The study of some physical properties of selected rare Earth compounds

The work set out in this thesis relates to studies of europium(II) and europium(III) containing solids. The primary mode of study was Mossbauer spectroscopy supported where useful, by information from chemical analysis, x-ray diffractometry, electrical conductivity, thermogravimetry, infra-red and mass spectrometry.

The thermal dehydration of EuCl3.6H2O and EuBr3.6H2O and subsequent conversion to oxyhaliae and oxide was studied initially by thermogravimetry to define stable intermediates; these were afterwards prepared in amounts enabling their characterization. The chloride system gave a sequence of well-defined intermediate hydrates; but those for the bromide system were less well-defined. Neither anhydrous europium(III) halide was observed in contrast with previous reports. However in the absence of oxygen the thermal decomposition of the hydrated bromide resulted in the formation of some europium(II) at temperatures above 300°C.

The non-stoichiometric phase formed in the initial stages of the thermal reaction

EuBr3 —» EuBr2 + 1/2 Br2

has been studied. The temperature dependent Mossbauer spectra are consistent with an "electron hopping" mechanism similar to that involved previously to describe non-stoichiometric EuCl3. Electrical conductivity measurements on both chloride and bromide systems revealed two regions for each system; they were separated by a characteristic temperature of 198 - 2K and 222 - 2K respectively. Below these temperatures charge ordering in the lattice is considered to occur while above, electron hopping predominates. Activation energies calculated for electron hopping from spectral and conductivity data are in agreement.

Several europium-mercury compounds (EuHg where x = 0.8,1,2,3,3.6) have been prepared and studied. Correlation was found between the europium isomer shift and (n/v) where n is the number of europium ions in a unit cell of volume v. The electron density at the europium nucleus together with an approximate electron configuration of the europium ion core have been determined for each of the samples. The compounds were found to be metallic in nature with some degree of normal valence character in bonding. The ionic core of europium was found to be divalent with the conduction electron density at the nucleus decreasing with decreasing europium concentration in the crystal lattice.