Elastic Softening in the Tetrahedrite Cu12Sb4S13

The tetrahedrite Cu12Sb4S13 has a cubic structure above 100 K and is paid attention as thermoelectric materials. At 85 K, a metal-semiconductor phase transition is reported by the electrical resistivity and magnetic susceptibility measurements. The origin of the phase transition is not determined yet. To investigate the phase transition at 85 K, we measured elastic modulus using an ultrasonic technique on polycrystalline samples. The longitudinal modulus shows elastic softening from above 300 K, suggesting that there is an instability of the structure or the electronic state. The softening continues down to 85 K, and it turns into increase below 85 K accompanied by hysteresis. The phase transition is of the first-order. The elastic hardening indicates disappearance of the instability by the phase transition.