Valence crossover of Ce ions in CeCu₂Si₂under high pressure-pressure dependence of the unit cell volume and the NQR frequency

The pressure dependence of the valence of Ce ions in CeCu ₂Si₂has 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 12 K for CeCu ₂Si₂and 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 ⁶³Cu-NQR frequency v_qmeasured in CeCu₂Si₂was compared with that calculated from the measured lattice parameters. The vq's were calculated for the two cases of LaCu₂Si₂and CeCu ₂Si₂, corresponding to localized and itinerant cases for 4f electrons, respectively. The measured and calculated slopes dv_Q/dP show good agreement. A small but significant deviation from the pressure-linear dependence in v_qwas observed in the narrow pressure range of 3.9-4.5 GPa, indicating a valence crossover adjacent to the critical end point.