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.