Atomistic simulation of the crystal structures and bulk moduli of TiO2 polymorphs

The crystal structures and bulk moduli of the known or proposed TiO2 polymorphs (which include the low-pressure forms rutile, anatase, brookite, TiO2(B), TiO2(H) and TiO2(R), and the high-pressure columbite-, baddeleyite-, cotunnite-, pyrite-, and fluorite-structured forms) have been simulated using static lattice energy minimisation and two independent forcefields, namely a variable charge Morse potential and a fixed charge rigid ion potential. The results obtained demonstrate that the variable charge model is capable of describing the crystal structures of all the polymorphs with Ti-O coordination ranging from six to eight, but fails for the nine-coordinated cotunnite structure. The fixed-charge model is successful in predicting the crystal structures of most of the polymorphs including the cotunnite-type, but fails for the ramsdellite-structured TiO2(R) and is poor for baddeleyite-type TiO2. The variable charge model is more successful in predicting the bulk moduli of the low-pressure polymorphs, whereas the fixed charge model is marginally better in predicting the bulk moduli of the high-pressure phases. The relative phase stabilities of the low-pressure phases and the high-pressure phase diagram obtained with the variable charge model are also in broad qualitative agreement with available calorimetric, phase equilibrium, and simulation data. (C) 2001 Elsevier Science Ltd. All rights reserved.