SYNTHESIS, POLYMORPHISM AND MAGNETIC-PROPERTIES OF NOVEL TB2BAMOO5 (M-CO, OR NI) OXIDES

Tb(2)BaMO(5) oxides with M=Co or Ni have been synthesized and characte rized by X-ray powder diffraction. The new Tb2BaCoO5 is a dimorphic co mpound and crystallizes in two different orthorhombic structures. Synt hesis at 1100 degrees C yields a green phase that belongs to the Pnma space group, with the lattice parameters a=12.3683(9) Angstrom, b=5.73 94(7) Angstrom and c=7.1021(9) Angstrom. At higher temperatures a brow n oxide, isostructural with the Tb2BaNiO5, is obtained and corresponds to the Immm space group, with lattice constants a=3.7581(1) Angstrom, b=5.8278(2) Angstrom and c=11.5605(4) Angstrom. Magnetic susceptibili ty measurements reveal the existence of one-dimensional antiferromagne tic correlations above the room temperature in the M(2+) sublattice fo r both the Ni and the Co Immm oxides. Below 60 K, three-dimensional an tiferromagnetism, in which both the M(2+) and the Tb3+ sublattices are involved, has been shown. In the Pnma Tb2BaCoO5 no magnetic order has been detected down to 4.5 K.