  Recently colossal positive volume thermal expansion has been found in the
framework compounds Ag3Co(CN)6 and Ag3Fe(CN)6. Phonon spectra have been
measured using the inelastic neutron scattering technique as a function of
temperature and pressure. The data has been analyzed using ab-initio
calculations. We find that the bonding is very similar in both compounds. At
ambient pressure modes in the intermediate frequency part of the vibrational
spectra in the Co compound are shifted to slightly higher energies as compared
to the Fe compound. The temperature dependence of the phonon spectra gives
evidence for large explicit anharmonic contribution to the total anharmonicity
for low-energy modes below 5 meV. We found that modes are mainly affected by
the change in the size of unit cell, which in turn changes the bond lengths and
vibrational frequencies. Thermal expansion has been calculated via the volume
dependence of phonon spectra. Our analysis indicates that Ag phonon modes in
the energy range from 2 to 5 meV are strongly anharmonic and major contributors
to thermal expansion in both compounds. The application of pressure hardens the
low-energy part of the phonon spectra involving Ag vibrations and confirms the
highly anharmonic nature of these modes.
