High-temperature structural study of the P2 1 /a A2/a phase transition in synthetic titanite, CaTiSiO 5

Synthetic CaTiSiOu has been shown to undergo a reversible, displacive phase transition at 220" + 20oC from space group P2r/a to A2/a, using high-temperature single crystal X-ray diffraction techniques. The transition is signaled by disappearance of reflections of the type hkl: k + / odd. The structure of a synthetic P2r/a sphene has been refined at 25"C, 165"C' 27O.C.5l5oC. and 740oC to conventional R-factors of 0.048, 0.047,0.033,0.039, and 0.043' respectively. The principal structural change characterizing the phase transition involves a movement of the titanium atom from a position displaced toward one corner of the TiOu octahedron to a position at the geometric center of the octahedron, a shift of 0.10 A. Ti-O bonds to the corner-shared oxygens change from 1.77 and 1.97 A at room temperature to 1.87 A at 270'C. Above the transition further atomic movement is slight and is consistent with simple cell expansion. The transition mechanism is similar to other displacive transitions in silicates and perovskite structure ferroelectrics and may lead to domain formation.