A Moessbauer spectroscopy study of an anhydrous non-stoichiometric europium(III) chloride phase

Anhydrous europium(III) chloride, when heated under vacuum at temperatures above 570 K, decomposes according to the overall reaction EuCl3(s)→EuCl2(s)+1/2Cl2(g). A nonstoichiometric chlorine-deficient phase with the hexagonal UCl3-type structure is observed down to at least a europium-to-chlorine ratio of 1:2.84. Samples of composition EuClsub(>2.8) were examined by Moessbauer spectroscopy, X-ray diffraction and electrical conductivity techniques. The temperature dependent Moessbauer spectra provide, for the first time, evidence for electron exchange between neighbouring europium sites in a non-stoichiometric europium compound. The activation energy Esub(a) for electron mobility above about 200 K is 0.083 +- 0.003 eV for EuClsub(2.96) from electrical conductivity measurements. This value for Esub(a) is in agreement with others derived from the Moessbauer spectra using an electron hopping model according to which a potentially mobile electron is bound at each site for an average time which is a function of temperature. Chemical isomer shifts and ratios of recoil-free fractions (Lamb-Moessbauer factors) for europium(II) to europium(III) are also derived from the spectra. Lattice parameters obtained from X-ray data are discussed in relation to those for the stoichiometric EuClsub(3.00) phase. (Auth.)