THE ELECTRIC-FIELD GRADIENT AT THE NUCLEUS IN HCP ZR AND HF

We use the linear muffin-tin orbital formalism, in the atomic sphere a pproximation,to obtain the electric held gradient (BG) at the nucleus for metallic Zr and Hf, in the HCP structure. Combined corrections are included in the calculations. To evaluate the importance of relativis tic effects, non-relativistic and scalar relativistic calculations wer e performed. In agreement with experiment, we find that the EFG for H- f is larger than for Zr by a factor of more than 2. We show that this is mainly due to the behaviour of metallic p and d orbitals, which for Hf have comparatively larger values close to the nuclear region. Beca use H-f and Zr are in the same group of the periodic table, Hf is ofte n used as a probe in perturbed angular correlation experiments, when i nvestigating the EFG for Zr alloys. We use our results to note that, f or similar values of orbital occupations, the EFG at a Hf probe is lar ger than the EFG at a Zr atom placed at the same site.