Light diffraction byKTiOPO4, a quasi-one-dimensional ionic conductor, under a dc field

Single crystals of ${\mathrm{KTiOPO}}_4$ (Pna$2_1$) possess a quasi-one-dimensional (1D) ionic conductance with ${\sigma }_{?}$/${\sigma }_{\perp }$∼${10}^4$, where ${\sigma }_{?}$ and ${\sigma }_{\perp }$ are, respectively, the ionic conductivity parallel and normal to the c axis. A specimen was cut into a rectangular parallelepiped with edges parallel to the a, b, and c axes. Under a dc voltage applied to the c surfaces and a He-Ne laser beam incident normally to the a (or b) surface, a wide-angle diffraction band extending normal to the c axis was observed. If the electric displacement of the incident beam was parallel to the b (a) axis, that of the diffraction was parallel to the c axis, and vice versa. There was a delay time between the initial application of voltage and the appearance of the diffraction, and also between the removal of voltage and the disappearance of diffraction. This relaxation behavior suggests that the optical diffraction band and related phenomena are affected not directly by the ionic transportation but by the space charges left behind by the flow of charge carriers. Our theoretical analysis based on phase-type gratings composed of Fourier components of the distribution of space charges and on the electro-optic effect of the related internal field successfully explains all the interesting features of the diffraction band.