Photoactive defect centers in photorefractive Sn$_{\mathrm{2}}$P$_{\mathrm{2}}$S$_{\mathrm{6}}$.

A variety of intrinsic and extrinsic defect centers were identified through photo-EPR studies of photorefractive Sn$_{\mathrm{2}}$P$_{\mathrm{2}}$S$_{\mathrm{6}}$ crystals: tin and sulfur vacancies, intrinsic small hole polarons (Sn$^{\mathrm{3+}}$ ions), dopant antimony ions on tin sites -- isolated (with no nearby perturbing defects) and with a charge-compensating tin vacancy at a nearest-neighbor tin site. All these centers are photo-rechargeable: their charge states alter under optical irradiation (for example, optical irradiation causes conversion of Sb$^{\mathrm{3+}}$ to Sb$^{\mathrm{2+}})$, so that they can serve as donors or traps of free charge carriers thus providing photo-refraction. The potential of the photo-rechargeable defect centers for photorefraction applications was evaluated through the measurements of the spectral range where photo-recharging occurs, the thermal stability of the photo-recharged states, the kinetic characters and activation energies of thermal relaxation (``general order'' kinetics with b$=$1.68 and activation energy of 0.28 eV for singly ionized tin vacancy; pure second order kinetics (b$=$2) or, in other words, hyperbolic decay 1/(1$+$t/$\tau )$ and activation energy 0.42 eV for isolated Sb$^{\mathrm{2+}}$ ions).