Valence Crossover of Ce Ions in CeCu2Si2 under High Pressure - Pressure Dependence of the Unit Cell Volume and the NQR Frequency

The pressure dependence of the valence of Ce ions in CeCu2Si2 has been investigated by means of X-ray powder diffraction (XRPD) and nuclear quadrupole resonance (NQR) measurements. The high-pressure XRPD measurements were carried out at 12K for CeCu2Si2 and CeCu2Ge2. Precise measurements under smoothly-applied pressure were achieved through the selection of the gasket material of a diamond anvil cell. The jump of the unit cell volume, which was reported previously in CeCu2Ge2, was not observed within the experimental accuracy, indicating the absence of a first-order valance transition. The pressure dependence of Cu-63-NQR frequency nu(Q) measured in CeCu2Si2 was compared with that calculated from the measured lattice parameters. The nu(Q)'s were calculated for the two cases of LaCu2Si2 and CeCu2Si2, corresponding to localized and itinerant cases for 4f electrons, respectively. The measured and calculated slopes d nu(Q)/dP show good agreement. A small but significant deviation from the pressure-linear dependence in nu(Q) was observed in the narrow pressure range of 3.9-4.5 GPa, indicating a valence crossover adjacent to the critical end point.