Mössbauer Spectroscopic Evidence of Angle-Dependent Intersystem Crossing in LiNbO3:Fe3+

Mössbauer emission spectra of LiNb${\mathrm{O}}_3$:${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ single crystals in magnetic fields up to 5 T at 4.2 K show initial populations of the ${}_{}{}^6{}_{}{}^{}A_{1\mathrm{g}}^{}{}_{}{}^{}$ Zeeman substates of ${\mathrm{Fe}}^{3+}$ which depend strongly on the angle between the crystallographic $c$ axis and the magnetic field. This is interpreted in terms of a crystal-field effect on excited states which influences the initial populations of the Zeeman sublevels of the ${}_{}{}^6{}_{}{}^{}A_{1\mathrm{g}}^{}{}_{}{}^{}$ ground term after the electron-capture decay of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$. An intersystem crossing process due to orbit-lattice interaction can fairly well explain the angular dependence.