Magnetic transitions in exotic perovskites stabilized by chemical and physical pressure

Exotic perovskites significantly enrich materials for multiferroic and magnetoelectric applications. However, their design and synthesis is a challenge due to the mostly required recipe conditions at extremely high pressure. Herein, we presented the Ca_2-xMn_xMnTaO₆ (0 ≤ x ≤ 1.0) solid solutions stabilized by chemical pressure assisted with intermediate physical pressure up to 7 GPa. The incorporation of Mn²⁺ into the A-site neither drives any cationic ordering nor modifies the orthorhombic Pbnm structure, namely written as (Ca_1-x/2Mn_x/2)(Mn_1/2Ta_1/2)O₃ with disordered A and B site cationic arrangements. The increment of x is accompanied by a ferromagnetic to antiferromagnetic transition around x = 0.2, which is attributed to the double-exchange interactions between A-site Mn²⁺ and B-site Mn³⁺. Partial charge disproportionation of the B-site Mn³⁺ into Mn²⁺ and Mn⁴⁺ occurs for x above 0.8 samples as manifested by X-ray spectrum and magnetic behaviors. The coexistence of B-site Mn³⁺ (Jahn-Teller distortion ion) and B′-site Ta⁵⁺ (second-order Jahn-Teller distortion ion) could be energetically responsible for the absence of A-site columnar ordering as observed in other quadruple perovskites with half of the A-sites occupied by small transition-metal cations. These exceptional findings indicate that exotic perovskites can be successfully stabilized at chemical and intermediate physical pressure, and the presence of Jahn-Teller distortion cations at the same lattice should be avoided to enable cationic ordering.