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

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 Pna2(1)), 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: sigma(parallel to c) = 1.0 x 10(-5) S/cm at 573 K. Wherein the characteristics of the anisotropy of ionic conductivity of the crystals is equal to sigma(parallel to c) /sigma(parallel to a) = 3 and sigma(parallel to c) /sigma(parallel to b) = 24. The thermal annealing at 1000 K for 10 h in vacuum increases the magnitude of sigma(parallel to c) of KTiOPO4 by a factor of 28 and leads to an increase in the ratio sigma(parallel to c) /sigma(parallel to b) = 2.1 x 10(3) at 573 K. A crystal-physical model of ionic transport in KTiOPO4 crystals has been proposed.