NEUTRON-DIFFRACTION STUDY OF THE METAL-INSULATOR-TRANSITION IN PRNIO3

The modifications of the crystallographic structure of PrNiO3 associat ed to the decrease of the metal insulator temperature T-M-I with exter nal pressure (P-max approximate to 5 kbar) have been investigated by m eans of neutron powder diffraction. The most interesting finding is th e unexpected evolution of the tilt angle phi of the NiO6 octahedra whi ch, in contrast to the results reported for other isostructural compou nds, decreases under pressure. A simple charge-transfer scheme is used to relate the evolution of phi to the decrease of T-M-I upon pressure .