Ferroelectric and structural instability in double perovskites Me¹⁺Bi³⁺Me³⁺Nb⁵⁺O₆ (Me¹⁺ = Na, K, Rb; Me³⁺ = Sc, Ga, In, Lu)

Abstract

Within the Gordon-Kim generalized model with regard to the polarizabilities of ions, the lattice constants, the high-frequency permittivity, the Born dynamic charges, and the vibration constants of the crystal lattice are calculated for cation-ordered double perovskites Me¹⁺Bi³⁺Me³⁺Nb⁵⁺O₆. The vibration spectra of all the compounds exhibit two types of instabilities: instability associated with the rotation of the oxygen octahedron and ferroelectric instability. Various combinations of distortions with respect to the rotation mode yield five energetically most favorable distorted phases. The symmetry and the energy characteristics of these phases are discussed. In four of the five phases, the distortions associated with the oxygen octahedron rotation lead to polar phases, thus allowing one to speak of improper ferroelectricity in these compounds. One phase turns out to be nonpolar; however, it contains unstable polar modes such that a displacement along the eigenvectors of these modes gives rise to polarization in the crystal.