The Mössbauer absorption spectra of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ have been measured in the rare-gas matrices argon, krypton, and xenon with iron atomic concentrations from 0.3 to 3% and matrix temperatures between 1.45 and 20.5 K. All of the spectra show an absorption line with an isomer shift of $\delta =-0.75\mathrm{}\pm 0.03$ mm/sec with respect to an iron foil at 300 K. This isomer shift is independent of rare-gas matrix, iron concentration, and matrix temperature. This line is ascribed to an isolated ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ atom (monomer) with an atomic configuration of $3d^{6}4s^2$. The measured isomer shift gives a new calibration point in the isomer-shift-versus-electron-density plot for ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$. The observed $\frac{1}{T}$ temperature dependence of the monomer linewidth shows that the direct phonon process is dominant in the spin-lattice relaxation mechanism. Spin-lattice relaxation times of the order of 2.5 × ${10}^{-10\mathrm{}}$ sec are obtained by assuming a hyperfine field of 1.1 × ${10}^6$ Oe at the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nucleus due to an iron atom with unquenched orbital momentum. From the temperature dependence of the Mössbauer $f$ factor, the Mössbauer temperatures ${\Theta }_M$ in the Debye model are calculated and compared with the values expected from specific-heat measurements. The Mössbauer spectra show, in addition to the monomer absorption line, a pair of narrow lines ($\Gamma =0.22\mathrm{}-0.3$ mm/sec), which is intensified with increasing iron concentrations. These lines can be interpreted as the result of the quadrupole splitting of the $I=\frac{3}{2}$ excited state of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ in the axial field produced by an iron nearest neighbor (dimer). The measured quadrupole splitting is $\Delta E_Q=4.05\mathrm{}\pm 0.04$ mm/sec, and the isomer shift for the dimer is $\delta =-0.14\mathrm{}\pm 0.02$ mm/sec, corresponding to an effective atomic configuration of $3d^{6}4s^x$ with $x=1.47\mathrm{}\pm 0.04$. For ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ in krypton and xenon, the measured dimer/monomer ratio is that which one would expect from probability considerations, but in argon it is a factor of approximately 3 higher than expected.

• Received 2 June 1971

DOI:https://doi.org/10.1103/PhysRevB.4.3787