High-pressure properties of materials
Ab initio calculations are used to predict a pressure-induced phase transition of molecular CO2 into non-molecular carbonate phases based on CO4 tetrahedra, at high pressures.
The problem: Depending on temperature and pressure conditions, Carbon-dioxide (CO2) shows different crystal structure:
· A molecular phase stable at ambient condition (Fig. a);
· Non-molecular solid phases, based on well-defined CO4 tetrahedra (Fig. b), at temperature of 1000 K and pressures in the range of 35 to 60 Gigapascals.
For different temperature and pressure conditions, the simulation performs:
· Optimization of the crystal structure.
· Total energy calculations.
Results: · For the optimized CO2 molecular lattice structure (Fig. a), errors are smaller than 0.3% with respect to the experimental data at 12 GPa. · The angle of the molecule relative to the vertical axis is calculate to be 52.5°, very close to the experimental value of 52°. · The structure transforms spontaneously into the non-molecular, CO4 tetrahedra-based solid (Fig. b), when the temperature is raised to 1000 K and the pressure slowly increased up to 100 GPa, · Experiments confirm the prediction for the non-molecular solid (Iota and Yoo, 2001).
Tools: First Principle Molecular Dynamics (FPMD) code, available within Quantum Espresso package, to carry out molecular dynamics simulations in the constant-pressure ensemble, by means of a variable cell algorithm (Bernasconi et al., 1995).
References: · Bernasconi M., Chiarotti G. L., Focher P., Scandolo S., Tosatti E., Parrinello M.: First-Principle constant pressure molecular dynamics. J. Phys. Chem. Solids 56 (1995) 501. · Serra, S., Cavazzoni C., Chiarotti G. L., Scandolo S., Tosatti E.: Pressure-induced solid carbonates from molecular CO2 by computer siulation. Science 284 (1999) 788-790.