tetrahedral pseudo-ionic metal halogen bond and a sta- ble halide framework that .... The crucial point is that the Cu-Xbond is not completely ionic and thus ...
Indian Journal of Pure & Applied Physics Vol. 37, April 1999, pp. 325-330
Cuprous halides: Old facts and new developments C S Sunandana*, Y L Saraswathi & P Senth il Kumar School of Physics, Uni versity of Hyderabad, Hyderabad 500 046 Received 3 February 1999 Cuprous halides are characterised by (a) large amplitude Cu + motion relative to immobile halide ion cage and (b) non-integral electron transfer from Cu to halogen . Recent theoretical work on Cu+ ion dynamics in Cu-halides derived from two body interatomic potentials is reviewed in the perspective of experimental diff~sion and conduction of CuBr & CuI. Calorim etri c data on phase transitions in CuI have been analysed. Thermal expansion in stron g anharmonic systems and its intimate connection to EXAFS has been pointed out.
1 Introduction
Cuprous halides, CUX (X = CI, Br, I) are solids with tetrahedral pseudo-ionic metal halogen bond and a stable halide framework that supports considerable cationic disorder even at moderate temperatures. CuCI transforms from zinc-blende to wurtzite at 681 K, which is stable up to the melting poi nt (703 K) . CuBr transforms from zinc-blende to wurtzite at 664 K and from wurtzite to superionic bcc structure at 744 K , which is stable up to the melting point (765 K). And CuI transforms from zinc- blende to a wurtzite-like phase at 642 K and back to a di sordered fcc superionic phase at 680 K, which is stable up to the melting point (878 K). In the case of CuBrJ and Cu I2, the temperature dependence of ionic cond uctivity (log (J vs. IIT plots) is highl y non-
linear, symptom atic of the unu sua l ionic diffu sion modes (Fig. I). The 'diffusion anoma ly ' in Cu-halides namely, the anomalously low diffusi on observed in Cui relative to CuBr and CuC I has been discussed recentl / in terms of interionic potentials. Simp le free-volume picture, however, predicts increasing diffusion fro m CuC I to CuI through CuBr, as the anion size increases 4 from CI-Br-1. Molecular Dynamics (MD) studies of the ionic cond uction mechanism in CuI conc luded that the intermediate range interactions control the crystal structure through polarization terms in the two-body potential. These potentials contro l the diffusion constants and phonon density of states in Cu-ha lides. Highly correlated diffusional motion of ' Cu + -chains' seems to occur in y-Cul. Furthermore, the rms displacement of cations 1500
