THEORY OF INSTABILITY PHENOMENA AND ORDER-DISORDER TRANSITION IN CSCL-TYPE CRYSTALS

For CsCl-type crystals consisting of two kinds of atoms, instability i n the whole crystal accompanied by atomic interchange is studied by ad opting the Morse potential as the interaction between atoms. The insta bility and disordering temperatures are derived using the variational principle at finite temperature and the Bragg-Williams approximation. It is found that the instability temperature is higher in crystals tha t retain the ordered state for atomic interchange and the disordering temperature is higher in crystals with a smaller frequency parameter o f the Morse potential. Further, the phase diagram for the instability in the whole crystal combined with the disordering is shown as a funct ion of the ordering energy for atomic interchange.