Polymorphism of the Co–Te nanophases in mechanochemical synthesis

Abstract

The mechanochemical synthesis of all cobalt tellurides' phases is demonstrated in this work. The samples had their structural, microstructural, and magnetic characterizations performed by X-ray powder diffraction, transmission electron microscopy, and magnetometry techniques. The initial atomic stoichiometries tested of Co₃₂Te₆₈ and Co₄₀Te₆₀ resulted in the synthesis of the γ-CoTe₂ Pnnm (marcasite), α-CoTe₂ Pa (pyrite), α-CoTe₂ Pm1 (CdI₂-like), and β-CoTe P6₃/mmc phases with different weight proportions in the sample. Modeling of the X-ray diffractograms employed conventional double-Voigt and crystallite shape-anisotropy DV approaches to show that the volumetric diameter average and true crystallite size of the diffraction domains are in the range of tens of nanometers. Transmission electron microscopy measurements also allowed distribution counting of the crystallite sizes via maximum caliper diameter. Electron diffraction experiments presented comparable structural parameters with Rietveld via the analysis of the Debye rings. A model using the Langevin approaches showed the phases to present both ferromagnetic and superparamagnetic contributions attributed to weakly-interacting metallic Co grains with magnetic domain sizes ranging between 2.3 and 4.0 nm. The phases' evolution with storage time was analyzed over two years and revealed to be stable regarding their structural and microstructural properties.