Lattice dynamics and thermal expansion behavior in the metal cyanidesMCN(M=Cu, Ag, Au): Neutron inelastic scattering and first-principles calculations

Abstract

We report measurement of temperature dependence of phonon spectra in quasi one-dimensional metal cyanides MCN (M=Cu, Ag and Au). Ab-initio lattice dynamics calculations have been performed to interpret the phonon-spectra as well as to understand the anamolous thermal expansion behavior in these compounds. We bring out the differences in the phonon mode behavior to explain the differences in the thermal expansion behavior among the three compounds. The chain-sliding modes are found to contribute maximum to the negative thermal expansion along "c" axis in the Cu-and Ag-compounds, while the same modes contribute to positive thermal expansion in the Au-compound. Several low energy transverse modes lead to positive thermal expansion along "a" and "b" axis in all the compounds.The calculated elastic constants and Born effective charges are correlated with the difference in nature of bonding among these metal cyanides.

Anomalous Lattice Dynamics in AgC4N3: Insights From Inelastic Neutron Scattering and Density Functional Calculations

“…The calculated (0 K) phonon density of state is compared with the experimentally measured inelastic neutron scattering spectra at 150 K. The calculated neutron weighted phonon density of statesa grees very well with the experimental data (Figure 2 ). The level of agreement between the calculated and experimental phonon density of states of AgC 4 N 3 is similar to that in many previous studies on metal cyanides framework compounds (Gupta et al, 2016 , 2017 ), which is quite good considering the use of incoherent approximation (Carpenter and Price, 1985 ; Price and Skold, 1986 ) in the calculation of neutron weighted phonon spectrum. Therefore, the calculations can be further used for the microscopic understanding and interpretation of the peaks in the phonon spectra.…”

“…The anisotropic character of elastic properties is a measure of the magnitude of NLC (or NAC) in a material. The anisotropy in elastic properties of a material together with mode Grüneisen parameters give rise to the anisotropic thermal expansion (Gupta et al, 2016 , 2017 ; Singh et al, 2017a ; Mittal et al, 2018 ). Therefore, there is a close relation between the nature and magnitude of anisotropic thermal expansion and compressibility.…”

“…However, the mechanism at the origin of this type of abnormal behavior can only be understood by studying the microscopic dynamics at the atomic level. Ab-initio density functional theory provides the pressure dependent phase diagram of these compounds and can be used to obtain the movements of atoms at different pressures, giving rise to negative linear or area compressibilities (Gupta et al, 2013 , 2016 , 2017 ; Singh et al, 2017a , b ; Mittal et al, 2018 ). On the other hand, phonons in the entire Brillouin zone calculated using the ab-initio lattice dynamics in combination with the elastic properties have successfully been used to reproduce the experimental values of anisotropic linear thermal expansion coefficients (Gupta et al, 2013 , 2016 , 2017 ; Singh et al, 2017a , b ; Mittal et al, 2018 ).…”

“…Recently, a combination of experimental phonon density of states coupled with ab-initio calculations are used for understanding the anomalous lattice response in a few metal organic framework compounds (Duyker et al, 2013 ; Hermet et al, 2013 ; Kamali et al, 2013 ; Gupta et al, 2016 , 2017 ). These calculations are found to provide fair agreement of calculated and experimental phonon spectra.…”

Facile one-pot synthesis of CuCN by pulsed laser ablation in nitrile solvents and mechanistic studies using quantum chemical calculations

“…Both compounds exist in the form of more than one polymorph. Unlike other cyanides, they are presumed to form –Cu–CN–Cu–CN– chains 4 , 5 . The basic structure of CuCN exhibits static and large thermal displacement of the chain composed of CN or NC groups, which makes the structure formation problematic 6 .…”