Thermal expansion and negative linear compressibility in ZnAu2(CN)4: An inelastic neutron scattering measurements and lattice dynamical studies

ZnAu₂(CN)₄ shows negative thermal expansion behavior as well as negative linear compressibility. The linear thermal expansion behavior is highly anisotropic (α_a~ 36.9×10^-6 K-¹, α_c~ -57.6×10^-6 K^-1). On application of hydrostatic pressure the c-axis expands along which the NTE behavior has also been reported. We have performed the inelastic neutron scattering measurment using IN4C spectrometer at ILL, Grenoble France. The measurments are performed at various temperatures ranging from 150 K to 400 K as shown in Fig 1(a). From the temperature dependence measurements we observed that modes below 20 meV show significant change in energy with temperature indicating the anharmonic nature of these modes. To understand the anamolus properties from microscpic view we have been calculated in the phonon in entire Brillouin zone using DFT. Our calculation suggest that these modes mostly involve motion of Zn(CN)₄ tetrahedral units connected via Au atoms. The calculated pressure dependence of the phonon energies have been used for thermal expansion calculation (Fig. 1(b)) of the compound. We observe that low energy modes below 5 meV contribute negative to the total thermal expansion (α_V) behavior. However modes around 10 meV have large positive contribution to α_V and results in positive volume thermal expansion behavior. The eigen vector of a zone centre phonon mode of energy around 4 meV energy with largest negative Grüneisen parameter of -9.2 suggest the twisting motion of the -Zn-NC-Au-CN-Zn- chain along with rotational motion of ZnN₄ tetrahedra. Such motion would lead to contraction along c direction.