Sn vacancies in photorefractive Sn2P2S6 crystals: An electron paramagnetic resonance study of an optically active hole trap

Electron paramagnetic resonance (EPR) is used to identify the singly ionized charge state of the Sn vacancy (V-Sn(-)) in single crystals of Sn2P2S6 (often referred to as SPS). These vacancies, acting as a hole trap, are expected to be important participants in the photorefractive effect observed in undoped SPS crystals. In as-grown crystals, the Sn vacancies are doubly ionized (V-Sn(-)) with no unpaired spins. They are then converted to a stable EPR-active state when an electron is removed (i.e., a hole is trapped) during an illumination below 100K with 633 nm laser light. The resulting EPR spectrum has g-matrix principal values of 2.0079, 2.0231, and 1.9717. There are resolved hyperfine interactions with two P neighbors and one Sn neighbor. The isotropic portions of these hyperfine matrices are 167 and 79 MHz for the two P-31 neighbors and 8504 MHz for the one Sn neighbor (this latter value is the average for Sn-117 and Sn-119). These V-Sn(-) vacancies are shallow acceptors with the hole occupying a diffuse wave function that overlaps the neighboring Sn2+ ion and (P2S6)(4-) anionic unit. Using a general-order kinetics approach, an analysis of isothermal decay curves of the V-Sn(-) EPR spectrum in the 107-115 K region gives an activation energy of 283 meV. Published by AIP Publishing.