QUASI-EQUILIBRIUM BEHAVIOR OF TIO2-SNO2 SOLID-SOLUTIONS AT HIGH-PRESSURES AND TEMPERATURES - THE INFLUENCE OF NON-HYDROSTATICITY

The system TiO2-SnO2 has been studied experimentally at pressures of 0 -6.5 GPa and temperatures of 1020-1430 degrees C. The extent of solid solution in the rutile structure was found to increase with pressure; the temperature of the solvus crest was fit as T-cr (degrees C) = 1430 - 61.9 P (GPa). The data are consistent with a symmetrical regular so lution model. The Margules parameter was fit as a linear function of p ressure, W (kJ/mol) = 28.92 - 1.03 P (GPa). However, theoretical solid solution models suggest that the non-ideality should increase with pr essure, which would be consistent with the observation that the volume of mixing at atmospheric pressure is positive. Therefore, the Margule s parameter would be expected to become more positive, and the solvus gap to widen, with increasing pressure. The behaviour observed may ref lect convergence towards a metastable-equilibrium state rather than th e global minimum of free energy. It is possible that the negative valu e derived for the apparent V-mix from the experimental data is due to 'solid solution hardening', i.e. the enhanced shear modulus of interme diate compositions relative to end-members. The likelihood of such beh aviour complicates considerably the analysis of high-pressure experime ntal data from solid solutions. Behaviour that appears to be internall y consistent may nevertheless correspond to a metastable state rather than to true equilibrium. Twinning on {101} in the rutile phase, and m etastable transformation to the alpha-PbO2 phase were observed in some samples. This was predominantly isochemical, and appears to have occu rred during cold, slightly non-hydrostatic pressurisation outside the true stability field of the alpha phase. Given that the atomic ar rang ement across a {101} twin boundary in rutile corresponds closely to th at in untwinned alpha-PbO2, it is likely that the twins acted as nucle i for growth of the alpha phase. The observed orientation relationship s between rutile structure matrix, twins and lamellae of alpha phase a re consistent with diffusionless transformation via a fluorite- or bad deleyite-like transition state, as proposed elsewhere.