Abstract
A brief review focuses on studies into the structural reasons for the nonlinear optical properties of crystals of the potassium titanyl phosphate family, performed at the Shubnikov Institute of Crystallography. Accurate X-ray diffraction data are discussed, providing evidence that the optical susceptibility of crystals is related not only to the alternation of long and short Ti–O bonds in the chains of TiO6 octahedra, but to the geometry of tetrahedral anions and the alkaline cation arrangement in the structure channels, as well. The contribution of each of the three structural components depends on the crystal composition.
Highlights
Crystals of the potassium titanyl phosphate KTiOPO4 family (KTP) are materials of a special type, namely, ferroelectrics-superionic conductors
The second harmonic generation (SHG) signal increases by 10% and 20% upon the partial (~5% and 4% respectively) substitution of niobium for titanium [10,11,12], by a factor of approximately two upon the partial substitution of zirconium for titanium [13,14], and by 30–40% in KTP crystals doped with hafnium [15]
A large number of studies of the atomic structure of crystals of the KTP family have been conducted, some of them at the Institute of Crystallography [33]. These studies were quite high level, but in order to analyze the defect structure of single crystals and structural reasons for their physical properties, a new, accurate approach to data collection was needed. This approach was used in the study of the atomic structure of crystals of pure KTP [34], specially selected single crystals (KTA [35], KTP, doped with zirconium (KTP:Zr) [36], hafnium (KTP:Hf) [37], and niobium (KTP:Nb) [38]) whose nonlinear susceptibility was higher than that of KTP crystals
Summary
Crystals of the potassium titanyl phosphate KTiOPO4 family (KTP) are materials of a special type, namely, ferroelectrics-superionic conductors. The results of a study of neutron total (Bragg and diffusion) scattering in KTP crystals [29] from room temperature to 900 ◦ C suggested that changes in the local arrangement of oxygen atoms around Ti4+ and the displacement of K+ were the reasons for the SHG signal decrease with increasing temperature, and a significant part of the SHG effect came from potassium cations. These studies were quite high level, but in order to analyze the defect structure of single crystals and structural reasons for their physical properties, a new, accurate approach to data collection was needed This approach was used in the study of the atomic structure of crystals of pure KTP [34], specially selected single crystals (KTA [35], KTP, doped with zirconium (KTP:Zr) [36], hafnium (KTP:Hf) [37], and niobium (KTP:Nb) [38]) whose nonlinear susceptibility was higher than that of KTP crystals.
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