The role of the lattice at the metal-semimetal transition in nickel sulphide

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Abstract
The characteristics of the lattice on either side of the metal-semimetal transition in Ni1- delta S have been determined using a panoply of experimental methods-specific heat, Mossbauer absorption inelastic neutron scattering, ultrasonic and X-ray diffraction. These different methods provide a coherent picture of the changes in lattice dynamics and thermodynamic properties at the first-order phase transition. Most significant is the softening of the acoustic modes, and in particular the decrease in the shear constant C44 in the high-temperature metallic phase. The transition as a function of temperature is mainly driven by the associated lattice entropy and not directly by electronic or magnetic entropy resulting from the change in electronic structure. Nevertheless, the change in electronic structure is probably indirectly responsible for the change in C44 in the metallic phase through modification of the population regions of the Fermi surface under shear stress.