Diffuse scattering anisotropy and the P2(1)/a <-> A2/a phase transition intitanite, CaTiOSiO4

Diffuse scattering in titanite has been measured at three temperatures, 0.9 51T(c), 1.053T(c) and 1.177T(c), using synchrotron radiation. T-c = 487 K i s the temperature of the antiferroelectric paraelectric phase transition. C harge-coupled device (CCD) intensity measurements were centred about the (4 ) over bar 01 reciprocal-lattice position (space-group setting P2(1)/a) and extended beyond the neighbouring (4) over bar 11 and (4) over bar(1) over bar1 fundamental positions. Planar diffuse scattering intensity is observed normal to [100] with a lens-shaped maximum centred at (4) over bar 01. On approaching T-c from above, the intensity of the maximum at (4) over bar 01 increases, while intensity scattered into the planes decreases at large di stances 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 octahedral ly coordinated Ti atoms parallel to [100].