Two new phenomena have been observed in Mössbauer spectra: a temperature-dependent shift of the center of gravity of the spectrum, and an asymmetric broadening of the spectrum peaks.Both phenomena were observed in thulium salts.In the temperature range 1˚K ≤ T ≤ 5˚K the observed shift has an approximate inverse temperature dependence.We explain this on the basis of a Van Vleck type of interaction between the magnetic moment of two nearly degenerate electronic levels and the magnetic moment of the nucleus.From the size of the shift we are able to deduce an “effective magnetic field” H = (6.0 ± 0.1) x 106 Gauss, which is proportional to ‹r-3›M‹G|J|E› where ‹r-3›M is an effective magnetic radial integral for the 4f electrons and |G› and |E› are the lowest 4f electronic states in Tm Cl3·6H2O.From the temperature dependence of the shift we have derived a preliminary value of 1 cm-1 for the splitting of these two states.The observed asymmetric line broadening is independent of temperature in the range 1˚K ≤ T ≤ 5˚K, but is dependent on the concentration of thulium ions in the crystal.We explain this broadening on the basis of spin-spin interactions between thulium ions.From size and concentration dependence of the broadening we are able to deduce a spin-spin relaxation time for Tm Cl3·6H2O of the order of 10-11 sec.
