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 Me 1+ Bi 3+ Me 3+ Nb 5+ O 6 . 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 rota tion mode yield five energetically most favorable distorted phases. The symmetry and the energy characteris tics of these phases are discussed. In four of the five phases, the distortions associated with the oxygen octa hedron rotation lead to polar phases, thus allowing one to speak of improper ferroelectricity in these com pounds. 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.

First-principles calculations of ferroelectric properties in AA′BB′O6 double perovskites with different types of cation ordering

“…Compounds with the perovskite structure belong to a class of functional materials due to a wide variety of their properties. Ferroelectric and other properties of such compounds change widely depending on their structure and composition . Recent development of new technologies of crystals and film growth results in a growing interest in new perspective materials.…”

“…These and other properties can be found in various compounds with the double‐perovskite structure AA′BB′O 6 . There are a lot of experimental and theoretical works devoted to the study of double‐perovskite properties. Compounds with general formula A 2 BB′O 6 , where B and B′ cations have rocksalt ordering, are well known.…”

“…Compounds with large spontaneous polarization were predicted in based on unstable phonon modes eigenvectors and energy landscape calculations. Lattice dynamics and the energy of different phases related to rotation of oxygen octahedra were calculated for a number of well‐ordered double perovskites A Bi B NbO 6 ( A – alkali metal Na, K, Rb, and B – trivalent metal Sc, Ga, In, Lu) in in the framework of the polarizable‐ion model and it was shown that spontaneous polarization may appear in some low‐symmetry phases as a secondary effect.…”

“…In a low‐temperature phase this crystal is monoclinic with P 2 1 symmetry, Na and La ions are ordered in layers and Mn and W ions are ordered in a rocksalt structure. It was shown in that additional ordering of A‐site cations leads to the appearance of improper ferroelectricity induced by oxygen octahedra rotations.…”

“…In the present work the study of double perovskite with the general formula A Bi B NbO 6 , where A – alkali metal Na, K, Rb, B – trivalent metal Sc, Ga, In, Lu started in is continued. Other types of possible ordering of A , A ′ and B , B ′ cations has been considered for four compounds from this series when A = Na, Rb and B = Sc, Lu.…”