Photoinduced EPR study of Sb2+ions in photorefractive Sn2P2S

Abstract

Single crystals of Sn 2 P 2 S 6 are both ferroelectric and photorefractive. Antimony (Sb) ions are optically active in this material and play an important role in optimizing the photorefractive response. Electron paramagnetic resonance (EPR) is used to determine the site and charge states of the Sb ions in Sn 2 P 2 S 6 and to illustrate the photocharging behavior of these ions. In as-grown crystals, Sb 3+ ions substitute for Sn 2+ ions. A multiline EPR spectrum from Sb 2+ ions is observed after exposing a crystal at 30 K to either 633-or 442-nm laser light. These Sb 2+ ions are thermally stable at low temperature after the light is removed. They revert back to Sb 3+ ions when the crystal is warmed above 250 K for a few minutes. The EPR spectrum has S = 1/2 and consists of well-resolved sets of hyperfine lines from 121 Sb and 123 Sb nuclei. Spin Hamiltonian parameters are obtained from the angular dependence of the spectrum (principal values are 1.810, 1.868, and 1.887 for the g matrix and 1404, 1687, and 1849 MHz for the 121 Sb hyperfine matrix). These parameters provide evidence that the wave function for the unpaired spin has significant p-like character and has overlap with neighboring ions.

Dual role of Sb ions as electron traps and hole traps in photorefractive Sn_2P_2S_6 crystals

“…After returning to 42 K following this brief warming, the spectrum in Fig. 1 (blue stick diagrams) has been previously reported [12]. In the present paper, our focus is on the newly observed (Sb-V Sn ) 0 trapped-hole defects.…”

“…Corresponding principal values for the 123 Sb hyperfine matrix can be extrapolated from the 121 Sb results using the known nuclear magnetic moments. Results for the Sb 2+ electron center [12] are also included in Table 2. For the (Sb-V Sn ) 0 center, the 12 × 12 matrix form of the spin Hamiltonian (H = βS·g·B + I·A·S) was repeatedly diagonalized in a least-squares fitting process as the g and A matrix parameters were varied.…”

“…These Sb 3+ ions and Sn 2+ vacancies may be well separated and thus act as isolated electron and hole traps, or they may thermally migrate to each other and form a close-associate pair (i.e., a donoracceptor pair) that acts only as a hole trap. A recent electron paramagnetic resonance (EPR) study has shown that isolated Sb 3+ ions trap an electron when an SPS crystal is illuminated at low temperature with light near the band edge [12]. The resulting Sb 2+ (5s 2 5p 1 ) ions are stable for sample temperatures below approximately 200 K. Their thermal decay at higher temperatures is described by second-order kinetics with an activation energy of 420 meV [13].…”

Light induced absorption and optical sensitizing of Sn 2 P 2 S 6 :Sb

“…The latter technique of SPS sensitizing appeared to be quite effective for antimony doped crystals [5], when the preexposure was achieved with the intense light of the same wavelength as the recording light (633 nm line of He-Ne laser which is traditionally used for photorefractive grating recording in SPS). It was established recently that the photogeneration of the secondary photorefractive centers is somehow related to recharging of the antimony defect center from an initially trivalent to a divalent state [10].…”

Hyperbolic decay of photo-created Sb2+ ions in Sn2P2S6:Sb crystals detected with electron paramagnetic resonance

“…3 The Sn 2 P 2 S 6 :Sb crystals were grown at the Uzhgorod National University, Ukraine, by the chemical vapor transport method. 6 where the hole is localized on a sulfur ion adjacent to a Sn 2þ vacancy. There is only a small number of the published reports where photo-EPR has been utilized for isothermal trap decay measurements.…”