The Raman spectrum of NaCN under hydrostatic pressure

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Abstract
We have measured the Raman spectrum of NaCN in the two orthorhombic low temperature phases under hydrostatic pressure up to 6 kbar. At the order–disorder phase transition from the disordered phase D252h to the ordered phase D132h the unit cell doubles and new phonon bands fold from the zone boundary to the center and appear in the Raman spectrum. We analyze the transition group theoretically and assign the different phonon bands in the Raman spectrum. The pressure shift of the CN− librational mode is larger than the shift of the lattice modes, which supports our band assignment. We estimate the mode Grüneisen parameters using an approximated value for the bulk modulus of NaCN and compare the results with the averaged Grüneisen parameter of NaCl.

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