Charge trapping by iodine ions in photorefractive Sn₂P₂S₆ crystals

Electron paramagnetic resonance (EPR) is used to establish the role of iodine as an electron trap in tin hypothiodiphosphate (Sn₂P₂S₆) crystals. Iodine ions are unintentionally incorporated when the crystals are grown by the chemical-vapor-transport method with SnI₄ as the transport agent. The Sn₂P₂S₆ crystals consist of Sn²⁺ ions and (P₂S₆)^4- anionic groups. During growth, an iodine ion replaces a phosphorus in a few of the anionic groups, thus forming (IPS₆)^4- molecular ions. Following an exposure at low temperature to 633 nm laser light, these (IPS₆)^4- ions trap an electron and convert to EPR-active (IPS₆)^5- groups with S = 1/2. A concentration near 1.1 × 10¹⁷ cm^-3 is produced. The EPR spectrum from the (IPS₆)^5- ions has well-resolved structure resulting from large hyperfine interactions with the ¹²⁷I and ³¹P nuclei. Analysis of the angular dependence of the spectrum gives principal values of 1.9795, 2.0123, and 2.0581 for the g matrix, 232 MHz, 263 MHz, and 663 MHz for the ¹²⁷I hyperfine matrix, and 1507 MHz, 1803 MHz, and 1997 MHz for the ³¹P hyperfine matrix. Results from quantum-chemistry modeling (unrestricted Hartree-Fock/second-order Møller-Plesset perturbation theory) support the (IPS₆)^5- assignment for the EPR spectrum. The transient two-beam coupling gain can be improved in these photorefractive Sn₂P₂S₆ crystals by better controlling the point defects that trap charge.