Photocreation and hyperbolic decay of Sb$^{\mathrm{2+}}$ in Sn$_{\mathrm{2}}$P$_{\mathrm{2}}$S$_{\mathrm{6}}$:Sb.

In Sn$_{\mathrm{2}}$P$_{\mathrm{2}}$S$_{\mathrm{6}}$:Sb, photorecharging of Sb$^{\mathrm{3+}}$ ions to Sb$^{\mathrm{2+}}$ causes a new interesting phenomenon -- photo-sensitizing of photorefraction.[1,2] The decay of the optically produced Sb$^{\mathrm{2+}}$ ions was directly measured through EPR and was found to have a hyperbolic character: \textasciitilde 1/(t/$\tau +$1) with an activation energy of 0.42 eV. This decay character and a very similar activation energy were also found in photorefraction and optically induced absorption experiments. The observed hyperbolic decay was explained through the set of rate equations that takes into account the EPR result: only Sb$^{\mathrm{3+}}$ ions are present in Sn$_{\mathrm{2}}$P$_{\mathrm{2}}$S$_{\mathrm{6}}$:Sb in thermal equilibrium. The long-wavelength onset of the EPR-measured Sb$^{\mathrm{2+}}$ ``photocreation'' spectrum together with the activation energy of the Sb$^{\mathrm{2+}}$ decay allowed to firmly locate the position of the Sb$^{\mathrm{2+/3+}}$ electron level in the bandgap of Sn$_{\mathrm{2}}$P$_{\mathrm{2}}$S$_{\mathrm{6}}$: 0.42 eV below the conduction band bottom. [1] D. R. Evans, A. Shuymelyuk, G. Cook, S. Odoulov. Opt. Lett. 36, 454 (2011). [2] Y. Skrypka, A. Shumelyuk, S. Odoulov, S. Basun, D. Evans, Opt. Comm. 356, 208 (2015).