A new method to calculate the lattice contribution to electric field gradients at a nuclear site in tetragonal crystals is developed. The crystal is regarded as an assembly of positive ions at lattice points embedded in a uniform background of negative charge (point charge model). The method uses Euler-Maclaurin formula and makes the plane-wise summation in the direct crystal space unlike most of the previous methods utilising Fourier transform to reciprocal space. The numerical values obtained using the above approach agree well with previous results.

Mössbauer studies of metallurgically prepared FeZn alloy reveal that it undergoes an irreversible phase change on heating. The new phase starts precipitating at temperatures below 403 K and causes an apparent decrease in quadrupole splitting. The results are analysed in the framework of charge shift model and it is suggested that the vibrations of iron ion in zinc are likely to be much less anisotropic than those of zinc ions in pure zinc.

The charge shift model has been used to calculate electric field gradients (efg) for various probe-host combinations. The comparison with the corresponding experimental values indicates that the validity of the model depends on sign and magnitude of the charge shift. The model agrees fairly well with experiments for the hosts having positive charge shift. The agreement is good for hosts having negative charge shift only if the magnitude is small (< 0·012).

Electric field gradient (efg) is calculated at the centre of the cell in the simple tetragonal crystal. The method uses Euler-Maclaurin summation formula and makes the planewise summation in the direct crystal space without any special regrouping of charges in point charge model. The results are in fair agreement with previous results of de Wette on the same system using Fourier transform to reciprocal space.