An-Najah Staff

The structural phase transformations of semiconductors under high pressure have recently attracted a lot of attention. Experimental studies in this field are very difficult  and expensive, the computational physics programs make these studies very easy, very accurate and inexpensive. The computational approach enables us to know the atomic structures of materials, the electronic properties and give the chance to modify the bonding between atoms to create novel materials with predetermined properties. In the present study  the Full-Potential Linearized Augmented Plane-Wave (FP-LAPW) (which is included in a computer code WIEN2K) method depending on the Density Functional Theory (DFT) were used  to investigate the structural phase transformations of BeO and ZnO compounds under high pressure. In these calculations, the local density approximation(LDA), the gradient generalized approximation (GGA) and the modified Wu- Cohen-GGA approximation for the exchange correlation potential have been used. For BeO the equations of state (EOS’s) of wurtzite(WZ), zinc-blende(ZB) and rock salt (RS) have been calculated. From these (EOS’s) the transition under high pressure is occurred from wurtzite  to rock salt and from zinc-blende to rock salt structures, the transition pressure and the structural properties have also been calculated. The energy band gap for all phases of BeO have been calculated and a large band gap was found to be  (6 ~ 8 eV) which is  indicating that BeO is a good insulator. The same work was done for ZnO using the same method and the same approximations. A number of transition phases is predicted for ZnO, wurtzite to rock salt, wurtzite to cesium chloride, zinc-blende to rock salt, zinc-blende to cesium-chloride and rock salt to cesium-chloride. The transition pressure for each case was calculated. The structural properties have also been calculated and finally the energy band gap for each phase was investigated. Small energy band gap (0.3 ~1.5eV) is found, which means that ZnO behaves as  a semiconductor. The most important results of this study are: 1-The present calculations agree very well with the available experimental data and the other theoretical calculations. 2- The transition from structure to another is possible under high pressure. 3-BeO behave as an insulator in all its structures. 4-Wurtzite found to be the ground state for BeO compound at zero temperature. ZnO behave as a semiconductor in all its structures except in cesium-chloride structure it behaves as a semi-metal.