The phenomenon of conversion polymorphism in bismuth-containing perovskites stabilized by high-pressure

Ferroelectric activity is known to be characteristic of complex ABO₃ perovskites that contain lone-pair cations such as Pb²⁺ or Bi³⁺ in A-position. Lattice distortions caused by atomic displacements and/or oxygen octahedral tilting in a particular perovskite composition depend on the relative size of the constituent cations. Magnetic properties can be controlled through manipulation of content and/or ordering of transition metal cations (e.g., Fe³⁺, Mn³⁺) in B-position. In such a way, new perovskite multiferroics can be designed. However, numbers of promising compositions do not form perovskite structure at ambient pressure and exists as a mixture of other phases or as a less compact (non-perovskite) phase. In many cases, the perovskite compositions can be synthesized using high-pressure synthesis and obtained as metastable phase by means of quenching under pressure. These phases then can be studied using the same facilities applicable to the stable phases over wide temperature ranges at ambient pressure.

A phenomenon of annealing-stimulated irreversible transformations of the high-pressure stabilized phases (conversion polymorphism) [1] has recently been revealed. The same compound can be obtained in different polymorphs, providing a unique opportunity to explore structure-properties relationship. It was shown that the pattern of the phase diagram of the high-pressure prepared complex perovskite compositions, which demonstrates the effect of conversion polymorphism, depends on the maximum annealing temperature. This feature can be used to design new materials with morphotropic phase boundary since by means of controlled annealing, materials with different combinations of the perovskite phases can be obtained.

Here we report on reversible and irreversible transformations between metastable perovskite phases of the Bi-containing solid solutions BiFeO₃-BiScO₃ and BiMg_0.5Ti_0.5O₃-BiZn_0.5Ti_0.5O₃ below their decomposition temperature. New perovskite polymorphs with interesting combinations of ferroic orders are compared and discussed.

[1] D.D. Khalyavin, et al., Chem. Commun. 2019, 55, 4683-4686.