Anti-phase boundaries and phase transitions in titanite: An X-ray diffraction study

Abstract X-ray diffraction rocking curves of titanite, CaTiSiO5, were measured using a novel high-resolution diffractometer. Three Bragg reflections were recorded as a function of temperature and their profiles analyzed in terms of a Gaussian Bragg peak and a diffuse scattering component with an overall Lorentzian shape. The temperature dependence of the Gaussian intensities of the rocking peaks of superstructure reflections, hkl with k + l odd, scale with the long-range order parameter as I ∞ Q2 ∞ |T - Tc|2β, where β = 0.14(1) is the effective order parameter exponent. The diffuse scattering intensity changes little with temperature at T < Tc - 20 K. Strong diffuse scattering is found at T > Tc with centers of their Lorentzian diffraction profiles shifted by Δω ≈ 0.5° with respect to the position of the equivalent Bragg peak. A second phase transition at 825 K is confirmed and the possibility of a third transition at ~ 1150 K is discussed.