Abstract
The series of Mn2−xFexP1−ySiy types of compounds form one of the most promising families of magnetocaloric materials in term of performances and availability of the elemental components. Potential for large scale application needs to optimize the synthesis process, and an easy and rather fast process here described is based on the use of two main type of precursors, providing the Fe-P and Mn-Si proportions. The series of Mn2−xFexP1−ySiy compounds were synthesized and carefully investigated for their crystal structure versus temperature and compared interestingly with earlier results. A strong magnetoelastic effect accompanying the 1st order magnetic transition—as well as the parent phosphide–arsenides—was related to the relative stability of both the Fe magnetic polarization and the Fe–Fe exchange couplings. In order to better understand this effect, we propose a local distortion index of the non-metal tetrahedron hosting Fe atoms. Besides, from Mn-rich (Si-rich) to Fe-rich (P-rich) compositions, it is shown that the magnetocaloric phenomenon can be established on demand below and above room temperature. Excellent performance compounds were realized in terms of magnetic entropy ΔSm and adiabatic temperature ΔTad variations. Since from literature it was seen that the magnetic performances are very sensitive to the synthesis process, correspondingly here a new effective process is proposed. Mössbauer spectroscopy analysis was performed on Mn-rich, equi-atomic Mn-Fe, and Fe-rich compounds, allowing determination of the distribution of hyperfine fields setting on Fe in the tetrahedral and pyramidal sites, respectively. Electronic structure calculations confirmed the scheme of metal and non-metal preferential ordering, respectively. Moreover, the local magnetic moments were derived, in fair agreement with both the experimental magnetization and the Fe contributions, as determined by Mössbauer spectroscopy.
Highlights
The series of pnictides related to the Fe2 P type, which we have studied for a long time, exhibit very unusual magnetic properties, as detailed in a series of reports [1,2,3]
The used method to synthesize the ternary phosphides and arsenides was the conventional route of solid-state reaction, where the powder of the considered elements are intimately mixed placed in an evacuated silica ampoule and annealed at optimized temperatures of reaction that are usually higher than 1100 ◦ C [4,6,7,24]
In order to overpass the associated difficulty with the phosphorus vapor pressure, a group of Amsterdam University has successfully proceeded to low temperature solid-state synthesis by mechanical alloying using the ball milling (BM) method [25,26,27]
Summary
The series of pnictides related to the Fe2 P type, which we have studied for a long time, exhibit very unusual magnetic properties, as detailed in a series of reports [1,2,3] These ternary compounds TT’X (T, T’ = transition metals, X = P, As, Si, Ge etc.) crystalize in one of three types of structures: orthorhombic (SG Pnma: O4), hexagonal (SG P-62m: H3), and tetragonal (SG P42 /nmm: T2). For the intermediately dense compounds of hexagonal Fe2 P type structure, ferromagnetic (Ferro) and long-range magnetic structures are observed These typical behaviors were effectively found with the MnFeP1−x Asx [4] system, where in successive orthorhombic, hexagonal, and tetragonal structures, Mn occupies the CN5-coordinated site (pyramid: PYR) and Fe occupies the CN4-coordinated one (tetrahedron: TET).
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