We present a detailed study of the temperature evolution of the crystal
structure, specific heat, magnetic susceptibility and resistivity of single
crystals of the paradigmatic valence-fluctuating compound EuIr2Si2. A
comparison to stable-valent isostructural compounds EuCo2Si2 (with
Eu3+), and EuRh2Si2, (with Eu2+) reveals an anomalously large
thermal expansion indicative of the lattice softening associated to valence
fluctuations. A marked broad peak at temperatures around 65-75 K is observed in
specific heat, susceptibility and the derivative of resistivity, as thermal
energy becomes large enough to excite Eu into a divalent state, which localizes
one f electron and increases scattering of conduction electrons. In addition,
the intermediate valence at low temperatures manifests in a moderately
renormalized electron mass, with enhanced values of the Sommerfeld coefficient
in the specific heat and a Fermi-liquid-like dependence of resistivity at low
temperatures. The high residual magnetic susceptibility is mainly ascribed to a
Van Vleck contribution. Although the intermediate/fluctuating valence duality
is to some extent represented in the interconfiguration fluctuation model
commonly used to analyze data on valence-fluctuating systems, we show that this
model cannot describe the different physical properties of EuIr2Si2 with
a single set of parameters.Comment: 12 pages, 4 figures, 1 tabl