Investigation of New Routes for the Synthesis of Mn 4 Si 7
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ganese silicides (HMSs), commonly referred to as MnSix (x = 1.72–1.75), are among the most promising semiconductors suitable for applications in thermoelectric energy harvesting systems, novel optoelectronic[1–3] and photovoltaic devices.[4] Moreover, the HMSs constituents, Mn and Si, are environmental friendly and they are present in abundance in the Earth’s crust. HMSs are called ‘‘Nowotny chimney ladder phases.’’[5] They crystallize in a tetragonal structure and are composed of two separate sublattices. The first one is a tetragonal array of manganese, which forms a chimney, and the second one embedded into this chimney corresponds to an ALI ALLAM, Ph.D. Student, is with IM2NP, UMR 7334 CNRS, and with MADIREL, UMR 7246 CNRS, Universite´ Aix-Marseille, 13397 Marseille Cedex 20, France. PASCAL BOULET, Associate Professor, is with MADIREL, UMR 7246 CNRS, Universite´ AixMarseille. CARLOS A. NUNES, Professor, is with Departamento de Engenharia de Materiais (DEMAR), Escola de Engenharia de Lorena (EEL), Universidade de Sa˜o Paulo (USP), Lorena, SP 12600-970, Brazil. MARIE-CHRISTINE RECORD, Professor, is with IM2NP, UMR 7334 CNRS, Universite´ Aix-Marseille. Contact e-mail: [email protected] Manuscript submitted October 16, 2012. Article published online January 26, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
helical zigzag chain of silicon, which forms a ladder.[6] Several HMS phases have been reported in the literature: Mn4Si7, Mn11Si19, Mn15Si26, and Mn27Si47. These structures differ only by the c-parameter, which is in all cases very large in comparison to the a-parameter. Therefore, it is difficult to distinguish these phases from one another.[7–9] HMSs reported in the literature have been synthesized both as thin films and bulk materials. In a previous work, we investigated the stability range of HMS phases,[10] and we showed that in bulk material under atmospheric pressure, Mn27Si47 is the stable phase at least up to 1073 K (800 °C). The identification of the structure was possible since we proposed a way from the X-ray diffraction (XRD) patterns to distinguish the HMS phases. Only a few works mentioned the existence of Mn4Si7, and they all concern thin films.[11–14] However, Migas et al.[9] showed by ab initio calculations that Mn4Si7 is the most interesting phase for thermoelectric application. In effect, by contrast to the other ones that are degenerate semiconductors, this phase is a semiconductor with an indirect band gap of 0.77 eV. Therefore, the aim of this work was to determine different ways of obtaining the Mn4Si7 phase. Our first objective was to confirm the formation of Mn4Si7 as thin films and to propose a route of fabrication easily transferable to industries. The second objective was to find a route to obtain this phase as a bulk material. Thin films were prepared by depositing manganese layers with different thicknesses onto Si (100) and Si (111) substrates by sputtering. These samples were then subjected to rapid thermal annealing (RTA) at different temperatures and characterized by X-ray diffraction. The
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