Using time-differential perturbed angular correlation spectroscopy we have measured the electric field gradient ͑EFG͒ at 111 Cd probe nuclei in solid Ce in a pressure range up to 8 GPa. Covering various allotropic phases of Ce, we find that the value of the EFG in the cubic ␣ phase is almost four times larger than in the cubic ␥ phase and close to values in the noncubic phases ␣Ј and ␣Љ. These results together with the differences in time modulation of the spectra are interpreted as evidence for quadrupolar electronic charge-density ordering and symmetry lowering at the ␥ → ␣ transition while the lattice remains face-centered cubic.

AbstracLThe specimens of Fei-,Cos alloys obtained by electrodeposition onlo graphite were tested by MBsbauer spedrawnw and x-ray diffraction. "0 stluctural transformations from the todysentmd cubic to the h e a n t r e d cubic and huther to the hexagonal, as the CO mncentration inaeases. were observed. How the hypersne field depends on the occupancies of the %I and 4s stales was deduced. Changes in the preferred direction d domain magnetization mused by Ihe variable Co mncentration have also k e n o b s e d .

Abstract. Small particles of magnetite, i.e. 7.5, 13.4 and 14.1 nm in diameter, were obtained by the method of co-precipitation. The crystal structure and size distributions were determined by means of transmission electron microscopy and X-ray diffraction. The magnetic properties of the nanoparticles were tested by Mössbauer spectroscopy within the temperature range from 3 K to room temperature (RT). The Mössbauer study of magnetic nanoparticles reveals relaxation behaviour related to the existence of the superparamagnetic phase. The blocking temperature depends on the sizes of the nanoparticles and the ammonia concentration.

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