Change from sixfold to fivefold coordination of silicate polyhedra: insights from first-principles calculations of (formula presented)

Recent x-ray-diffraction observations found a triclinic phase of (Formula presented) stable under ambient conditions, related to a monoclinic phase with the titanite structure. It differs from the titanite structure by the elongation and loss of a Si-O bond within a (Formula presented) octahedron, thus having (Formula presented) polyhedra. We have studied both these phases using density-functional theory in the generalized gradients approximation, to investigate the relative stability and chemical changes between the phases. Mulliken analysis is used to calculate covalent bond populations and atomic charges. A series of intermediate structures are studied to approximate a prototype transition pathway.