Identification of a radiation-induced hole center inKTiOPO4

Electron-paramagnetic-resonance (EPR) and electron-nuclear double-resonance (ENDOR) techniques have been used to determine the structure of a radiation-induced trapped-hole center in potassium titanyl phosphate (${\mathrm{KTiOPO}}_4$ or KTP). A single crystal of KTP was irradiated with 60-kV x rays at 77 K to produce the S=1/2 hole center. Data were then taken near 30 K. Spin-Hamiltonian parameters describing the g matrix and three ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ hyperfine matrices were obtained from the angular dependence of the EPR and ENDOR spectra, respectively. The principal values of the g matrix are 2.0008, 2.0250, and 2.0422 and the principal values of the largest phosphorus interaction are -6.809, -8.125, and -3.848 MHz. An analysis of the spin-Hamiltonian matrices (i.e., the magnitudes of their principal values and the directions of their principal axes) leads to a model wherein the hole is localized on a bridging oxygen ion between two titanium ions. The hole is stabilized at this site by a nearest-neighbor potassium vacancy. This defect is observed in both hydrothermally grown and flux-grown KTP, and it is stable only up to approximately 160 K.