The structural pressure dependence of potassium titanyl phosphate (KTP) to 8 GPa

The crystal structure of KTiOPO4 (KTP) has been determined as a function of pressure to 8 GPa at ambient temperature, using single-crystal x-ray diffraction techniques. The first-order isosymmetric phase transition, at 5.8 GPa, appears to have a strong effect on the compression mechanism of the structure: in the low-pressure phase the structure compresses by polyhedral tilting within the cross-linking TiO6-PO4 chains while in the high-pressure phase relative translations and rotations of the chains themselves account for a significant proportion of the unit-cell volume reduction both at the transition and on further pressure increase. The distortion of the framework at the phase transition is accompanied by relatively large displacements of the potassium atoms to positions more closely related to the Pnan symmetry of the high-temperature phase. A comparison of the pressure-induced changes of the highly distorted TiO6 octahedra with recent high-pressure optical measurements indicates that neither the lengths nor the orientations of the Ti-O bonds correlate, at least directly, with the observed variation of the second-harmonic-generating (SHG) power of the crystal. This strongly suggests that other regions of the structure may be important for the SHG properties.