Phase Transitions in the Metastable Perovskite Multiferroics BiCrO and BiCrScO: 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 % Cr-to-Sc substitution were studied and compared with those of the unmodified compound. Using a high-pressure synthesis, BiCrO and BiCrScO were obtained as metastable perovskite phases which are monoclinic 2/ with the √6 × √2 × √6 superstructure (where is the primitive perovskite unit-cell parameter) under ambient conditions. At room temperature, the unit cell volume of BiCrScO is ∼1.3% larger than that of BiCrO. Both perovskites undergo a reversible structural transition into a nonpolar GdFeO-type phase (orthorhombic , √2 × 2 × √2) in the temperature ranges of 410-420 K (BiCrO) and 470-520 K (BiCrScO) 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 -to-2/ crossover in BiCrO is accompanied by a decrease in the direct current (dc) conductivity, while in BiCrScO the conductivity increases. The onset of an antiferromagnetic order in BiCrScO is observed at the Néel temperature () of about 85 K as compared with = 110 K in BiCrO. In contrast to BiCrO, which exhibits a spin reorientation at ∼ 72 K, no such a transition occurs in BiCrScO.