STRUCTURAL PHASE-TRANSITION NEAR 825 K IN TITANITE - EVIDENCE FROM INFRARED SPECTROSCOPIC OBSERVATIONS

We report the direct experimental observation of a structural anomaly near 825 K in synthetic and natural titanite samples by high-temperatu re, hard-mode infrared spectroscopy. The anomaly in titanite, CaTiSiO5 , is characterized by a break of the temperature dependence of the 562 cm(-1) Si-O bending mode, the 675 cm(-1) Ti-O band, and the 900 cm(-1 ) Si-O stretching modes, and the rapid decrease of the IR signal at 87 3 cm(-1). The order parameter, as determined from the temperature evol ution of the frequencies of the absorption bands in the middle infrare d (MIR) region follows a second-order Landau behavior with an order-pa rameter exponent beta = 1/2. At T > 825 K, the Ti-O band shows further softening, whereas the Si-O bands at 562 and 900 cm(-1) show hardenin g with increasing temperature. in natural samples, the effects of impu rities on the high-temperature transition are weak. For pure titanite, the transition temperature, T-c, is about 825 K and increases with in creasing impurity concentration.