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 its constituent polyhedra through two different approaches: the so-called polyhedral approach and that based on the pressure dependence of particular Raman-active modes. In order to investigate the reliability of the results obtained for Na0.5Bi0.5TiO3, the two approaches have also been applied to perovskite-type titanates, ATiO(3) (A = Ca, Ba, Sr, Pb), and the results are compared with compressibility data reported in the literature. The comparative study shows that (i) ATiO(3) compressibilities are overestimated by the polyhedral approach, bur can be roughly corrected by an empirical scaling factor, (ii) the A cation has a smaller influence in the compressibility than would be expected in view of the weakly bonded large AO(12) polyhedra, (iii) TiO6 compressibilities deduced from Raman spectra are remarkably uniform among different titanates ((4.3 +/- 0.2) x 10(-3) GPa(-1)), (iv) the Raman approach yields compressibilities within 10% of the experimental values.