Calorimetric study of high pressure polymorphism in FeTiO₃: Stability of the perovskite phase

A calorimetric study of the ilmenite and lithium niobate polymorphs of FeTiO₃ was undertaken to assess the high-pressure stabilities of these phases. Ilmenite is known to be the stable phase at ambient pressure, but the lithium niobate form may be a quench phase from a perovskite form which has been previously observed in situ at high pressure. In this study, the lithium niobate phase of FeTiO₃ was synthesized from an ilmenite starting material at 15– 16 GPa and 1473 K, using a uniaxial split-sphere high-pressure apparatus (USSA 2000). The energetics of the ilmenite to lithium niobate transformation were investigated through transposed-temperature drop calorimetry. The heat of back-transformation of lithium niobate to ilmenite was measured by dropping the sample in argon from ambient conditions to a temperature where the transformation occurs spontaneously. In drops made at 977 K, an intermediate x-ray amorphous phase was encountered. At 1273 K, the transformation went to completion. A value of -13.5±1.2 kJ/mol was obtained for the heat of transformation. A diamond anvil cell compression measurement on the lithium niobate phase was also performed, and indicated that the volume change for the ilmenite to lithium niobate transition is nearly pressure-independent, at-0.34cm³/mol. Thermodynamic calculations using the enthalpy of the ilmenite to lithium niobate transition, along with the volume change for the transition and previously published phase equilibrium data, indicate that the ilmenite to lithium niobate phase boundary is metastable with respect to an ilmenite to perovskite phase boundary, and the stable phase at high pressure is probably the perovskite phase.