Correlation between Uniaxial Negative Thermal Expansion and Negative Linear Compressibility in Ag₃[Co(CN)₆]

Density functional theory (DFT) calculations are used to investigate the correlation between the uniaxial negative thermal expansion (NTE) and negative linear compressibility (NLC) behaviors in Ag₃[Co(CN)₆]. First, we reproduce the uniaxial-NTE and NLC behaviors under temperature- and pressure-field. And then the temperature dependence of elasticity is studied. The abnormal nature of elastic constants C₃₃ and C₁₁+C₁₂ as the function of temperature is predicted. The hardening of phonon modes (below 568.7 cm^-1) with increasing temperature can be as an indicative for abnormal physical properties of Ag₃[Co(CN)₆]. Through analyzing the vibration mode with the strongest phonon hardening, the deformation of wine-rack motif in anisotropic framework can be identified as the mechanism that leads to the coexistence of uniaxial-NTE and NLC in Ag₃[Co(CN)₆]. The response of phonon group velocity along c-axis is intense and quicker than that of a-axis on heating and on compression, facilitating c-axis itself to be the carrier of abnormal uniaxial-NTE and NLC properties.