Structural and electronic instabilities in AgMo 6Se 8

Abstract

The structural parameters, superconducting critical temperatures, and magnetic susceptibilities of the solid solution AgMo 6S 8− x Se x are reported. The behavior of the physical properties across the solid solution are similar to those observed for other ternary molybdenum chalcogenide solid solutions, except for the end member, AgMo 6Se 8, which has an anomalously low superconducting critical temperature relative to those observed in the solid solution. A discontinuity in the magnetic susceptibility of AgMo 6Se 8 was observed at 110 K. The decrease in the Pauli susceptibility at this temperature reflects a reduction in the density of states at the Fermi level which leads to the low T c observed for this compound. Since structural distortions are a likely cause of such anomalies, the temperature dependence of the lattice parameters of this material was investigated. A discontinuity in thermal expansion was discovered at the same temperature observed for the discontinuity in the magnetic susceptibility. This suggests that a structural distortion may be responsible for the decrease in the density of states, although our X-ray study did not show any splitting of the Bragg reflections. The presence of a structural distortion, however, also affords an explanation for the positive pressure dependence of the superconducting critical temperature of this material previously reported. The application of pressure will stabilize the high-temperature structure, thereby increasing the density of states. The increased density of states results in an increased superconducting critical temperature.