Small polarons in selenium-doped ferroelectric Sn2P2S6 crystals

Abstract

Tin hypothiodiphosphate (Sn2P2S6) crystals, consisting of Sn2+ ions and (P2S6)4− anionic groups, are ferroelectric semiconductors below 337 K. Doping with Se forms (P2SeS5)4− anions and reduces the transition temperature, as isovalent Se ions replace S ions. In the present work, a hole is trapped on a (P2SeS5)4− unit during an exposure at 55 K to 532 nm laser light. The resulting (P2SeS5)3− defect has an unpaired spin and is best described as a small polaron. Electron paramagnetic resonance (EPR) spectra show that this polaron (i.e., trapped hole) has principal g values near 2.01 and resolved hyperfine interactions with two phosphorous nuclei. Hyperfine splittings are 7.1, 7.0, and 6.3 mT for one phosphorous nucleus and 19.9, 27.7, and 25.7 mT for the other phosphorous nucleus when the magnetic field is along the a, b, and c directions, respectively. The absence of a large hyperfine interaction with a nearby Sn nucleus indicates that the (P2SeS5)3− polaron does not have an adjacent Sn vacancy [see Golden et al., J. Appl. Phys. 120, 133101 (2016)]. Comparisons are made with similar holelike small polarons previously observed with EPR at low temperature in undoped and Te-doped Sn2P2S6 crystals. Because of the significant differences found in the distributions of their unpaired spin densities, the small polarons in these three materials provide an experimental data set of g values and hyperfine that can be used to validate results from advanced density-functional-theory modeling studies.