Photoconductivity and Thermal Expansion of (TlInS2)1–x(TlGdS2)x Solid Solutions

Based on previously established concepts regarding chemical bonding in the crystal lattice of the semiconductor compound $${\text{TlIn}}{{{\text{S}}}_{2}}$$ and its solid solutions, an analysis of their thermal expansion and photoconductivity has been carried out. The thermal expansion and photoconductive properties of $${\text{TlIn}}{{{\text{S}}}_{2}}$$ and Bridgman-grown solid solutions $${{\left( {{\text{TlIn}}{{{\text{S}}}_{2}}} \right)}_{{1 - x}}}{{\left( {{\text{TlGd}}{{{\text{S}}}_{2}}} \right)}_{x}}$$ are experimentally investigated. The chemical composition and structural characteristics of the obtained crystals are determined. From the experimental thermal expansion data $$\left( {{{\alpha }_{L}}} \right)$$ ), the Debye temperatures $$\left( \theta \right)$$ , anharmonicity parameters $$\left( \gamma \right)$$ and root-mean-square dynamic atomic displacements $$\left( {\sqrt {{{{\bar {u}}}^{2}}} } \right)$$ in the crystal lattice of the $${{\left( {{\text{TlIn}}{{{\text{S}}}_{{\text{2}}}}} \right)}_{{1 - x}}}{{\left( {{\text{TlGd}}{{{\text{S}}}_{2}}} \right)}_{x}}$$ (0 < x ≤ 0.10) system are calculated. It is found that with increasing $${\text{TlGd}}{{{\text{S}}}_{2}}$$ concentration in $${\text{TlIn}}{{{\text{S}}}_{{\text{2}}}}$$ within the composition range $${{\left( {{\text{TlIn}}{{{\text{S}}}_{2}}} \right)}_{{1 - x}}}{{\left( {{\text{TlGd}}{{{\text{S}}}_{2}}} \right)}_{x}}$$ (0 ≤ x ≤ 0.06), the value of $$\sqrt {{{{\bar {u}}}^{2}}} ~$$ increases, while $${{\theta }_{{\text{D}}}}$$ decreases. This behavior indicates a significant weakening of chemical bonding due to the substitution of indium atoms with gadolinium atoms in the solid solutions, resulting in an enhanced degree of anharmonicity of thermal vibrations. For all investigated compositions, the spectral distribution of photocurrent measured at 300 K shows an impurity-related absorption region. The long-wavelength edge of the impurity region in the photoconductivity spectra shifts further into the infrared region with increasing $${\text{TlGd}}{{{\text{S}}}_{2}}$$ content. Based on the photoconductivity data, the temperature dependence of the bandgap energy $${{E}_{g}}\left( T \right)$$ is determined and theoretically fitted. A good agreement is obtained between the calculated and experimental results.