Unexpected value of the transition pressure in ionic layered BaFI compound observed by Raman scattering
Résumé
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 B1g, Eg, and A1g was measured in the tetragonal structure up to the phase transition near 55 GPa. The Grüneisen parameter of the low frequency A 1g 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 lower pressure 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 bi-dimensional characteristic of the structure which is shown to expand the lattice volume and to decrease the internal energy by a polarization contribution U pol .
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