Nonlinear Optical Materials Structural Studies of Alkali Titanium Phosphates

Doctoral thesis, 2003

Crystals are beautiful, not only as pieces of jewellery, but also for their intrinsic properties, which are of significant scientific and technological importance. Ferroelectric materials are extremely versatile, since they may be combined with piezoelectric or electrooptic devices for use as filters, oscillators, memories, light detectors, and displays. Their high dielectric constant makes them useful as capacitors, whereas the nonlinear optical effect finds uses in second harmonic generation, frequency mixing, and optical parametric oscillation. This thesis focuses on the crystal growth and structural characterisation of crystalline alkaline titanium phosphate compounds, and particular on ferroelectric crystals that are isostructural with potassium titanyl phosphate (KTiOPO4, KTP). A series of germanium-doped RbTiOPO4 crystals have been characterised by X-ray diffraction, and dielectric measurements. This has resulted in the discovery of a direct relationship between the crystal structure and the ferroelectric to paraelectric phase transition temperature in KTP isostructures. Structural characterisation of strontium, barium, and chromium dopant inclusion in KTP crystals has been carried out by the use of synchrotron radiation. Sodium-doped KTP has been investigated by X-ray diffraction at 10.5 K, and at room-temperature. This resulted in an expansion of the phase transition model for KTP isostructural compounds, since a correlation between temperature-induced cation shifts and the Abrahams-Jamieson-Kurtz Tc criteria for oxygen-framework ferroelectrics could be introduced. Synchrotron radiation has been used to determine the TiP2O7 superstructure, which differs rather substantially from the AP2O7 parent structure. The very first titanium phosphate langbeinites with mixed titanium/rare-earth metal sites, i.e., K2MTi(PO4)3 (M = Er, Y, or Yb), have been structurally characterised by conventional X-ray diffraction. They belong to the very few known structures with atomic sites shared by both titanium and a rare-earth metal.