First-order valence averaging phase transition and magnetism of europium in (formula presented)

Mössbauer (Formula presented), magnetization, resistivity, and x-ray diffraction measurements of (Formula presented) at temperatures ranging from 4.2 to 550 K have been carried out. The low-temperature measurements show that in (Formula presented) (space group (Formula presented) there are divalent and trivalent europium ions (1:1) distributed in equivalent sites. The divalent Eu ions order magnetically at (Formula presented)=8 K (evidenced by both Mössbauer and magnetization measurements) and the trivalent europium ions experience an exchange field, which produces a hyperfine field, 80 kOe, in quantitative agreement with theoretical expectations. On warming from 440 to 450 K there is an onset of fast electron hopping between (Formula presented) and (Formula presented) sites, which results in an averaging out of the Mössbauer spectrum. The Mössbauer spectra above 450 K can be fitted by assuming a sudden reduction of the activation energy to an average value of 0.33 eV, with a Gaussian or binomial distribution. The Mössbauer absorption area data follow a Debye curve with (Formula presented)=220 K, with an anomaly at the temperature of the valence-phase transition. The phase transition is evidenced also in the resistivity measurements which show a sharp increase in conductivity at the transition temperature. Powder x-ray diffraction studies show no crystallographic phase transition at the valence averaging phase transition, however a sharp increase in lattice parameters (1.1% in c and 0.26% in a) is observed.