Influence of the electron and gamma-irradiation on dielectric  properties of Sn2P2S6


In this work the results of our studies of the effect of high-energy irradiation of the nominally undoped Sn2P2S6 crystals grown by vapor-transport technique on their dielectric spectra are presented. The oriented (polar cut) samples were irradiated with gamma-rays with an average energy ~20 MeV with various exposition times (from 15 to 45 min). It was found that the obtained variations in dielectric spectra, caused by the irradiation, are unstable, and demonstrate slow changes: the measurements performed with time intervals of about 24 hours show that the relaxation to the initial state is reached during 2-3 weeks. Besides, an analysis of the dielectric spectra leads to the following conclusions: gamma-radiation stimulates an increase of the dielectric constant ε' at low frequencies (20-100 Hz), measured immediately after pre-illumination, approximately in five times. The dielectric losses also increase in 1.5-2 times. Most likely this occurs due to an appearance of additional charged defects, movements of which in measuring field produces an additional contribution to the ε' values. These charged defects produce stronger impacts at low frequencies, indicating on their relatively large size and/or their low movability, i.e. their localization. The measured dielectric spectra were compared with the same results obtained after radiation with fast electrons and intense white light. After lightening, as well as beta-irradiation, we observed decreasing of the value of low-frequency (~10 kHz) dielectric constant ε0 with time. In contrast, gamma-radiation leads to increasing of the dielectric constant, and subsequent relaxation with saturation after ~40 hours. The time evolution of the efficient parameters of the dielectric dispersion (dielectric contribution Δε, relaxation time τ and diffusion parameter α) were estimated. After irradiation by gamma-rays and fast electrons is observed a decreasing of the Δε, α and time constant values. After lighting is observed more complicate relaxation when these parameters initially are rising, and then are falling. This indicates that the white-light lightening has a stronger effect on the dielectric properties of the Sn2P2S6 crystals as compared with gamma- and beta-radiation. The results of our experiments with preliminary irradiated by gamma-rays, fast electrons or intense white light indicate on substantially different mechanisms of the induced defects, and relatively slow subsequent relaxation of the dielectric parameters. Note that variations of the optical density induced by the radiation were did not marked

Keywords: dielectric spectra, ferroelectric, β and γ irradiation