A neutron total scattering study of local coordination in KTiOPO ₄ from room temperature to 900 °c

Neutron total (Bragg plus diffuse) scattering data have been analysed by means of bond valence constrained reverse Monte Carlo modelling, using RMCProfile software, in order to simultaneously probe the natures of the short-and long-range structural correlations in potassium titanyl phosphate, KTiOPO₄ (KTP). The diffraction data were collected at room temperature, 300, 500, 700 and 900 °C, and the resulting partial radial distribution functions, g_jk(r), were thoroughly investigated in order to obtain information about the structural aspects that cause reduced second harmonic generation (SHG) with increasing temperature in KTP. The-TiO ₆-PO₄-lattice shows very little rigid unit motion up to 900 °C and the PO₄ tetrahedra were likewise unchanged. However, subtle changes regarding oxygen disordering around the TiO₆ octahedra, i.e.the number of short titanyl bonds, and K⁺ cation displacement along the c axis are carefully mapped out. The latter has a direct detrimental effect on the SHG response, whilst the anisotropic Ti-O bond distribution seems to be a prerequisite for the SHG effect.