THE 2-STEP PHASE-TRANSITION OF TITANITE, CATISIO5 - A SYNCHROTRON-RADIATION STUDY

In this study it is shown that the antiferrodistortive phase transitio n in titanite, CaTiSiO5, is a stepwise process. We report the occurren ce of an intermediate phase with Ca-disorder in split positions. Order ing of Ca triggers the displacement of Ti in the antiferroelectric pha se at lower temperatures. A group-theoretical analysis for the two-ste p subduction process is given. The irreduci ble representation Y-2+ ac counts for the intermediate phase A2/a (C2/c) with Ca cations in 8f Wy ckoff positions below 825 K as well as for the subduction of the isotr opy subgroup P2(1)/a below 496 K. 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 dif fraction data at 100 K, 295 K and 530 K on the 4-circle diffractometer at HASYLAB using a wavelength of 0.560(1) Angstrom. A total of 2292 r eflections 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 observatio ns respectively (sin theta/lambda less than or equal to 0.611 Angstrom (-1) for all data sets). Refinements with harmonic displacement parame ters 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 signif icant improvement of the fit could be achieved for the 530 K data usin g splitted calcium atoms with occupation probability 0.5 in 8f positio ns (R(w) = 0.039). k + l = 2n + 1 Bragg-peaks disapper at 496 K. Betwe en 496 K and 825 K anisotropic diffuse scattering of reflections k + l = odd is observed. APB's and deviations from a statistical distributi on of disordered Ca can lead to such diffuse reflections with their in tensity extended along b and c*.