High pressure studies on Thorium-Praseodymium mixed oxides

Abstract

Angle dispersive x-ray diffraction measurements on thorium-praseodymium mixed oxides, (Th1-yPry)O2-x [y = 0, 0.15, 0.4 and 1], have been carried out up to a pressure of around 36 GPa at ambient temperature. A structural transformation from cubic fluorite type to an orthorhombic α-PbCl2 type structure has been observed for y = 0, 0.4 and 1, at a pressure of Pt = 28.8 GPa, 28.8 GPa and 30.1 GPa respectively. No such phase transition could be observed for y = 0.15 oxide. This is attributed to the nano-crystallinity of the sample. In support of this, studies on nano-crystalline ThO2 shows no phase transition up to 42 GPa, indicating the increased structural stability with a decrease in crystallite size. The observed discontinuity in the pressure volume curve of nano ThO2 indicates a possible isostructural transition around 15 GPa. The choice of the equation of state to describe the compression data and derive the bulk moduli of these oxides is discussed. An increase in the bulk modulus with ThO2 substitution was observed for the bulk oxides. However, the nano-crystalline mixed oxides (y = 0.15) showed a reduction in the bulk modulus compared to their bulk counterpart.