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[R2-01] Prediction of low-temperature and high-pressure phase transition in keatite (SiO2)
Keywords:keatite, SiO2, phase transition, negative thermal expansion coefficient, molecular dynamics
Keatite is a naturally rare polymorph of SiO2 that has no stable region. Although keatite is not the official mineral name, it has been widely used for over 60 years and is used here as well. keatite shows negative volume thermal expansion from room temperature to 250 ºC (P. Keat, Science, 120, 328-339, 1954), which is unique among the low-pressure silica polymorphs. To explain this, we hypothesized that room temperature keatite is a high-temperature phase like beta-quartz. To test this, we performed DFT and classical MD calculations.
Vibrational mode calculations at ambient pressure of 0 K revealed a soft mode (see Figure), from which the structure changes from the original tetragonal keatite structure to a slightly distorted orthorhombic lattice. The transition to this orthorhombic phase occurs above 0.4 GPa. From MD calculations at ambient pressure, it was observed that the time-averaged a/b-axis ratio deviates from 1 at low temperatures. Even near room temperature, it oscillates in time between two equivalent tetragonal structures, suggesting that the tetragonal keatite is realized as a time average. Similar to beta-quartz, the negative thermal expansion coefficient in keatite may be due to its high-temperature phase. The hypothesis is supported by the calculations. However, the orthorhombic phase has not been experimentally confirmed at temperatures down to -100 ºC at this time. Further experiments are ongoing.
Vibrational mode calculations at ambient pressure of 0 K revealed a soft mode (see Figure), from which the structure changes from the original tetragonal keatite structure to a slightly distorted orthorhombic lattice. The transition to this orthorhombic phase occurs above 0.4 GPa. From MD calculations at ambient pressure, it was observed that the time-averaged a/b-axis ratio deviates from 1 at low temperatures. Even near room temperature, it oscillates in time between two equivalent tetragonal structures, suggesting that the tetragonal keatite is realized as a time average. Similar to beta-quartz, the negative thermal expansion coefficient in keatite may be due to its high-temperature phase. The hypothesis is supported by the calculations. However, the orthorhombic phase has not been experimentally confirmed at temperatures down to -100 ºC at this time. Further experiments are ongoing.