HIGH-PRESSURE NEUTRON-DIFFRACTION STUDY OF THE METALLIZATION PROCESS IN PRNIO3

The modifications of the crystallographic and magnetic structures of P rNiO3 associated with the decrease of the metal-insulator transition t emperature, T-MI, with external pressure (P-ext approximate to 5 kbar) have been investigated by means of neutron powder diffraction. Our re sults indicate that the symmetry of the unit cell (orthorhombic Pbnm a t ambient pressure), is preserved in the pressure range studied. The c hanges observed in the Ni-O distance and the Ni-O-Ni superexchange ang le are, however, similar in sign but very different in magnitude from those obtained by applying an equivalent ''internal'' pressure. The an tiferromagnetic ordering observed at ambient pressure for T < T-MI rem ains unchanged for P-ext < 5 kbar. This finding is in strong contrast with the results of high-pressure electrical resistance measurements, where a reentrant metallic behavior was observed for P > 4 kbar and T < 65 K.