High pressure Raman spectroscopic studies on nanocrystalline ThO2

Nanocrystalline thoria with average diameters of 9, 15, and 24 nm have been synthesised and their Raman spectra are compared with bulk crystal data in order to establish size-dependent changes to the phonon spectrum at ambient conditions. Micro-Raman spectroscopy has been used to investigate the structural stability of these samples up to pressures of ∼50 GPa in a diamond anvil cell at room temperature. The results are compared with the pressure dependence of Raman spectra of bulk thoria that undergoes a structural transition from fluorite to cotunnite structure around 38 GPa. We observed a linear dependence of the first order Raman band on pressure up to 20 GPa. The pressure dependence of this most prominent single Raman band is similar for nano and bulk phases suggesting that they have similar compressibilities. This band exhibits normal stiffening behaviour under compression. Phase transition pressures of different sized fluorite type nanocrystalline ThO2 are found to be less than that reported for bulk ThO2 except for the one with 9 nm average diameter. Under hydrostatic conditions, the compression and decompression cycles show large hysteresis. Moreover, the reduction of crystallite size results in a sluggish dynamic process in the pressure-induced phase transformation of nano-ThO2.