PREPARATION, CRYSTAL-STRUCTURE, AND METAL-TO-INSULATOR TRANSITION OF EUNIO3

Polycrystalline EuNiO3 has been prepared by a citrate technique and a subsequent thermal treatment at 1000 degrees C under 200 bar of oxygen pressure. The crystal structure was refined by the Rietveld method from X-ray powder diffraction data. EuNiO3 is an orthorhombic perovskite with a = 5.2938(1), b = 5.4560(1), c = 7.5359(2) Angstrom, space group Pbnm, Z = 4. A bond valence study shows that Eu and Ni cations are slightly over- and underbonded, respectively, giving rise to compressive/tensile stresses in the Eu-O/Ni-O bonds which are thought to be in the origin of the metastable character of the structure. The trend of Ni to increase its binding power leads to the observed distortion of the NiO6 octahedra. The first-order metal-to-insulator (MI) transition was studied by differential scanning calorimetry (T-MI = 463 or 459 K on the heating or cooling runs, respectively). The corresponding heat how is about half the value observed for NdNiO3, in which a magnetic transition is associated to the MI transition. (C) 1995 Academic Press,Inc.