TEMPERATURE-DEPENDENCE OF THE FE-57 MOSSBAUER PARAMETERS IN RIEBECKITE

Mossbauer spectra for two riebeckite miner als were collected at tempe ratures in the range 4.2 to 500 K. The magnetic-ordering temperatures were found to be 33+/-1 and 31+/-1 K respectively. Fitting the paramag netic spectra with a discrete number of doublets (three or four) did n ot lead to consistent results. Instead: a superposition of an Fe3+ (M2 ) doublet and one distributed ferrous component was found to produce a dequate fits with reasonable parameter values. For both samples, a min or fraction of ferrous ions was observed to be present at the M4 sites and for one of the samples at the M2 sites as well. The temperature v ariations of the center shifts were well reproduced using the Debye mo del of the lattice vibrational spectrum to evaluate the second-order D oppler shift. The characteristic Mossbauer temperatures were calculate d to be in the range 340-390 K for Fe2+, and 520 K for Fe3+. The tempe rature dependences of the various ferrous quadrupole splittings could not be explained in terms of the point-charge model and assuming a tem perature-independent energy-level scheme for the 5D term. It is sugges ted that a gradual change with temperature of the orbital-level splitt ings takes place. All calculations yielded a positive sign for the pri ncipal component of the electric field gradient (EFG). The spectrum re corded at 4.2 K for one of the riebeckites was fitted with a superposi tion of an Fe3+ and a Fe2+ hyperfine-field distribution, the latter on e primarily characterizing the Fe2+ (M1) cations. The following releva nt hyperfine data were calculated: H-hf=161 kOe, Delta E(Q)=3.11 mm/s, and V-zz <0, all referring to the maximum-propability values. For the second riebeckite at 4.2 K, an additional distributed ferrous compone nt could independently be resolved. The two maximum-probability hyperf ine fields were found to be 189 and 98 kOe and the corresponding Delta E(Q) values 3.10 and 2.67 mm/s. Both components exhibit a negative V- zz. The subspectra were attributed to M1 and M3 sites respectively. Th e Fe3+ hyperfine fields are 548 +/- 2 kOe for both riebeckites. The di fferent values found for the Fe3+ quadrupole shift 2 epsilon(Q) for th e two samples is explained by a different angle between the hyperfine field and the EFG's principal axis. The magnetic spectra recorded at 1 5 K and higher, could not be reproduced adequately with reasonable par ameter values.