  Quasiharmonic lattice dynamics calculations are carried out on SiO2-TiO2
glass mixtures (TiO2 less than or equal to 20 weight percent) showing that the
addition of TiO2 systematically lowers the thermal expansion of high purity
fused silica. Adding ten weight percent titania lowers the thermal expansion
coefficient by approximately 0.5 ppm/K in agreement with observations. The
driving force for lowering the thermal expansion coefficient in the model
system is broken down into mode-by-mode contributions. The driving force is
distributed broadly across the frequency spectrum and is not localized into
particular frequency ranges that can be associated with the coordination
structure of Ti4+ in the simulated glass.
