Columbite-type Fe_xMn_1−xNb₂O₆solid solution: structural and magnetic characterization

Abstract

This paper deals with the columbite Fe x Mn 1Àx Nb 2 O 6 (x ¼ 0, 0.25, 0.50, 0.75 and 1) solid solution. After synthesis under vacuum, structural and magnetic properties were studied by means of X-ray diffraction, Rietveld refinement, electron paramagnetic resonance and magnetization measurements. This investigation showed that (i) the homogeneity range of the solid solution is complete and the A (Fe and/or Mn) cations are randomly distributed in the A-octahedral sites of the structure; (ii) the oxidation state of Fe and Mn is always þ2; (iii) there is no cation disorder between the A and Nb sites; (iv) an antiferromagnetic transition is present only for the two end-members (FeNb 2 O 6 and MnNb 2 O 6 ).

Quantum magnetism in minerals

“…The noncollinearity of the spin structure is dictated by the direction of the local Ising axis in the distorted AO 6 octahedra. In all of the studied solid solutions with random distribution of the A cations, the non-stoichiometric composition rapidly suppressed long-range magnetic order [236,[239][240][241]. This evidences a substantial drop of the average exchange strength both within and between the chains and a tendency for enhanced frustration among the interchain interactions as a result of disorder.…”

Polarized Raman scattering and infrared spectroscopy of a natural manganocolumbite single crystal

“…For manganocolumbite, the AO 6 octahedra are in general more distorted than the BO 6 octahedra because of the stronger difference in ionic radii between Fe +2 (0.78 Å) and Mn +2 (0.83 Å) compared with Nb +5 and Ta +5 , which have nearly the same ionic radii of 0.64 Å. Octahedra with the same central cation share edges forming zig‐zag chains along the crystal c ‐axis. NbO 6 of adjacent octahedra chains share corners to form double layers parallel to the bc ‐plane, which are connected via AO 6 octahedra through common corners 22…”

“…divalent ions on the B site and pentavalent ones on the A site, the most probable scenario is a two‐mode behavior, as observed in complex perovskites 38. It is well known that while synthetic manganocolumbite are fully ordered, natural minerals could present disordered, partially ordered or A/B ordered ions 16, 18, 22, 23. In the (Mn, Fe)(Nb, Ta) 2 O 6 manganocolumbite system, the A/B ordering degree ( OD , in %) can be determined from the unit cell parameters by the empirical relation23 OD (%) = 1727 − 941.6( c = 0.2329 a ), with an accuracy of 5%.…”

“…a chemical combination between manganotantalite (MnTa 2 O 6 ) and ferrocolumbite (FeNb 2 O 6 ), deserve special attention due to their potential technological applications. Besides, in the natural state, these materials are found in granitic pegmatite and constitute the primary source of niobium 22. Regarding their structures, the iron‐tantalum rich side of the phase diagram ( x and/or y lower than ca 0.1) shows the tetragonal tapiolite ( P4 2 / mnm ) structure, while the remaining part of the diagram is largely dominated by the orthorhombic columbite structure (P bcn ) 16, 23–25.…”

Inorganic salt-assisted hydrothermal synthesis and excellent visible light-driven photocatalytic performance of 3D MnNb2O6flower-like nanostructures

“…NbO 6 octahedra of adjacent chains share corners to form double layers, parallel to the bc-plane, which are connected via MnO 6 octahedra through common corners. 35 Therefore, during the crystal growth, the 6 equivalently close-packed directions become the preferential growth directions. The dissolved Mn 2+ and Nb 2 O 5 •nH 2 O are surrounded by many sulfate ions, which, in the view of the dynamic process, would slow the collisions among the reacting ions and lead to preferential growth into hexagonal nanoparticles owing to the intrinsic anisotropic nature of MnNb 2 O 6 .…”