X-ray diffraction study of the structural phase transition in NaCN under high pressure

Pressure-induced structural phase transition in NaCN was studied by X-ray diffraction. A single crystal sample was loaded in a miniature diamond-anvil cell and diffraction patterns were recorded at room temperature by the precession method on a laboratory radiation source. It was confirmed that the cubic Bl-type structure stable under atmospheric pressure is transformed into the orthorhombic structure with the preferential alignment of the CN− molecules in the [110] direction of the cubic lattice. The orientation relationship is such that the (001) plane of the orthorhombic lattice is formed parallel to the (100) and equivalent planes of the cubic lattice and each orientation variant contains twin. Compression experiments were performed up to 5.0 GPa, but no clear indication was obtained of the occurrence of the previously suggested second transition at 4.0 GPa. Discussion is given on the structural phase transition in the alkali cyanides taking into account the repulsive interaction between ion cores.