The objective of this study was the application of Mossbauer effect to resolve the hyperfine structure of broadened by spin-spin relaxation and consequently to obtain information about its spin Hamiltonian. 57 Fe M~ssbauer absorption spectra were recorded for powdered and single-crystal solid solution samples of K3[(Al 1 _xFex)(C204) 3 ].3H20 with x in the range 0.002 to 0.25 at 4.2 K. Applied magnetic fields of up to l kOe were also used to facilitate the resolution of the hyperfine spectra,

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... particular the s 2 = ±1/2 Kramers' doublet. Our findings show that the resolved hyperfine spectra arise from a system of Kramers' doublets with the ±1/2 states lying lowest and small separation between Kramers' doublets because the zero-field splitting parameter Dis positive and very small (D = +0.20 ±0.05 cm-1 ). The site symmetry of the Fe(III) ion is not axial (A::: 0.05 ± 0.01). The magnetic field at the nucleus associated with the ±5/2 Kramers I doublet was found to be 540 ± 5 kOe. The quadrupole interaction energy (l/4e 2 qQ) -1s equal to 0.05 mm sec-1 and the asymmetry parameter (n) is equal to 0.15. Additional experiments were performed which confirmed .. the above results. First we measured the hyperfine spectra of frozen solutions. The zero-field spectra were found to be slightly different from those of the solid solutions, but the spectra in small magnetic fields (60-500 Oe) were essentially identical. This indicates that the states of the Fe(III) ions in these two diverse samples differ only slightly and therefore are determined predominantly by the ligands of the [Fe(C204)3] 3 -complex ion. The spectra of solid solutions and frozen solutions were also recorded at 1 .3 Kand revealed an interesting phenomenon. At l .3 K the magnetic hyperfine spectra exhibit a selective reduction in intensities and broadening of lines. A qualitative argument is presented which explains this unusual change in paramagnetic hyperfine structure as a consequence of the temperature-dependent electronic spin-spin relaxation time. The intralevel electronic spin-spin relaxation of the ±1/2 ground Kramers' doublet becomes faster as a result of depopulation of the excited Kramers' doublets at lower temperature.