes.

Raman spectroscopy
The Raman spectroscopy measurements were performed on the LabRAM HR Evolution spectrometer. The room temperature Raman spectra for Ca 4 GaNbO 8 and Sr 4 GaNbO 8 were measured over the range of 100-4000 cm À1 using the 325 nm line and with the light focus on the sample trough an optical lens. The spectral resolution was about 1-2 cm À1 .

Results and discussion
Phase identication Fig. 1, where the diffraction peaks continuous evolves to lower angles upon increasing Sr 2+ -content. This observation indicates the cell volume expansion of Ca 4Àx Sr x GaNbO 8 upon substituting Ca 2+ with Sr 2+ , which is consistent with the larger cationic radii of Sr 2+ (1.44 A in 12-fold coordination) in comparison with Ca 2+ (1.34 A in 12-fold coordination). 26 Previous study on Ca 4 GaNbO 8 revealed that this compound adopts a triple-cation B-site ordered perovskite-type structure and crystallizes in P2 1 /c with lattice parameters a z 11.18 A, b z 5.59 A, c z 14.07 A, and b z 121.55 . 24 Our preliminary Lebail tting performed on Ca 4Àx Sr x GaNbO 8 manifested that the XRD data for compositions with x < 0.65 could be indexed by this monoclinic cell. However, a group of weak reections for compositions with x $ 0.65, which is not ascribe to impurity phases, could not be tted with this monoclinic cell any more, indicating Sr 2+ -doping induced a change of lattice parameters. Indexing the XRD data of Ca 3 SrGaNbO 8 yield a monoclinic cell with lattice parameters a z c 1 /2, b z b 1 , c z O3a 1 , b z 97 , where a 1 , b 1 , and c 1 represent the cell dimension of Ca 4 GaNbO 8 (denoted as Ca 4 -type phase). Similar unit cell was also observed for Sr 4 AlNbO 8 (P2 1 /c), which also adopt a triple-cation B-site ordered perovskite-type structure with anionic vacancy ordering, suggesting Ca 4Àx Sr x GaNbO 8 (x $ 0.65) (denoted as Sr 4 -type phase) is isostructural to Sr 4 AlNbO 8 . 27 Preliminary Rietveld renements performed on Ca 4Àx Sr x -GaNbO 8 (x ¼ 0.5) using Ca 4 GaNbO 8 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 reections from the Sr 4 -type phase were observed for compositions x ¼ 0.3, 0.4 and 0.5. To clarify this clearly, the diffraction components from Cu Ka 2 were striped for comparison. As shown Fig. 1b, small shoulders, corresponding to contribution of the Sr 4 -type phases, of some characteristic reections were visually observed. Moreover, a representative Le-bail tting pattern for x ¼ 0.5 presented in Fig. S1 † demonstrated that all the reections could only be well reproduced by a two-phase model tting. These results indicate that the compositions for x ¼ 0.3, 0.4 and 0.5 comprise two phases. More importantly, single-phase for compositions x ¼ 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 Ca 4Àx Sr x GaNbO 8 (x ¼ 0-4) can be divided into three regions: (i) a single Ca 4 -type phase region (x < 0.3), (ii) a narrow two-phase region contains both Ca 4 -type and Sr 4 -type phases (0.3 # x < 0.65), and (iii) a single Sr 4 -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 Ba 3Àx Sr x ZnSb 2 O 9 (0.3 # x # 1.0). 28 Such a difference is attribute to the subtle distinction in crystal structures between Ca 4 -type and Sr 4 -type structures, which will be discussed later.
The evolution of normalized lattice parameters against Sr 2+content in Ca 4Àx Sr x GaNbO 8 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 Ca 4 -type and Sr 4 -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 coexistence of two phases in Ca 4Àx Sr x GaNbO 8 is thermodynamically favorable.

Rietveld renements
As described above, both Ca 4 -type and Sr 4 -type phases adopt oxygen-decient perovskite-type structure with 1:1:1 triplecation B-site ordering. Owing to their low structure symmetry (P2 1 /c), both structures exhibit three distinctly different A-sites (denoted as A1, A2, and A3, respectively), which could be readily discerned through their surrounding environments. As shown in Fig. 3, A1 cation is surrounded by three B1O 6 octahedra (B1 represents the B-site occupied by alkali earth cation),  four NbO 6 octahedra, and one GaO 4 tetrahedron; A2 is surrounded by three B1O 6 octahedra, one NbO 6 octahedron, and four GaO 4 tetrahedra; A3 is surrounded by two B1O 6 octahedra, three NbO 6 octahedra, and three GaO 4 tetrahedra. Such difference in surrounding environments for A-site cations results in the A1-site is much larger than A2-and A3-site, suggesting Sr 2+ is prone to occupy the A1-site at rst.
Careful Rietveld renements against laboratory XRD data were further performed on selected compositions Ca 4Àx Sr x GaNbO 8 (x ¼ 0, 1, 1.5, 2, 2.5, 3, 3.5 and 4) to get an insight picture of the site occupancy preference for Sr 2+ . The rened structure parameters for Ca 4 GaNbO 8 is in good agreement with that reported by Yang et al. using combined renements against neutron and synchrotron data (Table S1 †). For compositions with x $ 1.0, Sr 4 AlNbO 8 was used as the starting structure model for Rietveld renements. At rst, an A-site ordered structure model was constructed for preliminary renements, for instance, Sr 2+ occupies A1-site exclusively and the remaining two A-sites were occupied by Ca 2+ in Ca 3 SrGaNbO 8 . The renement proceeded smoothly with this completely ordered model, resulting in reliable agreement factors (R wp ¼ 7.625%, R p ¼ 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 Ca 3 SrGaNbO 8 . Consequently, the occupancy factor for Ca 2+ and Sr 2+ cations at B1-site and three A-sites were rened freely during the subsequent renements. It turned out that A1-site was dominantly by Sr 2+ , and A2, A3, and B1 sites were mainly occupied by Ca 2+ . Despite the occupancies for Ca 2+ and Sr 2+ at four independent sites were rened freely, the rened composition Ca 2.92 Sr 1.08 GaNbO 8 agrees well with the nominal formula Ca 3 SrGaNbO 8 . Moreover, the reliable factors were improved signicantly to R wp ¼ 5.543% and R p ¼ 4.074%. These results manifest that the Ca 2+ and Sr 2+ are partially ordered in Ca 3 SrGaNbO 8 . Rietveld renements performed on other compositions further demonstrated that all the compositions with mixed A-site cations were partially ordered. We Fig. 4 Rietveld refinement plots of XRD data for Ca 4Àx Sr x GaNbO 8 ( x ¼ 0, 1, 2, 3, 4).
should note that the inversion between A-and B-sites, which is commonly observed in spines, 29 was not considered in Ca 4Àx Sr x -GaNbO 8 because of the large differences in cationic size and coordination environment preference between (Ca, Sr) and (Ga, Nb). The nal crystallographic parameters and selected bond lengths for Ca 4Àx Sr x GaNbO 8 are summarized in Table S1 and S2. † The Rietveld renement patterns are presented in Fig. 4 and S3. † Structure evolution of Ca 4Àx Sr x GaNbO 8 The rened crystal structures for Ca 4 GaNbO 8 and Sr 4 GaNbO 8 are presented in Fig. 5, where both structures view along the same cubic-perovskite direction [110] p . Ca 4 GaNbO 8 and Sr 4 -GaNbO 8 share the similar structure features with 1:2 layerordered hexagonal perovskites i.e. Ca 4 Nb 2 O 9 , however the structure for Ca 4 GaNbO 8 and Sr 4 GaNbO 8 are more complex because there is an another B-site cation (Ga 3+ ) and ordered anionic deciency. 30 As shown in Fig. 5, the removal of oxygen within the AO 2 layers creates tetrahedral cavities that could only accommodate smaller Ga 3+ cations, which resulted in layered stacking sequence of (Ca/Sr)-(Ga 1/2 Nb 1/2 )-(Ga 1/2 Nb 1/2 )-(Ca/Sr) for B-site cations along the closed-packing direction [111] p . Owing to the ordered removal of every other oxygen-only O 2 columns within the AO 2 layers, Ga 3+ and Nb 5+ within (Ga 1/2 Nb 1/ revealed that the structural difference between Ca 4 GaNbO 8 and Sr 4 GaNbO 8 stems from the distinct orientations of GaO 4 tetrahedra along-side of (Ca/Sr)O 6 octahedra. As highlighted in Fig. 5, the GaO 4 tetrahedra in Ca 4 GaNbO 8 point to the same direction, however, the GaO 4 tetrahedra in Sr 4 GaNbO 8 point to opposite directions. Such a difference resulted in a doubled cell dimension along [111] p for Ca 4 GaNbO 8 in comparison with Sr 4 GaNbO 8 (see Fig. 5). The structural transformation between two-type structures requires the rearrangement of the orientations of GaO 4 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 Ca 4Àx -Sr x GaNbO 8 , and continuous structural transition is usually observed for simple cubic-type perovskites. We thus can speculate that this subtle structural difference in GaO 4 orientations is the origin of the formation of the two-phase region in Ca 4Àx Sr x GaNbO 8 .
The evolution of occupancy factors for Sr 2+ cations at both A and B sites in Ca 4Àx Sr x GaNbO 8 is elucidated in Fig. 6a, where a site-selective doping behaviour is observed clearly. In detail, Sr 2+ prefers to occupy the A1-site, which can be deduced from the sharp increase of occupancy factor to 0.926 (6) for Sr 2+ at A1site when x # 1.5, whereas the increase of occupancy factors at A2 and A3 sites are relatively slow. The occupancy factors for Sr 2+ at A2 and A3 sites show a synchronized increase behaviour in the range of 1.5 # x # 3.0, where the Sr 2+ -occupancy at A1 site manifests a slight increase. Further incorporation of Sr 2+ into Ca 4Àx Sr x GaNbO 8 lead to a sharp increase of occupancy factor for Sr 2+ at B1 site when x $ 3.0, whereas a slight increase is observed for x # 3.0. Such a sharp increase of occupancy of Sr 2+ at B1-site lead to a signicant deviation of the lattice parameters, especially for a, from the Vegard's law (see Fig. 2). Given the layered structure nature of Ca 4Àx Sr x GaNbO 8 , when viewed along [110] p , the sharp increase of Sr 2+ -content in B1-site would unambiguously result in a sharp expansion along [111] p , namely a-axis of Sr 4 -type structure (see Fig. 5).
As described above, Sr 2+ 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 hA-Oi and hB-Oi bond lengths are elucidated in Fig. 6b and c, where the change trends for both hA-Oi and hB1-Oi bonds are in good agreement with that of occupancy factors. The hGa-Oi and hNb-Oi bond lengths are almost kept in constant at $1.85 A and $2.0 A, respectively, for all compositions (Fig. 6c). The Ga-O bond lengths in all compounds are in the range of 1.76-1.90 A, which are comparable with four-coordinated Ga 3+ in LaAGa 3 O 7 (A ¼ Ca 2+ , Sr 2+ , Ba 2+ ). [31][32][33] Detailed inspection of the Nb-O bonds revealed that the Nb 5+ exhibits a distorted coordination environment with three Nb-O atomic distances shorter than 2.0 A and the remaining three bond lengths longer than 2.0 A (Table  S2 †), indicating Nb 5+ displaced from the centre of the octahedral cavity due to the second-order Jahn-Teller (SOJT) effect. 34,35 Though both Ga 3+ and Nb 5+ cations can adopt four-and six-fold coordinations, incorporation of Sr 2+ 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 Sr 2+ -doping, Raman spectra for Ca 4 GaNbO 8 and Sr 4 GaNbO 8 were measured. Raman spectra of Ca 4 GaNbO 8 and Sr 4 GaNbO 8 show similar features (Fig. S4 †), which is consistent with their similar crystal structures. The intensive peaks with wavelength numbers higher than 700 cm À1 , especially for the strongest peaks with frequency near 800 cm À1 , 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 À1 is assigned to be the stretching vibration of Nb-O bonds due to the displacement of Nb 5+ from the centre of NbO 6 octahedra. 37 All these observations from the Raman spectra are coherent with the crystal structures for Ca 4 -GaNbO 8 and Sr 4 GaNbO 8 obtained by Rietveld renements.

Transport properties
Typical AC impedance spectra at different temperatures for Ca 4Àx Sr x GaNbO 8 (x ¼ 0, 2, and 4) are presented in Fig. S5 † and  7a, where the well-resolved semicircle in the high frequency rang can be modelled with parallel resistance (R) and capacity (C) element. This large semicircle is associated with a capacity close to 1 Â 10 À12 F cm À1 , implying the contribution of bulk response. The absence of inclined-like spike in low frequency range indicates the absence of oxide ionic conductivity. This observation is consistent with the structural features that there are no terminal oxygens, which is capable of immigration at high temperature, boned to only one B-site cation in both Ca 4and Sr 4 -type structures. Moreover, the oxygen-vacancy ordering is coupled with the B-site cation ordering, which could not be disrupted upon warming. Therefore, the absence of oxide ionic conductivity for Ca 4Àx Sr x GaNbO 8 is understandable. The plots of conductivities for Ca 4Àx Sr x GaNbO 8 (x ¼ 0, 2, and 4) against temperature is elucidated in Fig. 7b. All the compounds exhibit typical semi-conductive behaviour with comparable bulk conductivities (10 À8 -10 À4 S cm À1 ) in the measured temperature range. Fitting the s-T curves with Arrhenius equation gives active energy of $ 0.85 eV for all compounds, which in turn corroborated that the B-site cationic ordering is not disrupted by Sr 2+ -to-Ca 2+ replacement.

Conclusions
Substitution of Ca 2+ in Ca 4Àx Sr x GaNbO 8 with larger Sr 2+ did not lead to oxygen-vacancy and B-site cation ordering-disordering transition but a Ca 4 -type (x < 0.3) to Sr 4 -type (x $ 0.65) structure  transition across a narrow two-phase region (0.3 # x < 0.65). Rietveld renements revealed that two-type structures possess similar anionic ordering and identical B-site ordering. The structural difference only lies in different orientations of GaO 4 tetrahedra, which is responsible for the formation of the narrow twophase region. In the process of changing the Ca 4 -phase to Sr 4phase, the Sr 2+ cations were mainly incorporated into A1-site for x < 1.5, and then doped into A2/A3-site for x < 3, and nally doped into B1-site (x $ 3). Such site-selective doping behaviour observed in Ca 4Àx Sr x GaNbO 8 was driven by the distinctly large difference in size for A1, A2/A3, and B1 sites. Incorporation of Sr 2+ cations into both A and B sites had no inuence on the arrangement of Ga 3+ / Nb 5+ cations and oxygen defects, thus no signicant change in electronic properties of Ca 4Àx Sr x GaNbO 8 was detected.

Conflicts of interest
There are no conicts to declare.