Similarly to metal sulfides and selenides, binary and ternary tellurium compounds, in which Te atoms bind to electropositive elements, possess exemplary properties that favor their application in various technological fields. Such desirable attributes include the recently discovered high temperature superconducting property of the iron-based FeSe and FeTe phases. In these studies, it was confirmed that besides crystal chemistry and structure, even material microstructure (size, shape and defectiveness) plays a role in driving important functionality. Nonetheless, many iron/tellurium phases remain poorly investigated, both on the structural and microstructural aspects. Worse off, the only iron-rich phase reported to date in the literature is Fe3Te2 which does not even appear in the published phase diagrams, and the crystal structure of which is unknown.
Recently, an international research team led by Professors Norberto Masciocchi at University of Insubria in Italy,Antonietta Guagliardi of the Italian CNR and Carlos E.M. Campos at Federal University of Santa Catarina in Brazil conducted an innovative study and isolated a novel iron rich telluride, Fe5Te4, easily prepared by mechanochemical techniques. In addition, they engaged in a thorough structural and microstructural characterization of the nanosized Fe5Te4 material. Drs. Kelli de Fátima Ulbrich, Federica Bertolottiand Antonio Cervellino also contributed to the study. Their work is currently published in the research journal, Journal of Material Chemistry C.
Briefly, the research activity commenced with the solid-state synthesis, using elemental iron and tellurium powders with molar compositions lying close (or slightly above) 1:1, mechanochemically treated in steel mills and annealed in vacuum. Next, the researchers characterized the as-milled and thermally treated samples, aiming at determining their stability ranges. X-Ray powder diffraction experiments, performed in the lab and at a synchrotron source (SLS in Switzerland) on powders of the FexTe materials, followed by the innovative use of the Debye scattering equation method, enabled the complete structural and microstructural study. Lastly, magnetic and Mössbauer measurements were undertaken. The Figure sketches several steps of the sample preparation and data analysis.
The authors observed that annealing the samples promoted crystal growth, thereby increasing the crystal domain size and partially healing defects. It was also observed that prolonged heating induces undesired phase transformations to β-FeTe and FeTe2. In addition, the researchers also noted that the Fe5Te4 phase, the iron-richest telluride ever characterized, was isostructural to a few transition metal chalcogenides, from which it separates thanks to the presence of an unusually short intermetallic contact, as low as 2.52 Å.
In summary, the research team successfully demonstrated that a new crystal phase, Fe5Te4, could be very easily prepared mechanochemically as a nanosized material, starting from elemental powder mixtures. In this study, the correct stoichiometry and structure of an originally purported, but manifestly non-existing,Fe3Te2 phase was determined. By utilizing the Debye Scattering Equation, the researchers here provided a quantitative estimate of the defectiveness of this nanosized material. Altogether, a newly developed protocol for integrating microstrain analysis within the Debye Scattering Equation method of characterization was demonstrated, opening the way to new, and unexpected, outcomes of advanced Total Scattering experiments.
This research activity was indeed possible thanks to the skills, and software development, available at the Total Scattering Laboratory in Como, Italy, jointly founded by University of Insubria and the Institute of Crystallography of the Italian CNR in Como (toscalab.uninsubria.it), and the amazing collaboration with UFSC in Florianópolis, where a Summer School of Total Scattering will soon be held (December 2018, see http://pcemc.paginas.ufsc.br/eventos/to-sca-lat-1-0).
K. F. Ulbrich, F. Bertolotti, N. Masciocchi,A. Cervellino,A. Guagliardiand C. E. M. Campos. A comprehensive structural and microstructural investigation of a new iron–telluride nano phase. Journal of Material Chemistry C, 2018, volume 6, page 3047Go To Journal of Material Chemistry