Structure and Magnetic Properties of Bulk Synthesized Mn2−xFexP1−ySiy Compounds from Magnetization, 57Fe Mössbauer Spectroscopy, and Electronic Structure Calculations

Abstract

† Deceased end October 2018 when completing this work dedicated to Stanek, a high level and gentleman scientist.Abstract: The series of Mn 2−x Fe x P 1−y Si y 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 Mn 2−x Fe x P 1−y Si y 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 ∆S m and adiabatic temperature ∆T ad 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.The series of pnictides related to the Fe 2 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]. In fact, 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 P4 2 /nmm: T2). The polytype structures, from the most to the least dense compacting modes of a unique Rhombus (R) block (with 4, 3, and 2 blocks successively), are formed from two X-tetrahedrons and two-X-pyramids coordinating T and T' [1,4]. The compaction level of the structure directly impacts the magnetic characteristic of the polytypic series when T and T' share magnetic trends. In fact, the tetragonal compounds, being mostly arsenides (X = As), exhibit antiferromagnetic (AF) characteristics [4,5], while the orthorhombic compounds, being mostly phosphides, exhibit typically non-collinear and l...

Effect of Co and Ni Doping on the Structure, Magnetic and Magnetocaloric Properties of Fe-Rich (Mn,Fe)2(P,Si) Compounds

“…1. As proven by density functional theory (DFT) calculations, the magnetic moment of Fe at the 3g site is lower than for the preferential Mn atoms at the same site [32,33].…”

“…In the (Mn,Fe) 2 (P,Si) family of compounds, Mn atoms show a preference to the pyramidal 3g site, which is marked by a larger magnetic moment, whereas Fe atoms preferentially occupy 3f sites, associated with a smaller magnetic moment [7,31]. In the Fe-rich region, the 3f site is fully occupied by Fe, and the 3g site is partially occupied by Mn and partially by Fe [32]. A schematic representation of the crystal structure is shown in Fig.…”