Dual Role of Sb Ions as Electron Traps and Hole Traps in Photorefractive Sn₂P₂S₆ Crystals

Doping photorefractive single crystals of Sn2P2S6 with antimony introduces both electron and hole traps. In as-grown crystals, Sb (5s) ions replace Sn ions. These Sb ions are either isolated (with no nearby perturbing defects) or they have a chargecompensating Sn vacancy at a nearest-neighbor Sn site. When illuminated with 633 nm laser light, isolated Sb ions trap electrons and become Sb (5s5p) ions. In contrast, Sb ions with an adjacent Sn vacancy trap holes during illumination. The hole is primarily localized on the (P2S6) anionic unit next to the Sb ion and Sn vacancy. These trapped electrons and holes are thermally stable below ∼200 K, and they are observed with electron paramagnetic resonance (EPR) at temperatures below 150 K. 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