CALCULATION OF THE RECOILLESS GAMMA-RAY EMISSION-SPECTRA FROM A SUBSTITUTIONAL CATION IMPURITY DIFFUSING VIA THE (001) CHANNELS IN THE RUTILE STRUCTURE

The large anisotropy of the diffusion coefficient for a substitutional randomly diluted cationic impurity in the rutile structure, with the diffusivity being much larger along the tetragonal axis as compared to the diffusivity within a tetragonal plane, indicates that open and al most empty [001] channels are involved along with an interstitialcy me chanism to originate diffusion events lending an impurity from one to another substitutional cationic site. This model is used to evaluate t he resulting emission Mossbauer spectra of the impurity staying locall y at equilibrium with the lattice and embedded in a single-crystalline sample. Final results are shown for the 14.4 keV Mossbauer line in Co -57 (Fe). It is shown that channel diffusivity is governed by the expo nential law. and details of the electric-field-gradient relaxation due to the diffusive motion of the probe are described from first princip les.