Behaviour of the non-linear optical material KTiOPO4 in the temperature range 293-973 K studied by x-ray diffractometry at high resolution:: alkaline displacements

The crystal structure of potassium titanyl phosphate, KTiOPO4 (space group Pna2(1)), has been refined at room temperature, at 673 K, and at 973 K, by using accurate single-crystal x-ray diffraction techniques at high resolution (d(min) = 0.35 Angstrom). The data show a large amount of anharmonic motion of the potassium ions, increasing with temperature. To describe this motion, two models are developed: a normal refinement including potassium anharmonic thermal displacement parameters, which describes the average motion of the alkaline sites, and another model in which the potassium sites are split within the harmonic approximation and the displacements of the potassium ions versus temperature are described more precisely.