V. Antonietti

Structure refinement of mixed oxides FexCo1−xTa2O6

Abstract We report X-ray powder diffraction measurements performed on synthetic samples of the tapiolite-like Fe x Co 1− x Ta 2 O 6 solid solution. The crystal structure has been refined by the Rietveld method, by using the program fullprof . All the samples are tetragonal and were indexed to the space group P 4 2 / mnm . The end-members (FeTa 2 O 6 and CoTa 2 O 6 ) have unit-cell parameters quite similar to those previously published. The unit-cell parameters a and c for the mixed samples vary linearly with the cation substitution (Co→Fe) according to the Vegards law in all the range of substitution.

Crystal chemistry and structure of the orthorhombic (Fe,Mn)(Ta,Nb)2O6 family of compounds

In this paper we review the crystal chemistry and structure data from the literature and our results reported in the last decade for natural and synthetic samples of the tantalite-columbite series. Order-disorder transitions and oxidation reactions on the ixiolite-columbite-wodginite system are reported for one Brazilian sample. It is shown that reports on these subjects are sometimes dubious and incomplete. In addition to the reaction atmosphere (air or vacuum), order-disorder and oxidation are strongly dependent on the physical state of the samples (powdered or crystal). Structure and chemical composition have been investigated in our laboratory using X-ray diffraction (XRD), electron probe microanalysis (EPMA) and Mossbauer spectroscopy (MS).

Iron oxidation and order-disorder in the (Fe2+, Mn)(Ta, Nb)2O6(Fe2+, Mn)Fe3+(Ta, Nb)2O8 transition

Heat treatments in air and in vacuum have been performed on crystal and powder of the natural tantalite (Mn2+0.88Fe2+0.09)(Ta5+0.86Nb5+0.14)2O2-6, as well as on powder of synthetic Fe2+(Nb5+0.6Ta5+0.4)2O6. Crystal parameters and hyperfine interactions were obtained by use of x-ray diffraction and Mossbauer 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 [(Mn2+0.88Fe2+0.09)(Ta5+0.86Nb5+0.14)2O2-6]wodginite [(Mn2+,Fe2+)Fe3+(Ta,Nb)2O8] 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 (Mn2+0.88Fe2+0.09)(Ta5+0.86Nb0.145+)2O62-, the other, in minor amount, is (Mn2+0.88,Fe2+0.09)Fe3+(Ta,Nb)2O8. 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 Fe3+(Nb0.6Ta0.4)O4 with a minor amount of (Nb,Ta)2O5.

Cation ordering observed on (Mn0.88Fe0.09)(Ta0.86Nb0.14)2O6: A Mössbauer and X-ray study

Abstract Mossbauer spectroscopy and X-ray powder diffraction are used to show cation ordering on a natural manganotantalite sample. By using electron microprobe analysis the chemical composition (Mn0.88Fe0.09)(Ta0.86Nb0.14)2O6 has been determined. The as-received sample is partially ordered, as suggested by the low intensities of the superstructure reflections in the X-ray diffraction pattern and by the existence of two doublets in the Mossbauer spectrum. Heat treatment in vacuum (P ≅ 10−5 Pa) at temperature T ≅ 1320 K induces cation ordering as shown by the Mossbauer spectrum, and also evidenced by the X-ray diffraction pattern.