Unexpected value of transition pressure in the ionic layered BaFI compound observed by Raman scattering

Abstract

The high-pressure behavior of optical phonons in layered BaFI has been studied at ambient temperature using Raman spectroscopy up to 61 GPa. The pressure dependence of the zone-center phonons B l g , E g , and A l g was measured in the tetragonal structure up to the phase transition near 55 GPa. The Gruneisen parameter of the low-frequency A 1 g mode has a very large variation between 0 and 10 GPa. This evolution is attributed to a gradual layer-nonlayer transformation of the structure under pressure. For ionic compounds, and with respect to the classical hard-sphere model, phase transition tends to occur at about the same effective hard-sphere packing fraction. Consequently, the phase transition in BaFI could be thought to occur at a pressure lower than in BaFCl or BaFBr, in contradiction with our experimental data. Using thermodynamics arguments, the unexpected increase of the phase-transition pressure with increasing halogen size in BaFX(X=Cl, Br or I) is attributed to the bidimensional characteristics of the structure, which is shown to expand the lattice volume and to decrease the internal energy by a polarization contribution U p o l .