Conductivity Phase Transition with Shift of Valence-Band Edge and with Structural Distortion in Cu₄Sn_1-xGe_xS₄

X-ray diffraction, electrical resistivity, DSC and XPS investigations were carried out on Cu₄Sn_1-xGe_xS₄ (0≦x≦1.0). At room temperature, the orthorhombic structure changes to a monoclinic one at a transition concentration of x_t=0.6 as x increases. This transition also exhibits the expansions in the energy width between the top of the valence band and the Fermi level and in the lattice parameter B. The transition has the characteristics of the first-order high (high temperature)-to-low (low temperature) conductivity phase transition as reported on Cu₄SnS₄. The transition temperature T_t linearly increases with x. These features are briefly compared with the electronic and structural phase transition reported on SnTe and Pb_1-xGe_xTe.