SYNTHESIS AND CRYSTAL STRUCTURE OF THE NEW CUPRATES La4Sr4Cu8-xMxO18 (M = Ni, Co)
Gennadi V. Bazuev, Tat'yana I. Chupakhina, Vladimir N. Krasilnikov, Vladimir G. Zubkov
Institute of
Solid State Chemistry, Ural Branch of Russian Academy of Sciences,
91 Pervomaiskaya str.,
Ekaterinburg GSP-145, 620219 Russia
E-mail: zubkov@ihim.uran.ru
Keywords: cuprates,
synthesis, crystal structure
Preparation of new copper complex oxides La4Sr4Cu8-xMxO18+ (M = Ni, Co) has been provided by selective doping of cation sites method. This method was used by us previously for stabilization of cuprates with 8-8-20 structure type of composition Ln8xAxCu8yMyO20, where Ln = La, Pr, Nd; A = Sr, Ca; M = Ni, Co, Fe [1]. M-elements in these compounds act as stabilizing elements, with out which the above phases are not formed under ordinary conditions. X-ray profile analysis of stabilized phase La6,4Ca1,6Cu6,4Ni1,6O20 [2] showed the presence of Ni in octahedral coordination. In results the substitution of Ni for Cu decreases distortion of octahedron, which are compressed in parent compound 8-8-20.
The stabilized phases La4Sr4Cu8-xMxO18+ (M = Ni, Co; 1 <= x <= 2 for Ni and x = 1 for Co) were synthesized from samples of nominal composition La4Sr4Cu8-xMxO20 treated in inert gas (He) at 970-1020oC. X-ray powder diffraction of the products revealed a tetragonal cells (space group P4/mbm) with parameters: a~2ap21/2, c~ap (a = 10.9378(13)A, c = 3.7721(7)A for the nickel substituted sample (x = 2) and a = 10.9419(19)A, c = 3.7656(7)A for cobalt substituted sample (x = 1).
Nickel-substitute cuprate La4Sr4Cu6Ni2O18 was subjected to Rietveld profile analysis by using date obtained with a STOE X-ray powder diffractometer. It was found that the crystal structure of this compound is characterized by fragments consisting of NiO6 octahedron, CuO5 pyramids and the linear linkages O-Cu-O. Ni-atoms are arranged inside the octahedron compressed along the c axis. Differed from 8-8-20 compounds there are none of CuO4 polyhedrons in the given structure being replaced by linear linkages O-Cu-O. Compared with known La2Sr6Cu8O18 octahedral sites occupied by Ni-atoms in the obtained cuprate are found in the structure. Most likely, the formation of these sites by the stabilizing element is cause of obtaining such kinds of compounds.
Electrical resistivity date show semiconductor-like (M = Co) or metallic (M = Ni) behavior.