Resonance Raman study of the high-pressure phase transition in chromium-doped titanite, CaTiOSiO4

Titanite, CaTiOSiO4, was doped with chromium and the resultant Cr4+ ions, which replaced the Si4+ ions in the lattice gave rise to resonance effects in the Raman spectra recorded with an argon ion laser. The resonance‐enhanced Raman bands of the Cr4+ ions, and other bands, were used as a probe to investigate the pressure‐induced phase transition that occurs in CaTiOSiO4. This transition occurs at 3.0±0.2 GPa at ambient temperatures and involves a change in the symmetry from P21/a to A2/a in the crystals. In the P21/a‐phase, the Cr—O and Si—O bonds strengthen under compression, in contrast to the softening of the Ti—O bonds. This is due to the centering of the Ti ion in the octahedron and is caused by overbonding effects of titanium under high pressure. In the high‐pressure A2/a phase, considerable strengthening occurs in the Si—O bonds, exemplified by changes in the Grüneisen parameters of particularly the asymmetric stretching and bending modes, while the Ti—O stretching mode continues to soften. The pressure‐ and temperature‐induced phase transitions in titanite are compared and contrasted. Copyright © 2000 John Wiley & Sons, Ltd.