Tuning the optical, electronic and thermal properties of Cu3NbS4−xSex through chemical substitution

Several compositions of the Cu3NbS4−xSex (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, and 4) series were synthesized through combination of the elements using conventional solid-state reactions. X-ray powder diffraction patterns suggest that the synthesized polycrystalline powder samples are isostructural to sulvanite (Cu3VS4). However, plotting the refined lattice parameters as well as the measured “true” density as a function of Se content (x) showed deviations from linearity, suggesting that the Cu3NbS4−xSex series is not a “true” solid solution. Differential scanning calorimetry data showed that all compounds incongruently melt at ∼1100 K and recrystallize with a minor impurity phase. Optical diffuse reflectance UV/Vis/NIR spectroscopy revealed a nearly linear contraction of the optical band gap with increasing Se content. The direct band gap decreases from 2.53 eV for x = 0.1 to 2.43 eV for x = 0.8. Electronic transport data indicate p-type semiconducting behavior for all samples. Thermal conductivity data showed a rather surprising trend, with the substituted compositions exhibiting higher thermal conductivity than the two end members.