Diffuse scattering anisotropy and theP21/a↔A2/aphase transition in titanite, CaTiOSiO4

Abstract

Diffuse scattering in titanite has been measured at three temperatures, 0.951Tc, 1.053Tcand 1.177Tc, using synchrotron radiation.Tc= 487 K is the temperature of the antiferroelectric paraelectric phase transition. Charge-coupled device (CCD) intensity measurements were centred about the \bar{4}01 reciprocal-lattice position (space-group settingP21/a) and extended beyond the neighbouring \bar{4}11 and \bar{4}\bar{1}1 fundamental positions. Planar diffuse scattering intensity is observed normal to [100] with a lens-shaped maximum centred at \bar{4}01. On approachingTcfrom above, the intensity of the maximum at \bar{4}01 increases, while intensity scattered into the planes decreases at large distances to the critical zone-boundary position. The intensity distribution is interpreted in terms of a two-dimensional spin-1/2 Ising model, in which individual spin states represent the collective displacement of octahedrally coordinated Ti atoms parallel to [100].