Iron oxidation and order-disorder in the (Fe<sup>2+</sup>, Mn)(Ta, Nb)<sub>2</sub>O<sub>6</sub>(Fe<sup>2+</sup>, Mn)Fe<sup>3+</sup>(Ta, Nb)<sub>2</sub>O<sub>8</sub> transition
C A dos Santos; L I Zawislak; V Antonietti; E J Kinast; J B M da Cunha; C A dos Santos; Instituto de Física, UFRGS, CP 15051, Campus do Vale, 91501-970 Porto Alegre, RS, Brazil; L I Zawislak; Instituto de Física, UFRGS, CP 15051, Campus do Vale, 91501-970 Porto Alegre, RS, Brazil; V Antonietti; Instituto de Física, UFRGS, CP 15051, Campus do Vale, 91501-970 Porto Alegre, RS, Brazil; E J Kinast; Instituto de Física, UFRGS, CP 15051, Campus do Vale, 91501-970 Porto Alegre, RS, Brazil; J B M da Cunha; Instituto de Física, UFRGS, CP 15051, Campus do Vale, 91501-970 Porto Alegre, RS, Brazil
Журнал:
Journal of Physics: Condensed Matter
Дата:
1999-09-13
Аннотация:
Heat treatments in air and in vacuum have been performed on crystal and powder of the natural tantalite (Mn<sup>2+</sup><sub>0.88</sub>Fe<sup>2+</sup><sub>0.09</sub>)(Ta<sup>5+</sup><sub>0.86</sub>Nb<sup>5+</sup><sub>0.14</sub>)<sub>2</sub>O<sup>2-</sup><sub>6</sub>, as well as on powder of synthetic Fe<sup>2+</sup>(Nb<sup>5+</sup><sub>0.6</sub>Ta<sup>5+</sup><sub>0.4</sub>)<sub>2</sub>O<sub>6</sub>. Crystal parameters and hyperfine interactions were obtained by use of x-ray diffraction and Mössbauer spectroscopy. It is shown that the partially ordered natural sample is completely ordered after heat treatment in vacuum. Conversely, heat treatment in air induced the tantalite [(Mn<sup>2+</sup><sub>0.88</sub>Fe<sup>2+</sup><sub>0.09</sub>)(Ta<sup>5+</sup><sub>0.86</sub>Nb<sup>5+</sup><sub>0.14</sub>)<sub>2</sub>O<sup>2-</sup><sub>6</sub>]wodginite [(Mn<sup>2+</sup>,Fe<sup>2+</sup>)Fe<sup>3+</sup>(Ta,Nb)<sub>2</sub>O<sub>8</sub>] transformation on the powdered sample. When applied to the crystal sample, the heat treatment in air produced a mixture of two phases: the one in large amount is the ordered (Mn<sup>2+</sup><sub>0.88</sub>Fe<sup>2+</sup><sub>0.09</sub>)(Ta<sup>5+</sup><sub>0.86</sub>Nb<sub>0.14</sub><sup>5+</sup>)<sub>2</sub>O<sub>6</sub><sup>2-</sup>, the other, in minor amount, is (Mn<sup>2+</sup><sub>0.88</sub>,Fe<sup>2+</sup><sub>0.09</sub>)Fe<sup>3+</sup>(Ta,Nb)<sub>2</sub>O<sub>8</sub>. The former arrives from cation ordering in the bulk portion of the sample, while the latter results from the near-surface oxidation. The Mn content as well as the oxidant atmosphere appears to play an important role in the transition mechanism. The same heat treatment applied to the synthetic columbite induces a different reaction: ferrocolumbite is transformed into the ixiolite Fe<sup>3+</sup>(Nb<sub>0.6</sub>Ta<sub>0.4</sub>)O<sub>4</sub> with a minor amount of (Nb,Ta)<sub>2</sub>O<sub>5</sub>.
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