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<p>The present work presents a study of single-crystal samples of Ag<sub>7+<i>x</i></sub>(P<sub>1-<i>x</i></sub>Si<sub><i>x</i></sub>)S<sub>6</sub> (<i>x</i> = 0.1, 0.25, 0.5, 0.75) solid solutions. These samples were studied using various techniques – XRD, SEM optical ellipsometry and spectroscopy methods. It has been determined that the compositions of Ag<sub>7+x</sub>(P<sub>1-x</sub>Si<sub>x</sub>)S<sub>6</sub> solid solutions crystallize in different space groups: P2<sub>1</sub>3 (Ag<sub>7.1</sub>P<sub>0.9</sub>Si<sub>0.1</sub>S<sub>6</sub>), F-43m (Ag<sub>7.25</sub>P<sub>0.75</sub>Si<sub>0.25</sub>S<sub>6</sub>) and Pna2<sub>1</sub> (Ag<sub>7.5</sub>P<sub>0.5</sub>Si<sub>0.5</sub>S<sub>6</sub>, Ag<sub>7.75</sub>P<sub>0.25</sub>Si<sub>0.75</sub>S<sub>6</sub>). The microstructural SEM investigation reveals the presence of microdefects in Ag<sub>7.25</sub>P<sub>0.75</sub>Si<sub>0.25</sub>S<sub>6</sub> and Ag<sub>7.75</sub>P<sub>0.25</sub>Si<sub>0.75</sub>S<sub>6</sub> samples. The refractive index <i>n</i> dispersion is nonlinear and the observed maximum is located in the region of the optical absorption edge of crystals. Compositional dependence of <i>n</i> shows nonlinear monotonic increase of <i>n</i> values in range of 2.58–2.68 with an increase in Si<sup>4+</sup> content. The dispersion dependences of refractive indices are described using the single-oscillator Wemple-DiDomenico model. Which established that the P<sup>5+</sup> → Si<sup>4+</sup> substitution leads to a monotonic nonlinear decrease in the values of single-oscillator and dispersion energies. The transmission spectra demonstrate a shift towards longer wavelengths with Si<sup>4+</sup> substitution and a shift towards shorter wavelengths with decreasing temperature. The determined absorption coefficient follows an exponential form and obeys to Urbach’s rule at all temperatures. The values of pseudo-band gap <i>E</i><sub><i>g</i></sub><sup><i>*</i></sup> (1.895–1.628&#xa0;eV) and Urbach energy <i>E</i><sub><i>U</i></sub><i> (</i>92.4–60.2&#xa0;meV) were determined and its monotonic decrease was observed with Si<sup>4+</sup> substitution. Established that for all single crystals, a decrease in temperature leads to a monotonic increase in the <i>E</i><sub><i>g</i></sub><sup><i>*</i></sup> values. The change in the <i>E</i><sub><i>U</i></sub> for Ag<sub>7+<i>x</i></sub>(P<sub>1-<i>x</i></sub>Si<sub><i>x</i></sub>)S<sub>6</sub> single crystals is complicated due to existing microstructural defects.</p>

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Unveiling the optical characteristics of Ag7+x(P1-xSix)S6 single crystals through combined ellipsometry and UV–Vis spectroscopy

  • 期刊论文

  • 发表日期：

    2025年7月4日

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  • Artem Pogodin,
  • Mykhailo Pop,
  • Iryna Shender,
  • Mykhailo Filep,
  • Tetyana Malakhovska,
  • Serhii Vorobiov,
  • Vladimir Komanicky,
  • Vitaliy Bilanych,
  • Oleksandr Kokhan,
  • Ruslan Mariychuk

摘要

The present work presents a study of single-crystal samples of Ag7+x(P1-xSix)S6 (x = 0.1, 0.25, 0.5, 0.75) solid solutions. These samples were studied using various techniques – XRD, SEM optical ellipsometry and spectroscopy methods. It has been determined that the compositions of Ag7+x(P1-xSix)S6 solid solutions crystallize in different space groups: P213 (Ag7.1P0.9Si0.1S6), F-43m (Ag7.25P0.75Si0.25S6) and Pna21 (Ag7.5P0.5Si0.5S6, Ag7.75P0.25Si0.75S6). The microstructural SEM investigation reveals the presence of microdefects in Ag7.25P0.75Si0.25S6 and Ag7.75P0.25Si0.75S6 samples. The refractive index n dispersion is nonlinear and the observed maximum is located in the region of the optical absorption edge of crystals. Compositional dependence of n shows nonlinear monotonic increase of n values in range of 2.58–2.68 with an increase in Si4+ content. The dispersion dependences of refractive indices are described using the single-oscillator Wemple-DiDomenico model. Which established that the P5+ → Si4+ substitution leads to a monotonic nonlinear decrease in the values of single-oscillator and dispersion energies. The transmission spectra demonstrate a shift towards longer wavelengths with Si4+ substitution and a shift towards shorter wavelengths with decreasing temperature. The determined absorption coefficient follows an exponential form and obeys to Urbach’s rule at all temperatures. The values of pseudo-band gap Eg* (1.895–1.628 eV) and Urbach energy EU (92.4–60.2 meV) were determined and its monotonic decrease was observed with Si4+ substitution. Established that for all single crystals, a decrease in temperature leads to a monotonic increase in the Eg* values. The change in the EU for Ag7+x(P1-xSix)S6 single crystals is complicated due to existing microstructural defects.