New insights into the structure, chemistry, and properties of Cu₄SnS₄

The ambient temperature structure of Cu₄SnS₄ has been revisited and the recently reported low temperature structure has been confirmed from single-crystal X-ray diffraction data. A structural phase transition from a large monoclinic unit cell at low temperature to a smaller orthorhombic unit cell at high temperature has been observed. The room temperature phase exhibited disorder in the two copper sites, which is a different finding from earlier reports. The low temperature monoclinic form crystallizes in P2₁/c space group, which is isostructural with Cu₄GeS₄. The phase transition has also been studied with variable temperature powder X-ray diffraction and ¹¹⁹Sn Mössbauer spectroscopy. The Seebeck coefficients and electrical resistivity of polycrystalline Cu₄SnS₄ are reported from 16 to 400 K on hot pressed pellets. Thermal conductivity measurements at high temperatures, 350 – 750 K exhibited very low thermal conductivities in the range 0.28 – 0.35 W K^–1 m^–1. In all the transport measurements the phase transition has been observed at around 232 K. Resistivity decreases, while Seebeck coefficient increases after the phase transition during warming up from low to high temperatures. This change in resistivity has been correlated with the results of first-principles electronic band structure calculations using highly-accurate screened-exchange local density approximation. It was found that both the low hole effective mass of 0.63 m_e for the Γ→Y crystallographic direction and small band gap, 0.49 eV, are likely to contribute to the observed higher conductivity of the orthorhombic phase. Cu₄SnS₄ is also electrochemically active and shows reversible reaction with lithium between 1.7 and 3.5 volts.