Paramagnetic ${\mathrm{Fe}}^{3+}$ in Li${\mathrm{Al}}_5$ ${\mathrm{O}}_8$: Magnetic Field Effects in the Mössbauer Spectrum

${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}_{}^{3+}{}_{}{}^{}$ Mössbauer absorption spectra were collected for powder specimens of ${\mathrm{LiFe}}_x{\mathrm{Al}}_{5-x}{\mathrm{O}}_8$ with $x=0.005\mathrm{}$. The effects on the spectra of applied magnetic fields with strength less than 1 kOe are interpreted in detail. A comparison of the calculated spectra using EPR parameters to the data is made. This comparison indicates (a) that the observed resonances which are sensitive to applied magnetic fields are associated with the ±1/2-spin Kramers's doublet of the ionic ground term; (b) that for zero applied field the field-sensitive resonances are affected drastically by dipolar fields originating from neighboring nuclei. The sign of the zero-field-splitting parameter $D$ is found to be negative. The value of the ground-state hyperfine-coupling constant is $A_g=(-2.45\mathrm{}\pm 0.03)$ mm/sec, and that of the quadrupole-coupling constant is $2P=(+0.67\pm 0.03)$ mm/sec.