PHASE-TRANSITIONS IN CDTE TO 5 GPA

Angle-dispersive powder-diffraction techniques using an image-plate area detector have been used to study the high-pressure phases of CdTe to 5 GPa. The recently discovered cinnabar phase between the zinc-blende and NaCl phases is found to be stable on both pressure increase and decrease. No pressure range of single-phase cinnabar has been found on pressure increase, but a single phase is obtained on pressure decrease, between 3.6 and 2.7 GPa. The cinnabar phase has an average bulk modulus of 32(1) GPa and is significantly more compressible than the zinc-blende and NaCl phases. All three phases are site ordered. Each atom in the cinnabar structure has two nearest-neighbor distances of approximately 2.74 angstrom (a bonds) and two of approximately 2.94 angstrom (b bonds), and the structure is thus close to fourfold coordinated. The next-nearest-neighbor distances are two of approximately 3.5 angstrom (c bonds). The a bonds are very similar in length to the nearest-neighbor distance in the zinc-blende phase, and the b bonds increase with pressure towards the nearest-neighbor distance in the NaCl phase. The c-bond distance and the interbond angles also become closer to the geometry of the NaCl structure with increasing pressure, but there are still large discontinuities in bond lengths and angles at the cinnabar-to-NaCl transition.