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

Abstract

Anhydrous europium (III) chloride, when heated under vacuum at temperatures above 570 K, decomposes according to the overall reaction EuCl 3(s) → EuCl 2(s) + 1 25 Cl 2(g) A non-stoichiometric chlorine-deficient phase with the hexagonal UCl 3-type structure is observed down to at least a europium-to-chlorine ratio of 1:2.84. Samples of composition EuCl > 2.8 were examined by Mössbauer spectroscopy, X-ray diffraction and electrical conductivity techniques. The temperature-dependent Mössbauer spectra provide, for the first time, evidence for electron exchange between neighbouring europium sites in a non-stoichiometric europium compound. The activation energy E a for electron mobility above about 200 K is 0.083 ± 0.003 eV for EuCl 2.96 from electrical conductivity measurements. This value for E a is in agreement with others derived from the Mössbauer 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-Mössbauer 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 EuCl 3.00 phase.