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 x 10(17) cm(-3) is produced. The EPR spectrum from the (IPS6)(5-) ions has well-resolved structure resulting from large hyperfine interactions with the I-127 and P-31 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 I-127 hyperfine matrix, and 1507 MHz, 1803 MHz, and 1997 MHz for the P-31 hyperfine matrix. Results from quantum-chemistry modeling (unrestricted Hartree-Fock/second-order MOller-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.