High pressure polymorphism of dicalcium silicate Ca2SiO4. A transmission electron microscopy study

Abstract

Ca2SiO4 dicalcium silicate has been transformed at high pressure in a diamond-anvil cell (DAC) coupled with a YAG laser heater, in order to study the high-pressure modifications of this compound. Starting material was the olivine form of Ca2SiO4 (γ-polymorph). Several samples have been synthesized at loading pressures of 4.5, 10 and 15 GPa respectively, at room temperature. Other samples have been obtained at pressures ranging between 4.5 and 45 GPa and temperatures estimated to be about 2500 °C. The study of the quenched high pressure and/or high temperature phases has been performed using analytical transmission electron microscopy (ATEM) and X-ray diffraction (XRD). All the polymorphs of Ca2SiO4 usually produced with high temperatures, including α-Ca2SiO4, have been observed in the samples recovered from the high-pressure experiments. The α′-Ca2SiO4 and α-Ca2SiO4 polymorphs have been obtained at ambient conditions for the first time without stabilizing impurities. A new modification of α′-Ca2SiO4 has also been synthesized. Finally, the breakdown at high-pressure and temperature of Ca2SiO4 into CaSiO3 and CaO is reported.