On the anomalous paramagnetism of Co(II) perchlorate hexahydrate at low temperatures

Abstract

Like the hexahydrated perchlorates of Fe2+and Ni2+, the isomorphous Co2+salt is observed to show thermally-reversible anomalous magnetic properties at Tc=(166+or-1)K. Mossbauer studies of this salt diluted with Fe2+ions, however, show an anomaly in the quadrupole splitting ( Delta EQ) in the range from 140K to 160K. This difference in the temperature of the anomaly is probably due to dilution. Again similar to the Fe2+salt, the authors find in Co(ClO4)26H2O a change in sign from positive to negative and an increase in magnitude of about 50% of the axial field parameter Delta below the transition temperature. As in the other isomorphous perchlorates of this group, it is observed for Co(ClO4)6H2O that K/sub ///>Kperpendicular toabove Tcand Kperpendicular to<K/sub /// below Tc. The experimental data in the region of transition can be satisfactorily fitted with theory only by reducing the values of Delta , keeping the other parameters more or less constant. This peculiar phase change in Co2+perchlorate is also associated with the appearance of anisotropy in the ab plane accompanied by a change of asymmetric field from trigonal (above Tc) to tetragonal (below Tc) symmetry.