A high pressure Raman study of ThO sub 2 to 40 GPa and pressure- induced phase transition from fluorite structure.

The pressure dependence of the first-order Raman peak and two second-order Raman features of ThO sub 2 crystallizing in the fluorite type structure are investigated using a diamond anvil cell, up to 40 GPa. A phase transition from the fluorite phase is observed near 30 GPa as evidenced by the appearance of seven new Raman peaks. The high pressure phases of ThO sub 2 and CeO sub 2 exhibit similar Raman features and from this it is believed that the two structures are the same, and have the PbCl sub 2-type structure. The pressure dependence (d omega/dP) of the observed phonons and their mode Gruneisen parameters are similar to the isostructural CeO sub 2. The observed second-order Raman features are also identified from the calculated phonon dispersion curves for ThO sub 2.