Photoinduced EPR study of Sb2+ions in photorefractive Sn2P2S6crystals

Single crystals of Sn${}_{2}$P${}_{2}$S${}_{6}$ are both ferroelectric and photorefractive. Antimony (Sb) ions are optically active in this material and play an important role in optimizing the photorefractive response. Electron paramagnetic resonance (EPR) is used to determine the site and charge states of the Sb ions in Sn${}_{2}$P${}_{2}$S${}_{6}$ and to illustrate the photocharging behavior of these ions. In as-grown crystals, Sb${}^{3+}$ ions substitute for Sn${}^{2+}$ ions. A multiline EPR spectrum from Sb${}^{2+}$ ions is observed after exposing a crystal at 30 K to either 633- or 442-nm laser light. These Sb${}^{2+}$ ions are thermally stable at low temperature after the light is removed. They revert back to Sb${}^{3+}$ ions when the crystal is warmed above 250 K for a few minutes. The EPR spectrum has $S$ $=$ 1/2 and consists of well-resolved sets of hyperfine lines from ${}^{121}$Sb and ${}^{123}$Sb nuclei. Spin Hamiltonian parameters are obtained from the angular dependence of the spectrum (principal values are 1.810, 1.868, and 1.887 for the $g$ matrix and 1404, 1687, and 1849 MHz for the ${}^{121}$Sb hyperfine matrix). These parameters provide evidence that the wave function for the unpaired spin has significant $p$-like character and has overlap with neighboring ions.