Structure of iron diniobium hexaoxide, FeNb2O6: an example of metal-disordered trirutile structure

One-pot hydrothermal synthesis of FeNbO4 microspheres for effective sonocatalysis

FeNbO4 sonocatalysts were successfully synthesized by a simple hydrothermal route at pH values of 3, 5, 7, 9 and 11. The catalysts were characterized by XRD, XPS, TEM, SEM, N2 adsorption and DRS to analyse the effect of pH parameters on the physicochemical properties of the materials during hydrothermal synthesis. The sonocatalytic activity of FeNbO4 microspheres was evaluated by using acid orange 7 (AO7) as the simulated contaminant. The experimental results showed that the best sonocatalytic degradation ratio (97.45%) of organic dyes could be obtained under the conditions of an initial AO7 concentration of 10 mg L-1, an ultrasonic power of 200 W, a catalyst dosage of 1.0 g L-1, and a pH of 3. Moreover, the sonocatalysts demonstrated consistent durability and stability across multiple test cycles. After active species capture experiments and calculation of the energy band, a possible mechanism was proposed based on the special Fenton-like mechanism and the dissociation of H2O2. This research shows that FeNbO4 microspheres can be used as sonocatalysts for the purification of organic wastewater, which has a promising application prospect.

Formation of contact and multiple cyclic cassiterite twins in SnO2-based ceramics co-doped with cobalt and niobium oxides

Contact and multiple cyclic twins of cassiterite commonly form in SnO₂-based ceramics when SnO₂ is sintered with small additions of cobalt and niobium oxides (dual doping). In this work, it is shown that the formation of twins is a two-stage process that starts with epitaxial growth of SnO₂ on CoNb₂O₆ and Co₄Nb₂O₉ seeds (twin nucleation stage) and continues with the fast growth of (101) twin contacts (twin growth stage). Both secondary phases form below the temperature of enhanced densification and SnO₂ grain growth; CoNb₂O₆ forms at ∼700°C and Co₄Nb₂O₉ at ∼900°C. They are structurally related to the rutile-type cassiterite and can thus trigger oriented (epitaxial) growth (local recrystallization) of SnO₂ domains in different orientations on a single seed particle. While oriented growth of cassiterite on columbite-type CoNb₂O₆ grains can only result in the formation of contact twins, the Co₄Nb₂O₉ grains with a structure comparable with that of corundum represent suitable sites for the nucleation of contact and multiple cyclic twins with coplanar or alternating morphology. The twin nucleation stage is followed by fast densification accompanied by significant SnO₂ grain growth above 1300°C. The twin nuclei coarsen to large twinned grains as a result of the preferential and fast growth of the low-energy (101) twin contacts. The solid-state diffusion processes during densification and SnO₂ grain growth are controlled by the formation of point defects and result in the dissolution of the twin nuclei and the incorporation of Nb⁵⁺ and Co²⁺ ions into the SnO₂ matrix in the form of a solid solution. In this process, the twin nuclei are erased and their role in the formation of twins is shown only by irregular segregation of Co and Nb to the twin boundaries and inside the cassiterite grains, and Co,Nb-enrichment in the cyclic twin cores.

Ten Thousand-Cycle Ultrafast Energy Storage of Wadsley-Roth Phase Fe-Nb Oxides with a Desolvation Promoting Interfacial Layer

Developing advanced electrode materials with enhanced charge-transfer kinetics is the key to realizing fast energy storage technologies. Commonly used modification strategies, such as nanoengineering and carbon coating, are mainly focused on electron transfer and bulk Li⁺ diffusion. Nonetheless, the desolvation behavior, which is considered as the rate-limiting process for charge-storage, is rarely studied. Herein, we designed a nitridation layer on the surface of Wadsley-Roth phase FeNb₁₁O₂₉ (FNO_-x@N) to act as a desolvation promoter. Theoretical calculations demonstrate that the adsorption and desolvation of solvated Li⁺ is efficiently improved at FNO_-x@N/electrolyte interphase, leading to the reduced desolvation energy barrier. Moreover, the nitridation layer can also help to prevent solvent cointercalation during Li⁺ insertion, leading to advantageous shrinkage of block area and reduced volume change of lattice cell during cycling. Consequently, FNO_-x@N exhibits a high-rate capacity of 129.7 mAh g^-1 with negligible capacity decay for 10 000 cycles.

Mn2TeO6: a Distorted Inverse Trirutile Structure

Inverse trirutile Mn₂TeO₆ was investigated using in situ neutron and X-ray powder diffraction between 700 °C and room temperature. When the temperature was decreased, a structural phase transition was observed around 400 °C, from a tetragonal (P4₂/mnm) to a monoclinic phase (P2₁/c), involving a doubling of the cell parameter along b. This complex monoclinic structure has been solved by combining electron, neutron, and synchrotron powder diffraction techniques at room temperature. It can be described as a distorted superstructure of the inverse trirutile structure, in which compressed and elongated MnO₆ octahedra alternate with more regular TeO₆ octahedra, forming a herringbone-like pattern. Rietveld refinements, carried out with symmetry-adapted modes, show that the structural transition, arguably of Jahn-Teller origin, is driven by a single primary mode.

A study on AB2O6 compounds, Part II: the branches of the hcp family

The crystal chemistry of many AB2O6 compounds is summarized in family trees comparing the crystal symmetry of several structures in group-subgroup relations. In this second part of a series the largest family derived from a hexagonally close packed arrangement of anions is described. On reducing the symmetry, different choices of octahedral sites for A and B cations and voids lead to four branches – the Rosiaite-, the ZnTa2O6-, the columbite- and the rutile-branch. We trace the reasons for the specific distribution patterns of the cations in the individual compounds by comparing long range contributions, i.e. the Madelung Part of Lattice Energy (MAPLE), and local effects represented by bond valence calculations according to I. D. Brown. Long range effects largely determine the choice of sites in the respective space groups, however, small local adjustments play a decisive role for the stability of a specific structural modification. Such mechanisms ultimately impose how the structure of an individual compound deviates from an ideally packed arrangement. We discuss the difference between the ideal structures and the real examples given in the family tree by defining a sort of deformation tensor and a measure of similarity, and we describe and depict the anisotropic deformation by a strain tensor.