Valence fluctuations of europium in the boride Eu₄Pd_29+xB₈

Abstract

We synthesized a high-quality sample of the boride Eu4Pd(29+x)B8 (x = 0.76) and studied its structural and physical properties. Its tetragonal structure was solved by direct methods and confirmed to belong to the Eu4Pd29B8 type. All studied physical properties indicate a valence fluctuating Eu state, with a valence decreasing continuously from about 2.9 at 5 K to 2.7 at 300 K. Maxima in the T dependence of the susceptibility and thermopower at around 135 K and 120 K, respectively, indicate a valence fluctuation energy scale on the order of 300 K. Analysis of the magnetic susceptibility evidences some inconsistencies when using the ionic interconfigurational fluctuation (ICF) model, thus suggesting a stronger relevance of hybridization between 4f and valence electrons compared to standard valence-fluctuating Eu systems.

Valence effect on the thermopower of Eu systems

“…In fact, the VF materials Eu 3 Pd 20 Ge 6 and Eu 4 Pd 29+x B 8 have rather weak temperature dependencies of their valence. 3,6 Therefore, S VF is expected to be small, which is in line with the relatively small thermopowers observed in these materials.…”

“…1 A non-integer average Eu valence may arise due to several physical reasons: (1) A trivial case are mixed-valent systems containing more than one crystallographic Eu site with different valence as, for example, Eu 3 O 4 . 2 More interesting are materials with a single Eu site: (2) Valencefluctuating (VF) systems such as Eu 4 Pd 29+x B 8 3 exhibit a non-integer valence due to thermal fluctuations between the two close-lying integer Eu configurations. The respective characteristic energy scale is given by the valence fluctuation temperature T VF .…”

“…The respective characteristic energy scale is given by the valence fluctuation temperature T VF . (3) In intermediate valent (IV) systems a non-integer Eu valence originates from hybridization between the Eu 4f and the conduction electron states due to Kondo interaction, for example in EuCu 2 (Ge 1−x Si x ) 2 close to x = 0.7. 4,5 In this case the characteristic energy scale corresponds to the Kondo temperature T K .…”