NEUTRON DIFFRACTION STUDIES OF THE METALLIZATION PROCESS IN RNiO₃ PEROVSKITES (R = RARE EARTH). M. Medarde¹, J. Mesot¹, S. Rosenkranz¹, P. Radaelli³, W. Marshall⁴, S. Klotz⁵, J. S. Loveday⁴, G. Hamel⁵, S. Hull⁶. ¹Laboratory for Neutron Scattering, ETH&PSI, 5232 Villigen PSI, Switzerland; ²Laboratoire des Fluorures, Universite du Maine, Avenue Olivier Messiaen, 72017 Le Mans CEDEX, France; ³Institut Laue Langevin, B.P. 156, 38042 Grenoble Cedes 9, France; ⁴University of Edinburgh, Mayfield Road, Edinburgh, EH9 3JZ, U.K.; ⁵University Paris VI, 4 pl. Jussieu, 75252 Paris Cedex 5, France; ⁶Rutheford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, U.K.

RNiO₃ perovskites (R = rare earth different of La) show a very sharp metal-insulator transition /1/ at a temperature TM-I which increases by decreasing the size of the R ion (TM-I = 130, 200, 400 and 420 K for R = Pr, Nd, Sm and Eu, respectively). By applying an external pressure P, TM-I decreases at a constant rate (R-independent) of about -4 K/kbar. In order to investigate the structural change s associated to the metallization of the system, several neutron diffraction experiments have been performed at high pressure (up to 80 kbar) both at continuous and spallation neutron sources. The results indicate that, in contrast to the behavior observed in other perovskite compounds displaying the same space group (Pbnm) /2/, /3/, the distortion of the structure decreases with P /4/. The reasons for this unusual behavior are presented and the implications for the metallization mechanism discussed.

/1/ P. Lacorre et al., J. Solid State Chem. 91,225 (1991).

/2/ A. Katrusiak et al., Solid State Commun. 84, 435 (1992).

/3/ H.K. Mao et al.. J. Geophys. Res. 96, 8069 (1991).

/4/ M. Medarde et al., Phys. Rev. B 52, 9248 (1995).