Chemical Substitution-Induced and Competitive Formation of 6H and 3C Perovskite Structures in Ba 3-x Sr x ZnSb 2 O 9 : The Coexistence of Two Perovskites in 0.3 ≤ x ≤ 1.0.

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 Ba3 ZnSb2 O9 . The 6H perovskite is only stable in the narrow range of x ≤ 0.2, which attributes to the impressibility of [Sb2 O9 ]. The preference of 90° Sb-O-Sb connection and the strong Sb5+ -Sb5+ electrostatic repulsion in [Sb2 O9 ] 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-Ba2.8 Sr0.2 ZnSb2 O9 and rock-salt ordered 3C-Ba2 SrZnSb2 O9 was observed (0.3 ≤ x ≤ 1.0). In the final, the successive symmetry descending was established 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). 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.