Superconductivity,lattice transformations,and electronic instabilities in CuxMo3S4

The variations of the superconducting transition temperature (T_c) and of the lattice transformation temperature (T_L) with composition are reported for the Chevrel-phase system Cu_xMo₃S₄ (1 ≤ × ≤ 2). A complex low temperature phase diagram was found, containing at least three distinct low temperature phases, each exhibiting its own characteristic T_c (11 K, 6.5 K, 4.5 K), T_L (280 K - 125 K), and electronic transport behavior. Pressure-induced transformations between these three phases were observed at pressures below 25 kbar. Temperature-induced lattice distortion for compositions near Cu₂Mo₃S₄ was found to increase the T_c above that (< 1.1 K) of the high temperature, higher symmetry rhombohedral phase. This is apparently the first time the latter behavior has been observed: in all previous studies of other materials, the opposite behavior was reported.