X-ray and neutron-diffraction, dc magnetic susceptibility, magnetization, and specific-heat measurements are reported for FexCo(1-x)Ta2O6 mixed oxides. X-ray refinement indicates homogeneous samples for all the reported concentrations. The neutron-diffraction measurements reveal magnetic structures with double propagation vectors (+/-1/4,1/4,1/4) for CoTa2O6, and (1/2,0,1/2) and (0,1/2,1/2) for FeTa2O6. The latter remain unchanged in the Fe-rich samples, for 0.46< or =x<1.00, while the Co-rich samples show propagation vectors (+/-1/4,1/4,0) for 0.09< or =x<0.46. The temperature vs x phase diagram exhibits a bicritical point at about T=4.9 K and x=0.46. For this concentration, and at low temperatures, the system shows coexistence of both magnetic structures. This novel bicritical behavior is interpreted as induced by competition between the different magnetic and crystallographic structures.
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 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 [(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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.