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 fr om 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 grou p Pbnm, Z = 4. A bond valence study shows that Eu and Ni cations are s lightly over- and underbonded, respectively, giving rise to compressiv e/tensile stresses in the Eu-O/Ni-O bonds which are thought to be in t he origin of the metastable character of the structure. The trend of N i to increase its binding power leads to the observed distortion of th e NiO6 octahedra. The first-order metal-to-insulator (MI) transition w as studied by differential scanning calorimetry (T-MI = 463 or 459 K o n the heating or cooling runs, respectively). The corresponding heat h ow is about half the value observed for NdNiO3, in which a magnetic tr ansition is associated to the MI transition. (C) 1995 Academic Press,I nc.