Direct observation of a magnetically ordered state in YbCu2Si2 under high pressure

The intermetallic compound YbCu2Si2 is a well-known nonmagnetic (NM) Yb int ermediate-valent compound with a Yb valence of 2.9 at ambient pressure and 300 K. In the present work we have investigated the effect of high pressure on the ground state properties of YbCu2Si2 on both microscopic and macrosc opic levels by using the Yb-170 Mossbauer effect, electrical resistance, an d x-ray diffraction techniques, respectively. High-pressure x-ray diffracti on data indicate that the lattice structure of YbCu2Si2 is stable up to 22. 2 GPa. The value of the bulk modulus [B-0 = 168(10) GPa(-1)] is found to be close to the value expected for trivalent RCu2Si2 compounds. The pressure dependence of the electrical resistance reveals evidence for a pressure-ind uced magnetic order for p greater than or equal to 8 GPa. From our Mossbaue r data, we conclude a crossover from the NM to a magnetically ordered state of localized Yb moments for p greater than or equal to 8 GPa and below 2 K . The pressure-induced change of the electric quadrupole splitting indicate s that this transition is accompanied by a valence change towards the Yb3state. [S0163-1829(99)01729-4].