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 $B_{1g},$ $E_g,$ and $A_{1g}$ 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 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 $\mathrm{BaF}X(X=\mathrm{Cl},$ 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 $U_{\mathrm{pol}}.$

• Received 30 January 2002

DOI:https://doi.org/10.1103/PhysRevB.66.024115