X-RAY-DIFFRACTION STUDY OF THE STRUCTURAL PHASE-TRANSITIONS IN RBCN UNDER HIGH-PRESSURE

Pressure-induced structural changes in RbCN were studied by X-ray diff raction. A single crystal sample was loaded in a miniature diamond-anv il cell and diffraction patterns were recorded at room temperature by the precession method on a laboratory radiation source; It was confirm ed 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 directio n of one of the body-diagonals of the CsCl lattice with its (010) plan e parallel to the {110} plane of the lattice. No definite conclusion w as obtained regarding the head-and-tail ordering of the CN molecules. (C) 1997 Elsevier Science Ltd. All rights reserved.