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The intermetallic compound YbCu 2 Si 2 is a well-known nonmagnetic ͑NM͒ Yb intermediate-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 YbCu 2 Si 2 on both microscopic and macroscopic levels by using the 170 Yb Mössbauer effect, electrical resistance, and x-ray diffraction techniques, respectively. High-pressure x-ray diffraction data indicate that the lattice structure of YbCu 2 Si 2 is stable up to 22.2 GPa. The value of the bulk modulus ͓B 0 ϭ168(10) GPa Ϫ1 ͔ is found to be close to the value expected for trivalent RCu 2 Si 2 compounds. The pressure dependence of the electrical resistance reveals evidence for a pressureinduced magnetic order for pу8 GPa. From our Mössbauer data, we conclude a crossover from the NM to a magnetically ordered state of localized Yb moments for pу8 GPa and below 2 K. The pressure-induced change of the electric quadrupole splitting indicates that this transition is accompanied by a valence change towards the Yb 3ϩ state. ͓S0163-1829͑99͒01729-4͔

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We have investigated the effect of pressure on the electronic, magnetic, and structural properties on a single crystal of conducting, ferromagnet (T C ϭ157 K) La 0.82 Sr 0.18 CoO 3 located near the boundary of the metalinsulator transition. Contrary to the results reported on related systems, we find a transition from the conducting state to an insulating state and a decrease of T C with increasing pressure while the lattice structure remains unchanged. We show that this unusual behavior is driven by a gradual change of the spin state of Co 3ϩ ions from magnetic intermediate-spin (t 2g 5 e g 1 ;Sϭ1) to a nonmagnetic low-spin (t 2g 6 e g 0 ;Sϭ0) state.