X-ray diffraction study on the structural phase transitions of (KCN)x(KBr)1-x mixed crystals

Structural phase transitions in the title mixed crystals with the concentration of x = 0.95, 0.90 and 0.80 having the cubic NaCl-type structure at room temperature were studied by Xray single crystal precession method in the temperature range between 300 K and 85 K. An x-T phase diagram has been determined, in which the relation among the cubic phase, ITP (a so-called intermediate temperature phase) and the orthorhombic phase (space group Immm) has been established. ITP consists of three crystal forms, monoclinic, triclinic I and triclinic II; forming a multidomain hybrid structure with a definite orientation relationship to the cubic lattice. The former two forms were the same as those found in the low temperature phase of RbCN as well as in the intermediate metastable phase of KCN, while the latter form is the one found in the mixed crystals. The orthorhombic phase is the same forming in KCN and NaCN at low temperatures. It has been found that, although sample crystal breaks up into the multidomain hybrid structure, it recovers the cubic single crystalline state when the temperature is raised above the phase transition temperature.