X-ray diffraction study of the structural phase transitions in RbCN under high pressure

Abstract Pressure-induced structural changes in RbCN were 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 a two-step structural transition takes place up to a pressure of 2 GPa: from the NaCl-type to the CsCl-type structure and further to the monoclinic structure. The CsCl-type structure forms with its (111) plane parallel to the three equivalent {100} planes of the NaCl-type structure. The monoclinic structure with the space group Cm forms as a result of preferential alignment of the CN molecules in the direction of one of the body-diagonals of the CsCl lattice with its (010) plane parallel to the {110} plane of the lattice. No definite conclusion was obtained regarding the head-and-tail ordering of the CN molecules.