Soft modes and anharmonicity inH3[Co(CN)6]: Raman spectroscopy and first-principles calculations

Abstract

In situ high-pressure Raman spectroscopy and ab initio calculations are carried out to investigate the phase stability and the thermal expansion behavior of ${\mathrm{H}}_{3}[\mathrm{Co}(\mathrm{CN}{)}_{6}]$. Raman studies at high pressures in a diamond anvil cell identify soft phonons and phase instability at 2.3 GPa. Evidence of pressure-induced amorphization is found at 11 GPa. The phonon frequencies and eigenvectors obtained from ab initio calculation are used to complement the observed phonon spectra and for assignment of Raman modes. The computed eigenvector displacement patterns indicate that the soft modes correspond to the CN-librational vibrations (${E}_{\mathrm{g}}$ mode) of the Co-CN-H-NC-Co linkages and their Gr\"uneisen parameters are found to be negative, in agreement with our measured values. The thermal expansion coefficient $(15.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$ calculated using our computed mode Gr\"uneisen parameters is found to be in good agreement with the reported value $(20\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$. Temperature-dependent phonon spectra down to 77 K are used to obtain the anharmonicities of different modes.