A new quaternary sphalerite-derivative compound for thermoelectric applications: Cu₇VSnS₈

Cu-S-based multinary compounds with sphalerite-derivative structures have received attention due to their potential as thermoelectric materials. Recently, we proposed a strategy to design Cu-S-based quaternary compounds, “the pseudo-binary approach”. Within the Cu₃SbS₄-Cu₄TiS₄ (Cu_3+xSb_1−xTi_xS₄) system, this new approach led us to discover Cu₃₀Ti₆Sb₂S₃₂, at x = 0.75. In this study, we adopted the same approach for the “Cu₃SnS₄”-“Cu₄VS₄” and (Cu_3+xSn_1−xV_xS₄) system. Although the ordered end-point compounds are not known, a semiconducting quaternary phase, Cu₇VSnS₈, was discovered at x = 0.5. Single crystal X-ray diffraction has revealed that this new compound crystallizes in a tetragonal sphalerite derivative structure with space group P4̄m2. Electronic and phonon/vibrational properties were investigated by combining experiments and theory. Cu₇VSnS₈ allows partial substitution of Ti for V, leading to the increase of the hole carrier concentration and the thermoelectric power factor. The relatively large power factor of 0.5 mW K⁻² m⁻¹, combined with a low lattice thermal conductivity of 0.5 W K⁻¹ m⁻¹, yields a large dimensionless figure of merit ZT = 0.6-0.7 at 673 K for unoptimized hot-pressed samples of Cu₇V_1−yTi_ySnS₈ (y = 0.25, 0.5, 0.75). High-temperature stability was also examined by thermogravimetry and X-ray diffraction.