Comparative study of A-site order in the lead-free bismuth titanates M sub(1/2)Bi sub(1/2)TiO sub(3) (M=Li, Na, K, Rb, Cs, Ag, Tl) from first-principles

We investigate the possibility of enhancing chemical order in the relaxor ferroelectric Na sub(1/2)Bi sub(1/2)TiO sub(3) upon substitution of Na super(+) by other monovalent cations M super(+) using total energy calculations based on density functional theory. All chemically available monovalent cations M super(+), which are Li, Na, Ag, K, Tl, Rb and Cs, are considered and an analysis of the structurally relaxed structures in terms of symmetry-adapted distortion modes is given in order to quantify the chemically induced structural distortions. We demonstrate that the replacement of Na super(+) by other monovalent cations can hardly alter the tendency of chemical order with respect to Na sub(1/2)Bi sub(1/2)TiO sub(3). Only Tl sub(1/2)Bi sub(1/2)TiO sub(3) and Ag sub(1/2)Bi sub(1/2)TiO sub(3) show enhanced tendency for chemical ordering. Both heavy metals behave similar to the light alkali metals in terms of structural relaxations and relative stabilities of the ordered configurations. Although a comparison of the Goldschmidt factors of components (M TiO sub(3)) super(-) reveals for Tl a value above the upper stability limit for perovskites, the additional lone-pair effect of Tl super(+) stabilizes the ordered structure.