X-ray-absorption study of CuBr at high pressure

The x-ray-absorption spectrum of cuprous bromide has been measured as a function of pressure. The x-ray-absorption near-edge structure proved to be an excellent indicator of high-pressure phase transitions in this material. The normalized ‘‘white-line’’ peak heights at both the Cu and Br K edges decreased on entering the tetragonal phase and increased in going to the NaCl structure. The zinc-blende to tetragonal phase transition took place over a very narrow pressure range centered at 46±5 kbar. The transformation from the tetragonal to the NaCl structure, on the other hand, showed a broad mixed-phase region, suggesting a nucleation-and-growth mechanism for the transition. The mixed-phase region was centered at 75±6 kbar. No evidence of a phase between the zinc-blende and tetragonal phases was observed, presumably because it does not exist. Analysis of the extended x-ray-absorption fine-structure (EXAFS) clearly showed that there is no change in coordination in going from the zinc-blende to the tetragonal phase although the nearest-neighbor distance increases slightly. A much larger increase in $R_1$ occurs at the transition to the NaCl structure, where the coordination increases from 4 to 6. The mean-square deviation in the nearest-neighbor bond length, ${\sigma }_1^2$, appears to be a fairly smooth function of nearest-neighbor distance, decreasing (or increasing) as $R_1$ decreases (or increases) more or less independent of structure. Evidence from the literature was presented to suggest that the zinc-blende to tetragonal transition in CuBr (and also CuCl) should occur by shear deformation. The zinc-blende lattice becomes unstable under pressure due to a decrease in the bond-bending force constant. This mechanism is consistent with the results of our EXAFS study. Unfortunately, the information obtained from the EXAFS, even at low temperature, was not sufficient to solve completely the structure of the tetragonal phase.