TRANSFORMATION OF FIVEFOLD-COORDINATED SILICON TO OCTAHEDRAL SILICON IN CALCIUM SILICATE, CASI2O5

A single-crystal high-pressure diffraction study in a diamond-anvil ce ll shows that under hydrostatic conditions triclinic CaSi2O5 transform s to a monoclinic structure at a pressure between 0.17(2) and 0.205(5) GPa. The transition is of the zone-boundary type and first order in c haracter with a 2.9% volume change. The monoclinic phase can be recove red from the diamond-anvil cell. Its room pressure unit-cell parameter s are a = 6.5430(6) Angstrom, b = 8.3918(4) Angstrom, c = 6.3416(5) An gstrom, beta = 113.175(6)degrees, V = 320.10(4) Angstrom(3). The struc ture was refined using single-crystal X-ray intensity data and is of t he titanite aristotype, space group A2/a. Unlike the triclinic phase, which contains SiO5 polyhedra in addition to SiO4 tetrahedra and SiO6 octahedra, the monoclinic phase contains only tetrahedra ([Si-O] = 1.6 30 Angstrom) and octahedra ([Si-O] = 1.796 Angstrom). The phase transi tion between the monoclinic and triclinic phases of CaSi2O5 is the fir st example of a displacive-like transition involving a coordination ch ange of Si to be observed directly in a crystalline oxide or silicate.