Temperature and pressure dependence of lattice parameters of the XCu2Si2 compounds (X=Ce,Eu, Yb,La, Gd,Lu, Ca,Th)

The RECU₂Si₂(RE=Rare Earth) series shows pronounced anomalies of the lattice parameters. In those stable valent compounds with 4f orbital angular momentumL0 thec/a ratio shows an anomalous temperature dependence due to crystal field effects. The compounds with unstable valence (RE=Ce, Eu, Yb) show less or no such effects but on the other hand show anomalies of the volume which are temperature dependent. Using these volume anomalies of the unstable valent systems and the measured bulk moduli of the reference compounds we have extracted the valence as function of temperature.Between 4K and 300K the valences of CeCu₂Si₂ and YbCu₂Si₂ change by only a few percent towards 3, while it changes for EuCu₂Si₂ by ca. 20% towards 2. A critical discussion of these valences will be given.