Estimation of the compressibility of Na0.5Bi0.5TiO3 and related perovskite-type titanates

We report on the estimation of the compressibility of Na0.5Bi0.5TiO3 and it s constituent polyhedra through two different approaches: the so-called pol yhedral approach and that based on the pressure dependence of particular Ra man-active modes. In order to investigate the reliability of the results ob tained for Na0.5Bi0.5TiO3, the two approaches have also been applied to per ovskite-type titanates, ATiO(3) (A = Ca, Ba, Sr, Pb), and the results are c ompared with compressibility data reported in the literature. The comparati ve study shows that (i) ATiO(3) compressibilities are overestimated by the polyhedral approach, bur can be roughly corrected by an empirical scaling f actor, (ii) the A cation has a smaller influence in the compressibility tha n would be expected in view of the weakly bonded large AO(12) polyhedra, (i ii) TiO6 compressibilities deduced from Raman spectra are remarkably unifor m among different titanates ((4.3 +/- 0.2) x 10(-3) GPa(-1)), (iv) the Rama n approach yields compressibilities within 10% of the experimental values.