Single crystals of Sn2P2S6, their optical and structural properties. (a) A photograph of single crystals of Sn2P2S6 studied in this work, (b) optical absorption spectrum of Sn2P2S6 near the absorption (α) edge. The same spectrum is presented in two dependencies of “ vs E” and “α 2 vs E” for determination of indirect and direct energy band gaps, respectively. ( c ): Full-profile Rietveld refinement in the ambient monoclinic Pn lattice (space group #7) of neutron diffraction pattern of Sn2P2S6 collected on crushed sample at ambient conditions. We used a structural model of this structure established in Refs. 16 and 17 . Points are the experimental data; solid line is a calculated profile; vertical bars mark the reflection positions for the monoclinic Pn lattice; and the lowermost curve gives the difference between the experimental and the calculated profiles. Inset in (c) shows a part of the same neutron diffraction pattern elongated along the x-axis. (d) Raman spectrum of Sn2P2S6 collected at ambient conditions. Insets display selected parts of the spectrum. Wave numbers are shown near each peak.
Photographs of Sn2P2S6 sample (it is pointed by arrow) inside diamond anvil cell in transmission light at different pressures during compression (the upper row of the photographs) and decompression cycles (the lower row of the photographs). One can see that the color of the sample reversibly changes with pressure from transparent yellow (at 1.1 GPa, it is already shifted to light orange) to black opaque and shiny. Sphere in the centre is a ruby ball used for pressure determination. Two black pieces near the sample of Sn2P2S6 are materials that were used as references for color change, silicon (small chip), and Mn3O4 (big one). Dashed circle on the photographs taken at 10.8 and 39.2 GPa points out silicon single crystal. At pressures above ∼12–14 GPa, silicon becomes metallic and changes its color from black silver to shiny white. One can see that at 39.2 GPa Sn2P2S6 shows the same luster as metallic silicon.
Pressure evolution of Raman spectra of Sn2P2S6 for pressurization ((a) and (b)) and decompression (c) cycles at ambient temperature. Pressure values are given in GPa units near each curve. (a) On the pressurization cycle, one can see apparent changes in the spectra collected at 6.7 GPa, 16.9–18.5 GPa, 28.7–30.2 GPa, and the signal loss at 39.2 GPa. Dashed circle indicates splitting of the main peak at a spectrum collected at 18.5 GPa. (b) Asterisks mark peaks form diamond anvil and ruby chip. (c) The decompression cycle shows both hysteresis in characteristic pressures and reversibility of pressure-driven changes.
Pressure dependencies of frequencies of basic phonons of Sn2P2S6 at ambient temperature for pressurization cycle. The inset shows a high-frequency range. Wave numbers together with their pressure coefficients are summarized in Table I . Vertical dashed lines separate different compression regimes that are most likely related to different phases labelled as I–VI.
Pressure evolution of frequency ((a) and (c)) and intensity (b) of the main phonon of Sn2P2S6 at ambient temperature. (a) Pressure values are given in GPa units near each curve. Spectra taken at pressures beyond 18.5 GPa are shown in 7.7-time-magnification for better visibility. (b) and (c) Filled symbols correspond to pressurization, and open ones to decompression cycles. Vertical dashed lines are the same as in Fig. 4 .
Pressure evolution of selected phonon frequencies of Sn2P2S6 at ambient temperature. Labels correspond to those in Figs. 4 and 5 .
Comparison of pressure dependencies of energy gaps of Sn2P2S6 and elemental sulfur. The curve for Sn2P2S6 is based on dEg /dP coefficients, 31 estimations of band gap at 20 GPa, 37 and band gap closure at 39 GPa observed in this work. The curve for sulfur is based on the data from Ref. 67 ; other studies reported metallization of sulfur already at 90 GPa. 57,58 Inset shows pressure dependence of electrical resistance of two samples of Sn2P2S6 labelled as #1 and #2 from Ref. 37 . Vertical dashed lines in the inset indicate different pressure phases, shown in Figs. 4–6 .
Wave numbers of basic phonons (ω 0) of Sn2P2S6 and estimations of their pressure coefficients and Grüneisen parameters ( , where B 0 is the bulk modulus; we assumed it to be similar to B 0 ∼ 44 GPa, as calculated for SnP2S6). 49
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