Phase Transitions in the Metastable Perovskite Multiferroics BiCrO₃ and BiCr_0.9Sc_0.1O₃: A Comparative Study

The temperature behavior of the crystal structure as well as dielectric and magnetic properties of the perovskite bismuth chromate ceramics with the 10 mol % Cr³⁺-to-Sc³⁺ substitution were studied and compared with those of the unmodified compound. Using a high-pressure synthesis, BiCrO₃ and BiCr_0.9Sc_0.1O₃ were obtained as metastable perovskite phases which are monoclinic C2/c with the √6a_p × √2a_p × √6a_p superstructure (where a_p is the primitive perovskite unit-cell parameter) under ambient conditions. At room temperature, the unit cell volume of BiCr_0.9Sc_0.1O₃ is ∼1.3% larger than that of BiCrO₃. Both perovskites undergo a reversible structural transition into a nonpolar GdFeO₃-type phase (orthorhombic Pnma, √2a_p × 2a_p × √2a_p) in the temperature ranges of 410-420 K (BiCrO₃) and 470-520 K (BiCr_0.9Sc_0.1O₃) with a relative jump of the primitive perovskite unit cell volume of about -1.6 and -2.0%, respectively. Temperature dependences of the complex dielectric permittivity demonstrate anomalies in the phase transition ranges. The Pnma-to-C2/c crossover in BiCrO₃ is accompanied by a decrease in the direct current (dc) conductivity, while in BiCr_0.9Sc_0.1O₃ the conductivity increases. The onset of an antiferromagnetic order in BiCr_0.9Sc_0.1O₃ is observed at the Néel temperature (T_N) of about 85 K as compared with T_N = 110 K in BiCrO₃. In contrast to BiCrO₃, which exhibits a spin reorientation at T_sr ∼ 72 K, no such a transition occurs in BiCr_0.9Sc_0.1O₃.