Crystal-Physical Model of Ion Transport in Nonlinear Optical Crystals of KTiOPO4

Abstract

The ionic conductivity along the principal axes a, b, and c of the unit cell of the nonlinear-optical high-resistance KTiOPO₄ single crystals (rhombic syngony, space group Pna2₁), which are as-grown and after thermal annealing in vacuum, has been investigated by the method of impedance spectroscopy. The crystals were grown from a solution-melt by the Czochralski method. The as-grown KTiOPO₄ crystals possess a quasi-one-dimensional conductivity along the crystallographic c axis, which is caused by the migration of K⁺ cations: σ_║c = 1.0 × 10^–5 S/cm at 573 K. Wherein the characteristics of the anisotropy of ionic conductivity of the crystals is equal to σ_║c/σ_║a= 3 and σ_║c/σ_║b= 24. The thermal annealing at 1000 K for 10 h in vacuum increases the magnitude of σ_║c of KTiOPO₄ by a factor of 28 and leads to an increase in the ratio σ_║c/σ_║b= 2.1 × 10³ at 573 K. A crystal-physical model of ionic transport in KTiOPO₄ crystals has been proposed.