Kationenverteilung und magnetische Ordnung in den Rutilen (Fe_0,5Ta_0,5)O₂und (Fe_0,45Nb_0,53)O₂

Abstract

The cation distribution of the compounds (Fe0.: Tao.5)O 2 and (Feo.45Nbo.53)O2 has been redetermined by the combined use of neutron and X-ray powder diffraction. Analysis of the powder patterns showed that the compounds are isostructural, their structure being of the rutile-type A O 2. The cations are statistically distributed. There exists no partial ordering, neither of Fe and Ta nor of Fe and Nb. Neutron diffraction patterns taken at 4.2 K showed only one magnetic reflection, which is strongly broadened. The magnetic structure of (Feo.sTa0.5)O 2 is of MnF2-type, whereas (Fe0.45Nbo.53)O2 orders in the BEY-structure. Profile analysis of these broadened reflections was carried out with the convolution integral of a Gaussian with a Lorentzian function, from which correlation lengths could be derived.

Synthesis and magnetic susceptibility of the rutile phase MnTaO4

“…The phases MTaO 4 with M~Ti, V, Cr show a paramagnetic temperature dependence of the magnetic susceptibility, while FeTaO 4 exhibits a broad maximum at ca. 25 K. 13,14 Similar diffuse NPD peaks are observed for MnTaO 4 and FeTaO 4 , 12 which suggests similar local magnetic orderings.…”

“…The (disordered) rutile modi®cation is usually the hightemperature/low-pressure form. It can be remarked that FeTaO 4 , contrary to FeNbO 4 , has so far only been reported with rutile, 12,18 tri-rutile 14 and, in small amounts, a-PbO 2type 18 structures, implying different behavior for FeNbO 4 and FeTaO 4 , despite the fact that Nb and Ta belong to the same group in the Mendeleev table. A comparison of c/a ratios and the x(O) parameter for MnTaO 4 and other rutile MTaO 4 phases shows that MnTaO 4 has a relatively lower c/a ratio, 0.632, and a larger x(O) parameter, 0.3075(2), compared with, respectively, 0.648±0.653 and 0.295±0.304 for the other phases.…”

“…The magnetic properties can be compared with those of the compositionally similar and isostructural compound FeTaO 4 . 12 The magnetic susceptibility of FeTaO 4 exhibits a broad maximum at ca. 25 K. 13,14 Below ca.…”

“…14 NPD patterns collected at 2±140 K show only a diffuse magnetic scattering without any distinct transition. 12 At 4.2 K, a 1.2³ broad peak is present that can be indexed as a 100 re¯ection 12 and this has been interpreted as indicating an anti-ferromagnetic ordering of the MnF 2 type, i.e. with spins aligned along the c-axis and those at the corner sites antiparallel to the ones on the centre sites.…”