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

The crystal structures and bulk moduli of the known or proposed TiO2 polymo rphs (which include the low-pressure forms rutile, anatase, brookite, TiO2( B), TiO2(H) and TiO2(R), and the high-pressure columbite-, baddeleyite-, co tunnite-, 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 coordi nation ranging from six to eight, but fails for the nine-coordinated cotunn ite structure. The fixed-charge model is successful in predicting the cryst al structures of most of the polymorphs including the cotunnite-type, but f ails for the ramsdellite-structured TiO2(R) and is poor for baddeleyite-typ e TiO2. The variable charge model is more successful in predicting the bulk moduli of the low-pressure polymorphs, whereas the fixed charge model is m arginally better in predicting the bulk moduli of the high-pressure phases. The relative phase stabilities of the low-pressure phases and the high-pre ssure phase diagram obtained with the variable charge model are also in bro ad qualitative agreement with available calorimetric, phase equilibrium, an d simulation data. (C) 2001 Elsevier Science Ltd. All rights reserved.