CRYSTAL CHEMISTRY OF OXONIOBATES WITH CONDENSED CLUSTERS

Vladimir G. Zubkov, Alexander P. Tyutyunnik, Vladlen P. Zhukov, Ivan F. Berger, Ol'ga V. Makarova

Institute of Solid State Chemistry, Ural Branch of Russian Academy of Sciences, 91 Pervomaiskaya str., Ekaterinburg GSP-145, 620219 Russia
E-mail: tyutyunnik@ihim.uran.ru

Keywords: condensed clusters, oxoniobates, crystal chemistry

The last years, the group of "metal oxides", possessing direct metal-metal bonding, have been essentially enlarged by including of the "reduced barium (strontium or europium) oxoniobates with condensed clusters Nb₆O₁₂". These compounds were named so complicated because of technological peculiarities of theirs preparation - they could be synthesized only, when mechanical mixtures of highest niobium oxide and barium (strontium or europium oxide) carbonate are reduced in presence of niobium metal or carbon (acetylene soot). Basic product of first stage of the reduction is alpha-phasoid, that is a disperse liaphobous system, being coherently connected and chaotically arranged micro-phases (4-20 A and higher) of niobium monoxide and perovskite. The alpha-phasoid exhibits a tendency towards coalescence, coagulation and mutual ordering of constitutive micro-phases. The above processes result in a final product, being a stable configuration, that corresponds to a specific macro-phase. Detailed structure investigations of the prepared compounds, carried out by means of X-ray, neutron and electron diffraction and HREM, showed they are block built. Basing on these results, the crystal structure of the reduced oxoniobates was proposed to be described as a combination of niobium monoxide blocks and perovskite ones with dimensions [p*q*r] and {s*r*u*}, respectively. The values in brackets and parenthesis may vary from 0 up to infinity. They correspond to the ratio between block sizes and unit cell constants of niobium monoxide and perovskite, respectively. In such terms, NbO may be described as a combination of [inf*inf*inf] and {0*0*0}, Ba(Sr,Eu)NbO₃ - [0*0*0] and {inf*inf*inf}. The following values have been obtained in real oxoniobates.

Two-dimensional:

Ba(Eu,Sr)₂Nb₅O₉ - [inf*inf*1], {inf*inf*2};
BaNb₄O₆ - [inf*inf*1], {inf*inf*1};
SrNb₄O₆ - [inf*inf*2], {inf*inf*2};
BaNb₇O₉ - [inf*inf*2], {inf*inf*1}.

One-dimensional:

BaNb₅O₈ - [1*1*inf], {1*1*inf};
Ba(Sr,Eu)₄Nb₁₇O₂₆ - [2*2*inf], {2*2*inf};
Ba₃Nb₁₆O₂₃ - [2*2*inf], {1*3*inf};
Ba₄Nb₁₄O₂₃ - [1*3*inf], {2*2*inf}.

Analysis of interatomic distances in these compounds showed direct niobium-niobium interaction into the niobium monoxide block to form a metal conductivity band. Subsequent investigation, including quantum chemistry calculation, showed the metal-like electric and magnetic properties of the reduced oxoniobates to depend directly on dimensions of the condensed clusters Nb₆O₁₂.

This work was supported by RFBR, grant 96-32015a.