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Article Cite This: Inorg. Chem. 2017, 56, 14335-14344

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Chemical Substitution-Induced and Competitive Formation of 6H and 3C Perovskite Structures in Ba3−xSrxZnSb2O9: The Coexistence of Two Perovskites in 0.3 ≤ x ≤ 1.0 Jing Li, Pengfei Jiang,* Wenliang Gao, Rihong Cong, and Tao Yang* College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 401331, P. R. China

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S Supporting Information *

ABSTRACT: 6H and 3C perovskites are important prototype structures in materials science. We systemically studied the structural evolution induced by the Sr2+-to-Ba2+ substitution to the parent 6H perovskite Ba3ZnSb2O9. The 6H perovskite is only stable in the narrow range of x ≤ 0.2, which attributes to the impressibility of [Sb2O9]. The preference of 90° Sb−O−Sb connection and the strong Sb5+-Sb5+ electrostatic repulsion in [Sb2O9] are competitive factors to stabilize or destabilize the 6H structure when chemical pressure was introduced by Sr2+ incorporation. Therefore, in the following, a wide two-phase region containing 1:2 ordered 6H−Ba2.8Sr0.2ZnSb2O9 and rock-salt ordered 3C− Ba2SrZnSb2O9 was observed (0.3 ≤ x ≤ 1.0). In the final, the successive symmetry descending was established from cubic (Fm3m ̅ , 1.3 ≤ x ≤ 1.8) to tetragonal (I4/m, 2.0 ≤ x ≤ 2.4), and finally to monoclinic (I2/m, 2.6 ≤ x ≤ 3.0). Here we proved that the electronic configurations of B-site cations, with either empty, partially, or fully filled d-shell, would also affect the structure stabilization, through the orientation preference of the B−O covalent bonding. Our investigation gives a deeper understanding of the factors to the competitive formation of perovskite structures, facilitating the fine manipulation on their physical properties.

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Sb5+-to-Nb5+ substitution in Ba3MNb2O9 resulted in the decrease of dielectric constant and temperature coefficient (from positive to negative), due to the dilution of average ionic polarizability. Moreover, Ba3MSb2O9 (M = Co2+, Ni2+, Cu2+) are good model compounds displaying low-dimensional quantum spin liquid behavior, because of the unique stacking sequence of transition-metal cations in Ba3MSb2O9.19−22 For example, Ni2+ cations (S = 1) in Ba3NiSb2O9 were distributed in a layered-type triangular lattice, which led to the formation of two different spin liquid phases at low temperatures;23 the disordered distribution of Cu2+ and Sb5+ in the face-sharing octahedra in 6H−Ba3CuSb2O9 created a honeycomb-based magnetic lattice; thus, a spin−orbital short-range ordering was observed at low temperatures.24 The structural stability of perovskites with the general formula ABO3 depends on the ionic radii of the metal cations, which is commonly measured by the tolerance factor t = (rA + rB)/√2(rB+ rO), where rA, rB, and rO are the ionic radii of respective ions. It is commonly accepted that perovskites with tolerance factor t larger than unity prefer to adopt hexagonaltype structure. The tolerance factors for Ba3ML2O9 (M = Mg2+, Zn2+, Ni2+; L = Nb5+, Ta5+, Sb5+) are all larger than 1, and it is interesting to observe two different but related perovskite structures. In detail, Nb5+- and Ta5+-containing compounds adopt the 3C-type structure, wherein the B-site cations are

INTRODUCTION Perovskites are able to accommodate various cations, covering a wide range of ionic sizes and charges, thus are widely studied in solid-state chemistry and physics. Accordingly, perovskites can offer chemists great opportunities to prepare new compounds and tailor the physical properties, such as ferroelectricity,1−3 superconductivity,4,5 ferromagnetism,6 and ionic conductivity,7−10 through chemical substitutions. Some typical and interesting properties for B-site ordered perovskites include colossal magnetoresistance (i.e., Sr2FeMoO6)11 and ferrimagnetism with a high TN (i.e., Sr2CrOsO6).12 The B-site cationic ordering in perovskites is usually driven by the size and/or charge differences. For example, the large charge and size difference between Zn2+ (r = 0.74 Å, coordination number (CN) = 6) and Sb5+ (r = 0.6 Å, CN = 6) leads to double-perovskite LaBaZnSbO6 with the rock-salt type ordering of Zn2+ and Sb5+.13 Layered and columnar-type cationic ordering in perovskites have also been observed in a few special cases.14 Layered ordered perovskites with 1:2 B-site ordering of general formula Ba3ML2O9 (M = Mg2+, Zn2+, Ni2+; L = Nb5+, Ta5+, Sb5+) have received considerable attention due to their intriguing physical properties and potential applications.15−18 For example, Ba3ZnL2O9 (L = Nb5+, Ta5+) both possess high dielectric constants and a low dielectric loss in the microwave region along with a near-zero temperature coefficient and thus are important dielectric materials and could be used as resonators in microwave telecommunication technology.15,18 © 2017 American Chemical Society

Received: September 23, 2017 Published: November 9, 2017 14335

DOI: 10.1021/acs.inorgchem.7b02429 Inorg. Chem. 2017, 56, 14335−14344

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Inorganic Chemistry

Figure 1. Final convergences of Rietveld refinements for Ba3ZnSb2O9. (blue ○) Observed data. (red solid line) Calculated pattern. (purple marks below the diffraction patterns) The expected reflection positions. The difference curve is also shown at the bottom. (inset) Enlargement of the highangle fitting.

Table 1. Atomic Coordinates, Isotropic Thermal Displacement, and Site Occupancy Factors for Ba3−xSrxZnSb2O9 (x = 0, 1.3, 2.0, 3.0) Obtained from Rietveld Refinements from Powder XRD Beq (Å2)

Ba3ZnSb2O9

x

y

z

Ba1 Ba2 Zn Sb O1 O2 Ba1.7Sr1.3ZnSb2O9 Ba/Sr Zn/Sb2 Sb1 O1 BaSr2ZnSb2O9 Ba/Sr Zn/Sb2 Sb1 O1 O2 Sr3ZnSb2O9 Sr Zn/Sb2 Sb1 O1 O2

0 0.3333 0 0.3333 0.4848(4) 0.1695(3)

0 0.6667 0 0.6667 0.5152(4) 0.8305(3)

0.25 0.9106(4) 0 0.15173(3) 0.25 0.4158(2)

1 1 1 1 1 1

0.0073(5) 0.0101(5) 0.0095(9) 0.0043(6) 0.0085(1) 0.0099(1)

0.25 0 0 0

0.25 0 0.5 0.2505(5)

0.25 0 0 0

0.567/0.433 2/3, 1/3 1 1

0.0127(6) 0.0038(6) 0.0064(6) 0.0182(8)

0 0 0 0 0.2802(2)

0.5 0 0 0 0.2310(2)

0.25 0 0.5 0.2563(3) 0

1/3, 2/3 2/3, 1/3 1 1 1

0.0131(5) 0.0054(7) 0.0043(6) 0.0268(5) 0.0083(2)

0.500(1) 0 0 0.9269(2) 0.2631(3)

0 0 0 0 0.2502(3)

0.2490(8) 0 0.5 0.2558(4) 0.0259(1)

1 2/3, 1/3 1 1 1

0.0153(3) 0.0048(6) 0.0046(5) 0.0153(2) 0.0153(2)

distributed along the ⟨111⟩p direction in the ordered sequence of M-L-L, forming trigonal symmetry (P3̅m1) in the ideal case. In contrast, Ba3MSb2O9 adopts the 6H-type structure (the ideal symmetry is P63/mmc) with the mixed cubic and hexagonal close-packing. M and L ions in both 3C- and 6H-perovsiktes are octahedrally coordinated, but the connections between the octahedra are different. It is exclusively corner-sharing in the 3C perovskite; differently, corner- and face-sharing coexist in 6H perovskite. Here in this work, we examined the effect of substituting Sr2+ for Ba2+ in the solid solution formula Ba3−xSrxZnSb2O9 to look at the resulting structures. As can be expected from Vegard’s law, a two-phase region mixed with 6H and 3C end-members with different percentages was observed for 0.3 ≤ x ≤ 1.0. Rietveld refinements revealed that the competitive formation of two perovskites was originated from the rigid characteristic of [Sb2O9], wherein there existed the competition between the strong Sb5+-Sb5+ electrostatic repulsion and the strong σ bonds

occupancy

due to 90° Sb−O−Sb connection. In addition, we observed the successive symmetry descending in 3C−Ba3−xSrxZnSb2O9 from cubic (Fm3̅m, 1.3 ≤ x ≤ 1.8) to tetragonal (I4/m, 2.0 ≤ x ≤ 2.4), and finally to monoclinic (I2/m, 2.6 ≤ x ≤ 3.0), resulting from the rotation and tilting of the metal−oxygen octahedra.

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EXPERIMENTAL SECTION

The polycrystalline samples Ba3−xSrxZnSb2O9 (0 ≤ x ≤ 3) were prepared by high-temperature solid-state reactions. Barium carbonate (BaCO3, Alfa Aesar, 99.9%), strontium carbonate (SrCO3, Alfa Aesar, 99.9%), zinc oxide (ZnO, Alfa Aesar, 99.9%), and antimonous oxide (Sb2O3, Alfa Aesar, 99.9%) were used as starting materials. Typically, stoichiometric starting reagents were weighed and mixed in an agate mortar and then heated at 600 °C for 5 h to ensure the complete oxidation of Sb3+ into Sb5+. After this precalcination, the powder sample was pressed into a pellet (dimeter, 13 mm) and annealed at 950 °C for 10 h and at 1250 °C for 20 h. Further regrinding, repressing, and annealing may be needed, until the final thermody14336


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Inorganic Chemistry namic equilibrium was achieved according to the powder X-ray diffraction (XRD). Powder XRD were performed on a PANalytical X’pert powder diffractometer equipped with a PIXcel 1D detector (Cu Kα radiation). The operation voltage and current were 40 kV and 40 mA, respectively. The data used for phase identification was collected with a setting of 30 s/0.0262°. High-quality XRD data, which were used for Rietveld refinements, were collected with a setting of 200 s/ 0.0131°. Rietveld refinements were performed using the TOPAS software package.25 Bond valence sums (BVS) were calculated by the Brown and Altermatt’s method.26

Table 2. Selected Bond Distances (Å) and Angles (deg) for Ba3−xSrxZnSb2O9 (x = 0, 1.3, 2.0, 3.0)a Ba3ZnSb2O9 Ba1−O1 Ba1−O2 Ba2−O2 Ba2−O1 Ba2−O2

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RESULTS 6H-Perovskite Ba3−xSrxZnSb2O9 (0 ≤ x ≤ 0.2). It is wellknown that perovskites with the tolerance factor larger than unity tend to adopt hexagonal structures. For Ba3ZnSb2O9, the tolerance factor t is 1.04 (Ba2+ 1.61 Å, Zn2+ 0.74 Å, Sb5+ 0.60 Å, O2− 1.40 Å).27 Indeed, the XRD pattern of Ba3ZnSb2O9 in Figure 1 can be readily assigned to the 6H perovskite in the space group P63/mmc. Ba2.80Sr0.20ZnSb2O9 adopts the hexagonal structure as well (see Figure S1), and the diffraction peaks slightly shift toward higher angles, because of the smaller ionic size of Sr2+ (1.44 Å in 12-coordiantion) in comparison with Ba2+ (1.61 Å in 12-coordiantion). Rietveld refinements were performed for hexagonal Ba3−xSrxZnSb2O9 (0 ≤ x ≤ 0.2). Since it is isostructural with Ba3MSb2O9 (M = Mg2+, Ni2+, Co2+), in which M and Sb are ordered, then the structure of Ba3MgSb2O9 was chosen to be the initial structural model. Here, attention was paid to the ordering or disordering in the B site. First, we refined freely the occupancy factors of Zn2+ and Sb5+ at two independent B sites (2a and 4f, respectively). The resultant antisite occupancy factors for Zn2+/Sb5+ was as small as 2%, indicating that Zn2+ and Sb5+ can be considered as fully ordered in Ba3ZnSb2O9. The final convergence of Rietveld refinements for Ba3ZnSb2O9 using the model with complete ordering of Zn/Sb was given in Figure 1. The very good agreement between the observed and calculated patterns together with the very low agreement factors suggested the correctness of the ordering model. The finally obtained cell parameters were in good agreement with those in literature,28 and the crystallographic data were given in Tables 1 and 2. Note that the calculated BVS for Zn2+ and Sb5+ were 2.0 and 5.1, respectively, which further supported the Zn2+/Sb5+ ordering. Similar strategy on refinements was done for Ba2.80Sr0.20ZnSb2O9, and the corresponding results are provided in the Supporting Information. The crystal structure of Ba3−xSrxZnSb2O9 (0 ≤ x ≤ 0.2) could be described as the typical six-layer perovskite with mixed hexagonal and cubic close packing of “BaO3” layers, which is presented in Figure 2. In this structure, the Zn2+ cations occupy the 2a site in a regular octahedral cavity (see Table 1), forming a planar triangular lattice perpendicular to the c-axis. Such a sublattice of transition-metal cations has attracted great attention when it was occupied by magnetic cations.19−24 The Sb5+ cations occupy the 4f site, forming [Sb2O9] dimers, which are linked through ZnO6 octahedra with corner-sharing. Obviously, there exists a strong Sb5+-Sb5+ electrostatic repulsion within the [Sb2O9] dimers composed of two face-sharing SbO6 octahedra, visualized by an opposite off-center shift of the Sb5+ along the c-axis in the respective octahedral cavities (see Figure 2 and Table 2). Usually, the stabilization of the hexagonal perovskites containing face-sharing octahedra benefits from the strong metal−metal bonding to overcome the cationic repulsion. In

6 6 6 3 3

2.9333(2) Sb−O1 × 3 2.950(3) Sb−O2 × 3 2.9303(1) BVS 2.966(2) Zn−O2 × 6 3.0116(3) BVS Ba1.7Sr1.3ZnSb2O9

2.094(3) 1.928(3) 5.1 2.107(3) 2.0

Ba/Sr−O1 × 12 Zn/Sb2−O1 × 6

2.8652(2) Sb1−O2 × 6 2.057(3) BaSr2ZnSb2O9

1.995(5)

Ba−O1 × 4 Ba−O2 × 4 Ba−O2 × 4 Zn/Sb2−O1 × 2 Zn/Sb2−O2 × 4

2.8478 (8) Sb1−O1 × 2 2.715(8) Sb1−O2 × 4 2.993(9) 2.07(2) Zn1−O1−Sb1 2.07(1) Zn1−O2−Sb1 Sr3ZnSb2O9

1.96(2) 1.98(1)

Sr−O1 Sr−O2 Sr−O2 Sr−O1 Sr−O2 Sr−O2 Sr−O1 a

× × × × ×

× × × × × × ×

1 2 2 2 2 2 1

2.42(2) 2.64(2) 2.727(2) 2.855(2) 2.94(2) 3.02(2) 3.24(2)

180° 168.744°

Zn/Sb2−O1 × 2 Zn/Sb2−O2 × 4 Sb1−O1 × 2 Sb1−O2 × 4

2.08(3) 2.06(2) 1.99(3) 1.96(2)

Zn1−O1−Sb1 Zn1−O2−Sb1

156.559° 167.803°

The calculated BVS values for Zn2+ and Sb5+ are also given.

Figure 2. Crystal structure views along the [010] directions for perovskite Ba3ZnSb2O9. Brown and blue octahedra represent ZnO6 and SbO6, respectively. Ba and O are shown as red and cyan spheres, respectively.

the structures containing metal−metal bonding, the BVS of the central metal cations should be smaller than its normal value. For example, there are [Ge2Te6] dimers in Cr2Ge2Te6, where the calculated BVS for Ge is 3.2. This means that three valence electrons of Ge are bonded with three Te ions, and the last valence electron of Ge is bonded with the neighboring Ge to form the typical metal−metal bonding.29 Here in our cases, the calculated BVS for Sb in Ba3ZnSb2O9 is 5.1, which means all five valence electrons are bonded to the surrounding oxygen atoms, and there is no extra electron to form any Sb−Sb metal bonding. Thus, for stabilization of the 6H perovksite structure, there must be another factor to at least partially suppress the strong Sb5+-Sb5+ cationic repulsion. Indeed, we observed the very short O1−O1 distance of 2.663(2) Å, which is much shorter than the O2−O2 distance (2.880(4) Å) and is comparable with the value in Ba2ScRuO6 (2.673(2) Å).30 14337


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Figure 3. Final convergences of Rietveld refinements for two-phase samples with initial loading Sr2+ content x = (a) 0.3, (b) 0.5, and (c) 1. Please note the purple and green marks below the diffraction patterns are the expected reflection positions for hexagonal and cubic perovskite phases. Agreement factors and the calculated weight percentages for both phases are also shown.

intensity of the 3C perovskite increased with increasing Sr2+ content, and the intensities for the 6H phase decreased. According to the Vegard’s law, it is as expected that the peak positions for the 6H and 3C perovskites were invariable and that only the relative mass percentage between these two endmember compositions changed in the region of 0.2 ≤ x ≤ 1.0 (see Figure S1). Please note that there was an obvious peak shift of the 3C phase for x = 1 and 1.2; however, a small amount of hexagonal phase was still observed for x = 1.2, suggesting that x = 1.2 is close to the two-phase boundary in the phase diagram. Rietveld refinements were performed on powder XRD using two structures simultaneously, including 1:2 ordered 6H

This means the small O1−O1−O1 triangular plane right in the middle of the [Sb2O9] dimer could shield the electrostatic repulsion of the Sb5+-Sb5+ pair. In addition, the orientation preference of the Sb5+−O2− covalent bond also helps the stabilization of the 6H structure, which will be discussed in later sections. The Two-Phase Region in 0.3 ≤ x ≤ 1.0. In addition to the 6H perovskite powder pattern, a series of peaks corresponding to the 3C perovskite emerged when x = 0.3 (see Figure S1). Increasing the reaction time would not lead to a single-phase product, which means the thermodynamic equilibrium state at x = 0.3 comprises inherently two phases, namely, 6H and 3C perovskites. The overall diffraction 14338


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Inorganic Chemistry perovskite (P63/mmc) and rock-salt ordered 3C perovskites (Fm3̅m), giving the formulas for both phases very close to Ba2.8Sr0.2ZnSb2O9 and Ba2SrZnSb2O9, respectively. In the final, we fixed the compositions of both phases, leaving the atomic coordinates, temperature displacement factors, and the proportion of the two phases being refined. The resultant crystallographic data are provided in Table S1 (Supporting Information). The final convergences of Rietveld refinements for x = 0.3, 0.5, 1.0 were shown in Figure 3, along with the respective weight percentages of two types of perovskites. It should be also pointed out that similar two-phase region was also observed in other Sb5+-containing perovskites, such as Sr2−xBaxFeSbO631 and Ba3MSb2−xNbxO9 (M = Zn2+, Mg2+, Ni2+).15 It is not surprising to see a two-phase region according to the Vegard’s law; however, this is a macroscopic observation. In a later section, detailed analyses will be given to the structural origin of such a coexistence of two different structure types in the final thermodynamic equilibrium state. 3C Perovskite Ba3−xSrxZnSb2O9 (1.3 ≤ x ≤ 3). The 6H perovskite phase completely disappeared as the doping content of Sr2+ above 1.2 (see Figure 4a,b). The single-phase Ba3−xSrxZnSb2O9 (1.3 ≤ x ≤ 3) was successfully synthesized with the continuous right-shift of the diffraction peaks due to the smaller cationic radius of Sr2+. People could observe super lattice diffraction peaks at ∼19°, 37°, and 49° for all samples, which is usually an indication of the rock-salt cationic ordering in double perovskites. In fact, these peaks could be indexed with (111), (311), and (331), respectively, in a faced-centered cubic unit cell (Fm3̅m). To verify the crystal symmetry, the diffraction component from Cu Kα2 was stripped for some characteristic peaks (see Figure 4c,d). For 1.3 ≤ x ≤ 1.8, no obvious broadening or peak splitting was observed; consequently, these samples were expected to be in the space group Fm3̅m. When x = 2.0, the peak at ∼45° with an index of (002)p (where the subscript “p” represents the prototype perovskite structure in the space group Pm3̅m) splits into two peaks with the indices of (004) and (220) in the tetragonal superlattice, respectively (see Figure 4c). Such a peak splitting suggests the symmetry for x ≥ 2 was lowered to tetragonal (I4/m). When x ≥ 2.6, a further symmetry lowering was confirmed by the change of relative peak positions for (004) and (220), which is usually a result of the change of tilting system from a0a0c+ to a−a−c0 or a−a−c+. In addition, we also observed the peak splitting of the peak at ∼83.5° with an index (044)t for x ≥ 2.6, where “t” represents the B-site ordered superstructure in space group I4/m (see Figure 4d). The above observations proved that the structure symmetry for x ≥ 2.6 should be lowered to orthorhombic or monoclinic. In 1993, Anderson et al. claimed that there is no orthorhombic structure for rock-salt B-site ordered double perovskite.33 Therefore, the structure symmetry for the samples with x ≥ 2.6 should be monoclinic. Woodward and his coworkers proposed that the allowed space groups for B-site ordered perovskite in the monoclinic system include C2/c, P21/ c, and C2/m (I2/m with a different cell choice).34 Considering the group−subgroup relationship and the extinction condition of the XRD patterns, we found the most probable space group in our case was I2/m, although it was rarely seen; for instance, only a few Sb5+-based perovskites LaBaMSbO6 (M = Mn2+, Ni2+, Cu2+, Zn2+) possess this space group.13,32,35,36 According to the above analyses on powder XRD patterns for Ba3−xSrxZnSb2O9 (1.3 ≤ x ≤ 3), Sr2+-doping-induced and successive symmetry lowering was observed from Fm3̅m to I4/

Figure 4. XRD patterns for Ba3−xSrxZnSb2O9 for (a) 1.3 ≤ x ≤ 2.0; (b) 2.2 ≤ x ≤ 3.0; (c, d) the evolution of peak splitting (note that the Kα2 diffraction was subtracted).

m and then to I2/m. The same symmetry-lowering sequence has been observed in Ba2−xSrxInTaO6,37 Ba2−xSrxYIrO6,38 and A2CoWO6 (A = Ba2+, Sr2+).39,40 To check the B-site ordering, Rietveld refinements were performed on all the 3C-type perovskites. We notice that one of the B sites was exclusively occupied by Sb5+, and the other one was occupied by Zn2+/Sb5+ in a ratio of 2:1. In addition, the refined occupancy factors for Ba2+ and Sr2+ at the A-site were consistent with the experimental values very well. During the final Rietveld refinement, the occupancy factors for all cations were fixed to be the expected values. The final convergence of the Rietveld refinements for Ba3−xSrxZnSb2O9 (1.3 ≤ x ≤ 3) are given in Figures 5 and S2. The refined crystallographic data are summarized in Tables 1, 2, S2, and S3. The unit-cell volume per formula obtained from the Rietveld refinements against the Sr2+ content for all compositions were presented in Figure 6. Clearly, the two-phase region in the range of 0.2 ≤ x ≤ 1.0 and the cell volume decreased 14339


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Figure 5. Final convergences of Rietveld refinements for Ba3−xSrxZnSb2O9 x = (a) 1.3, (b) 2.0, and (c) 3.0.

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monotonously in the range of 1.2 ≤ x ≤ 3. Representative

DISCUSSION Structural Origin to the Formation of the Two-Phase Region. In most cases, the 6H perovskite would be favored if the tolerance factor was larger than unity, because it is able to accommodate relative large A-site cations through the expansion along the c-axis. Just for example, the average distance between the closed packing layers in 6H−Ba3ZnSb2O9 is 2.41 Å, and it is 2.36 Å in the hypothetic “3C−Ba3ZnSb2O9” (this value was obtained by prolonging the cell volumes for 3C curve in Figure 6). This means the cell volume of the hexagonal phase is larger than that of cubic phase if the composition was the same. Here, the tolerance factor is 1.034 ≥ t ≥ 1.020 (0.3 ≤

structures of Ba3−xSrxZnSb2O9 (x = 1.3, 2, 3) are shown in Figure 7. Ba1.7Sr1.3ZnSb2O9 adopted the cubic structure and showed no octahedra tilting. It is common that smaller A-site cations would lead to the tilting or rotation of BO6 octahedra in 3C perovskites, which is helpful to improve the A-site coordination. BaSr 2 ZnSb 2 O 9 (space group I4/m) and Sr3ZnSb2O9 (space group I2/m) possessed a0a0c− and a−a−c0 tilt systems, respectively. 14340


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orbital, and this σ-type bonding could be optimized in facesharing [Sb2O9] dimers due to the nearly 90° Sb−O−Sb connection. This might also be the reason why the M−O−M angles always deviate from 180° in many Sb5+-containing double perovskites (i.e., La(Ba/Sr)MSbO6 (M = Zn2+, Mn2+).13,36,41 And it could also explain the quick descending rate of the symmetry from Fm3̅m to I2/m in 3C-type Ba3−xSrxZnSb2O9 (1.3 ≤ x ≤ 3.0), even though the tolerance factor exhibited only a minor decease (see Figure 6). Of course, there existed some exceptional but rare cases exhibiting the 180° Sb−O−Sb connection, that is, Ba 4 LiSb3 O 12 and Ba4NaSb3O12.42 In summary, the nearly 90° Sb−O−Sb connection in [Sb2O9] is likely one of the factors to the stabilization of the 6H structure. There was one more result to reinforce our speculation. The Nb5+-to-Sb5+ doping at B site in Ba3MSb2O9 would destabilize the 6H perovskite and led to the formation of a two-phase region, that is, Ba3MgSb2−xNbxO9 in the region of 0.5 ≤ x ≤ 1.25 and Ba3ZnSb2−xNbxO9 in the region of 0.5 ≤ x ≤ 1.55, although the t values are all larger than unity.15 Second, another commonly seen phenomenon in hexagonal perovskites is the metal−metal bonding in face-sharing MO6 octahedra, which definitely helps the stabilization of the columnar arrangement of the octahedra. For example, the Ru−Ru distances within face-sharing dimers in BaRuO343 and Ba2ScRuO630 are smaller than the shortest metal−metal distance in ruthenium metal, indicating a strong attractive interaction between. In these cases, Ru4+ and Ru5+ both possess partially filled d shell, and the d-orbitals could directly overlap with each other through π bonding (t2g-t2g), in which a quite small cation−cation distance would be observed. Apparently, this is not the same for Ba3ZnSb2O9, because the very long Sb− Sb distance (2.84 Å) in the [Sb2O9] dimer is a strong indication of the electrostatic repulsion, opposed to any possible Sb−Sb metal bonding. It was mentioned above that the strong Sb5+Sb5+ electrostatic repulsion could be partially shielded by the O1−O1−O1 anionic plane in the middle of [Sb2O9], but even so, this strong repulsion should be one of the factors to the destabilization of the 6H strucutre. In fact, the Sb−Sb distance in Ba2.8Sr0.2ZnSb2O9 (2.83 Å) is almost identical to the value in Ba3ZnSb2O9, which means the [Sb2O9] dimer is almost incompressible along the c-axis in this 6H structure. In the following, we demonstrate there is another way to suppress the electrostatic repulsion effect by incorporating lowvalent cations into the face-sharing dimer. Experimentally, Ba2.7Sr0.3Zn0.9Zr0.1Sb1.9Ga0.1O9 (t = 1.034), which has an identical tolerance factor with Ba2.7Sr0.3ZnSb2O9 (t = 1.034), was synthesized to be a single-phase 6H perovskite (see Figure S3), because some of the Sb5+ was replaced by Zr4+ or Ga3+. On the one hand, because of the competition between the preference of 90° Sb−O−Sb connection and the strong Sb5+Sb5+ repulsion, the upper limit of the Sr2+ content in 6H perovskite Ba3−xSrxZnSb2O9 was as small as x = 0.2. On the other hand, the cubic 3C perovskite “Ba3−xSrxZnSb2O9 (0.3 ≤ x ≤ 1.0)” was also not obtainable because of the overly high tfactor. Therefore, a wide range of two-phase region was observed in our study, wherein the mixture of 6H− Ba2.8Sr0.2ZnSb2O9 and 3C−Ba2SrZnSb2O9 was the optimal choice in energy to be the thermodynamic stable state. In general, high pressure benefits the formation of the highdensity phase, which is the 3C perovksite; in contrast, high temperature benefits the formation of the 6H perovskite. Here, we heated the samples with x = 0.3, 1.0, and 1.2 to 1450 °C;

Figure 6. Evolution of the volume per formula for Ba3−xSrxZnSb2O9.

Figure 7. (a) Structural views along the [001] directions for Ba1.7Sr1.3ZnSb2O9; structural views along the [110] (left) and [001] (right) directions for (b) BaSr2ZnSb2O9 and (c) Sr3ZnSb2O9, respectively. Brown octahedra represent Zn/SbO6 (the cavity was occupied by Zn2+ and Sb5+ in the ratio of 2:1), and blue octahedra represent SbO6. Ba/Sr cations are shown as purple spheres.

x ≤ 1.0); it is interesting not to observe the formation of a single-phase 6H perovskite but a mixture of both 6H and 3C structures (see Figure 6). First, we noticed that the 3C perovskite was the preferable structure in Ba3ML2O9 (M = Mg2+, Zn2+, Ni2+; L = Nb5+, Ta5+), while it was not the case in 6H−Ba3ZnSb2O9. Please also refer to the similar phenomenon of two-phase region in the Sbcontaining double perovskite (Sr1−xBax)2FeSbO6 (0.80 ≤ x ≤ 0.975).31 Apparently the nature of the B-site cations plays a key role to the competitive formation of either 6H or 3C perovskites. Since Nb5+/Ta5+ (0.64 Å) and Sb5+ (0.60 Å) have close ionic radii, we believe their difference in electronic configuration may be responsible. In detail, the empty d shell of Nb5+/Ta5+ and Jahn−Teller effect assist the π bonding with the O 2px,y orbitals, and the bonding could be optimized in cornersharing NbO6 octahedra (ideally 180° for Nb−O−Nb angle).15 Therefore, Ba3MNb2O9 prefers to 3C perovskite instead of the 6H structure. On the contrary, because of the fully filled d shell of Sb5+, no such π bonding to oxygen is possible, and the formation of linear Sb−O−Sb linkages are unfavorable. Instead, Sb5+ with d10 configuration prefers to σ bonding with the O 2pz 14341


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Figure 9a, where the values increased along with the doping content of Sr2+ as expected.

however, they did not transfer to the single-phase 6H perovskite but, instead, decomposed to a multiphasic mixture (see Figure S4). Nevertheless, it might be possible to prohibit the decomposition and induce the phase transition by applying an appropriate high pressure. Symmetry Evolution of 3C Perovskite Structures. People prefer to use the c/a value to evaluate the tetragonality of perovskites. Here, the c-axis length of the tetragonal phase 3C−Ba3−xSrxZnSb2O9 (2.0 ≤ x ≤ 2.4) showed an obvious elongation (see Figure 8), and the c/a ratio is larger than 1.

Figure 9. (a) Average tilting angle and (b) average ⟨B−O⟩ bond length plotted with doped Sr2+ content in 3C perovksites.

In cubic 3C−Ba3−xSrxZnSb2O9 (1.3 ≤ x ≤ 1.8), both Sb1O6 and Zn1/Sb2O6 are regular octahedra with six identical M−O bond distances (see Tables 2 and S3). In the tetragonal and monoclinic 3C structures, both Zn2+ and Sb5+ located in slightly distorted octahedral cavities. The average Sb1−O bond length was smaller than that of Zn1/Sb2−O bond length in all cases, which agreed well with the obvious difference in ionic radii between Zn2+ and Sb5+ (0.74 and 0.6 Å for Zn2+ and Sb5+, respectively). When looking into the average Sb1−O bond length in Ba3−xSrxZnSb2O9 (1.3 ≤ x ≤ 3), they were almost unchanged (see Figure 9b), and they were comparable with the observed values in typical B-site ordered perovskites, that is, Pb2TmSbO6,45 LaBa1−xSrxZnSbO6,13 and Sr3NiSb2O9.46 In addition, on the one hand, the calculated BVS for Sb1 agreed well with Sb5+, indicating that Sb1 site was exclusively occupied by Sb5+. On the other hand, the Zn1/Sb2−O average bond length was smaller than the typical Zn−O bond lengths in Ba2ZnWO6,47 Sr2ZnMoO6,48 and LaBa1−xSrxZnSbO6,13 which suggested that Zn1/Sb2 site was co-occupied by both Zn2+ and Sb 5+ . In literature, the Ba 2+ -to-Sr 2+ substitution in (Sr1−xBax)2FeSbO6 led to the partially disordering at the Bsite cations. This was not observed in our cases, because of the larger difference between Zn2+ and Sb5+ in both cationic radii and charge.

Figure 8. (a) Normalized lattice parameters and (b) c/a values plotted along with Sr2+ content for 3C-type Ba3−xSrxZnSb2O9.

This change of c/a value is usually due to the change of tilting system from a0a0a0 in Fm3̅m to a0a0c− in I4/m, which has been observed for many rock-salt B-site ordered perovskites, such as Ba2−xSrxInTaO633 and Ba2−xSrxYIrO6.39 Further increasing the Sr2+ content in Ba3−xSrxZnSb2O9 (2.6 ≤ x ≤ 3.0), a sharp shrinkage and elongation were observed for c- and a-axes, respectively, resulting into the smaller c/a ratio than 1 (see Figure 8). This reversal of c/a values has been observed for numerous perovskites and is often associated with a reorientation of the tilting system of BO6 octahedra. In Ba3−xSrxZnSb2O9, it was from tetragonal I4/m (a0a0c+) to monoclinic to I2/m (a−a−c0). Similar phenomenon was also observed in Ba2−xSrxYIrO6 and Ba2−xSrxYOsO6.38,44 In addition, the average tilting of the octahedra was represented by the deviation of B−O−B′ bond angles from 180° as shown in 14342


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CONCLUSIONS Ba3ZnSb2O9 and Sr3ZnSb2O9 have 6H and 3C perovskite structures, respectively, wherein the BO6 octahedra connection types (equally, the arrangement of ionic close packing) are different. In Ba3ZnSb2O9, there was no metal−metal bond in [Sb2O9] but a very strong Sb5+-Sb5+ electrostatic repulsion, indicated by the opposite shift of Sb5+ along the c-axis. Replacing the Ba2+ by smaller Sr2+ resulted in the compression of ionic lattice along the c-axis, contradicting the impressibility of [Sb2O9], so the 6H perovskite is only stable at a narrow range of x ≤ 0.2, followed by a very wide two-phase region (0.3 ≤ x ≤ 1.0). Phase-pure 3C perovskites Ba3−xSrxZnSb2O9 can only be obtained when x ≥ 1.3, and successive symmetry lowering was observed and analyzed by Rietveld refinements. Usually, the t-factor is the criteria to predict the perovskite structure type; here, we proved that the electronic configurations of B-site cations, with either empty, partially, or fully filled d-shell, would also affect the structure stabilization, through the orientation preference of the B−O covalent bonding; for instance, 180° Nb5+−O−Nb5+ and 90° Sb5+−O− Sb5+ are the respective favored connections, because the π bonding between Nb5+−O and σ bonding between Sb5+−O could be reinforced if so. Since both hexagonal and cubic perovskites are important prototype structures in materials science, our investigation gives a deeper understanding of the factors to their competitive formation, facilitating the fine manipulation of crystal structure and physical properties.

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REFERENCES

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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.7b02429. Refined crystallographic parameters for Ba3−xSrxZnSb2O9, XRD patterns for Ba3−xSrxZnSb2O9 (0 ⩽ x ⩽ 1.2), the final convergences of Rietveld refinements for Ba3−xSrxZnSb2O9 (x = 0.2, 1.2, 1.4, 1.6, 1.8, 2.2, 2.4, 2.6, 2.8), XRD patterns for Ba3−xSrxZnSb2O9 (0 ≤ x ≤ 1.2), and the XRD patterns for Ba2.7Sr0.3Zn0.9Ga0.1Sb1.9Zr0.1O9 and Ba3−xSrxZnSb2O9 (x = 0.3, 1.0, 1.3) heated at elevated temperatures (PDF)

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (P.-F.J.) *E-mail: [email protected]. (T.Y.) ORCID

Pengfei Jiang: 0000-0001-6832-7462 Rihong Cong: 0000-0002-9018-6819 Tao Yang: 0000-0002-2276-4023 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21671028 and 21771027). 14343


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