High-pressure phase transition in CaTiOSiO4 titanite

The P2(1)/a to A2/a transition in titanite has been studied at high pressur e (and room temperature) by X-ray powder diffraction. On the basis of the d isappearance of k + l = odd reflections from the diffraction pattern, the t ransition was located between 3.351(3) and 3.587(3) GPa. The variation with pressure of the spontaneous strain in the P2(1)/a phase indicates that the transition has an effective critical exponent significantly less than 1/2. Within the uncertainties of the data, the transition could be weakly first -order in character or be continuous with an effective critical exponent in the range of similar to 0.13-0.25. The A2/a to P2(1)/a transition is accom panied by a significant expansion of the a-axis as a result of the bond-val ence sum requirements of the Ti atoms: in the high-pressure phase they occu py the centres of the TiO6 octahedra, but they are displaced along the a-ax is in the P2(1)/a phase to form alternating short and long Ti-O bonds.