Lattice dynamics and hyperfine interactions in ZnF2 single crystals

Using the 93.31-keV Mössbauer resonance in Zn67 the anisotropy of the Lamb-Mössbauer factor (LMF), the quadrupole interaction, and the isomer shift in ZnF2 single crystals have been investigated between 1.1 and 4.2 K. The anisotropy of the LMF is large: fx=(2.4 0.1)%, fy=(2.3 0.2)%, and fz=(1.4 0.2)%. The main component of the electric-field gradient (EFG) tensor Vzz=+(2.18 0.22)×1017 V/cm2 and =0.29 0.03 is derived. Vzz is perpendicular to the c axis. Using a force-constant model (shell model) which includes short-range interactions we solved the eigenvalue problem of the dynamic matrix and quantitatively reproduced the anisotropy of the LMF. Within the same model we calculated the second-order Doppler shift. We performed ab initio Hartree Fock (HF) cluster calculations to give a detailed interpretation of the various electronic contributions to the EFGs at the Zn and F sites and to the s electron density at the Zn nucleus (isomer shift). The covalent part of the chemical bond essentially determines all hyperfine interactions. The agreement between HF calculations and experiment is very good.