The two-step phase transition of titanite, CaTiSiO5 : a synchrotron radiation study

In this study it is shown that the antiferrodistortive phase transition in titanite, CaTiSiO 5 , is a stepwise process. We report the occurrence of an intermediate phase with Ca-disorder in split positions. Ordering of Ca triggers the displacement of Ti in the antiferroelectric phase at lower temperatures. A group-theoretical analysis for the two-step subduction process is given. The irreducible representation Y 2 + accounts for the intermediate phase A2/a (C2/c) with Ca cations in 8f Wyckoff positions below 825 K as well as for the subduction of the isotropy subgroup P2 1 /a below 496K. A high-symmetry phase with Ca in 4e positions exists only above ca. 825 K. In order to study the stepwise structural changes synchrotron radiation was used to collect X-ray diffraction data at 100 K, 295 K and 530 K on the 4-circle diffractometer at HASYLAB using a wavelength of 0.560(1) A. A total of 2292 reflections were recorded at 100 K, 1540 at 295 K and 1442 at 530 K and reduced to 705 (100 K), 707 (295 K) and 358 (530 K) unique observations respectively (sin θ/λ ≤ 0.611 A -1 for all data sets). Refinements with harmonic displacement parameters converged to R w = 0.038 (100 K), R w = 0.039 (295 K), and R w = 0.045 (530 K). The value for R w at 530 K was further reduced to 0.042 by assuming anharmonic temperature factors for Ca. A final significant improvement of the fit could be achieved for the 530 K data using splitted calcium atoms with occupation probability 0.5 in 8f positions (R w = 0.039). k + l = 2n + 1 Bragg-peaks disapper at 496 K. Between 496 K and 825 K anisotropic diffuse scattering of reflections k + l = odd is observed. APB's and deviations from a statistical distribution of disordered Ca can lead to such diffuse reflections with their intensity extended along b * and c * .