THE CRYSTAL-STRUCTURE OF POTASSIUM TITANYL PHOSPHATE (KTP) IN ITS HIGH-PRESSURE PHASE

The structure of the high-pressure phase of KTP has been determined at 6.2 GPa using Patterson projections, Fourier difference maps and least-squares refinement. Although the phase transition is strongly first-order and involves a 4.2% reduction of the unit-cell volume, the Pna2(1) space-group symmetry of the low-pressure phase is retained. The main structural changes involve tilting of the PO4 tetrahedra relative to the TiO6 octahedra, while both units remain relatively unchanged in size and shape. In particular, the TiO6 octahedra remain highly distorted and retain their anomalously short Ti-OT bonds. The potassium atoms are observed to undergo relatively large movements of about 0.5 angstrom along the polar c-axis.