FLAPW calculations of the EFG in semimetallic arsenic and antimony under varying pressure

The variation of the electric field gradient (EFG) at nuclear sites with pr essure in semimetallic arsenic and antimony has been calculated using the f ull-potential linearized augmented plane wave (FLAPW) method. The results a re compared with NQR data, which are available up to a pressure of 2 GPa. T he calculated EFG depends sensitively on the values of the lattice paramete rs used. A theoretical investigation of the relationship between c/a and th e internal parameter z, using FLAPW methods, has been carried out and compa red with available X-ray diffraction data for arsenic and antimony. For ant imony, the FLAPW calculations give results which are consistent with the ex perimental pressure variation of both the lattice parameters and the EFG. I n the As case, the use of theoretical values of z leads to good agreement w ith the measured pressure variation of the EFG.