Light-scattering studies of the polymorphism of potassium cyanide under hydrostatic pressure

We measured the Raman spectrum of potassium cyanide under hydrostatic gas pressures up to 7 kbar at temperatures down to 75 K and found two new high-pressure phases with characteristic Raman spectra. The topology of the phase diagram of the alkali cyanides suggests that the orientation of the C${\mathrm{N}}^{-}$ molecules in these phases is probably a [111] direction of the original cubic lattice. We discuss possible structures of new phases and correlate them with the intermediate phase $\mathrm{Aa}$ in potassium cyanide at zero pressure. From the pressure shift of the Raman bands, we estimate the Grüneisen parameters of the lattice modes and compare them with the temperature-dependent bulk Grüneisen parameter of KCN and the Grüneisen parameter of the soft TA phonon above the cubic to orthorhombic phase transition.