THE MEAN-SQUARE RELATIVE DISPLACEMENT AND DISPLACEMENT CORRELATION-FUNCTIONS IN TETRAHEDRALLY AND OCTAHEDRALLY COORDINATED A(N)B(8-N) CRYSTALS

The Debye-Waller factors of atoms in Ge, GaN, ZnO, CdS, CdSe, CuBr, Cu Cl, Agl, CdO, AgBr, KBr and RbCl in order of Phillips ionicity (f(i)) are studied by both EXAFS and the diffraction method. The displacement correlation functions (DCF) are derived from the mean-square relative displacement (MSRD) and the mean-square displacement (MSD) at room te mperature. The magnitudes of the MSRD and MSD correlate well with the coordination number and ionicity. The MSRDs of tetrahedrally coordinat ed compounds show a gradual approach to those of octahedrally coordina ted ones as the ionicity increases. A divergent-like curve toward f(i) = 0.785 is observed for the MSD in the tetrahedrally coordinated comp ounds, which indicates the behavior of lattice instability. In the tet rahedrally coordinated covalent materials: the MSRD is nearly half the MSD(cation) or MSD(anion) and the ratio of the DCF to MSD given by 2D CF/(MSD(cation)+MSD(anion)) is about 80%. In the octahedrally coordina ted ionic materials, the MSRD is comparable to the MSD and 2DCF/(MSD(c ation)+MSD(anion))=50%. An anharmonic contribution to the Debye-Waller factor determined by EXAFS appears pronouncedly when the magnitude of sigma(2) is greater than 0.01 Angstrom(2). The MSRD for the tetrahedr ally coordinated compounds of silver and copper halides obeys the syst ematic ionicity dependence and pronounced specificity of anharmonicity was not observed: AgI, CuBr and CuCl of which the high-temperature fo rms are typical superionic conductors have a broad interatomic potenti al which is similar to that of the octahedrally coordinated compounds and strongly correlated displacement in thermal vibration between cati on and anion.