High P-T phase transformations and metastability in the Zr_0.5Hf_0.5O₂ solid-solution ceramic

High pressure-temperature (P-T) phases of the Zr_xHf_1-xO₂ (x=0.5) solid-solution have been stabilised in a CO₂ laser heated diamond anvil cell. At room-temperature the monoclinic to orthorhombic-I structural transformation is initiated at 5-8GPa. The X-ray diffraction (XRD) studies show these two phases coexist to above ∼15GPa. A progressive increase in the orthorhombic-I phase abundance occurs, to culminate in full conversion at ∼20GPa. At this lower threshold of ∼20GPa transformation to the orthorhombic-II (cotunnite) structure can be initiated by heating in the range of 600-1200°C. Substantial conversion to the cotunnite phase occurs in the same temperature range at 25-30GPa. Raman signatures have been assigned to the two orthorhombic high-pressure phases, aided by the qualitative assessment of the complementary XRD data. Decompression experiments show that phase mixture composites of these high pressure structures, possibly with enhanced tribological properties, can be recovered to ambient conditions.