Stabilization of a Metastable Fibrous Bi21.2(1)(Mn1–xCox)20 Phase with Pseudo-Pentagonal Symmetry Prepared Using a Bi Self-Flux

Abstract

Communication pubs.acs.org/cm Stabilization of a Metastable Fibrous Bi 21.2(1) (Mn 1−x Co x ) 20 Phase with Pseudo-Pentagonal Symmetry Prepared Using a Bi Self-Flux Srinivasa Thimmaiah,* ,† Valentin Taufour, †,§ Scott Saunders, § Stephen March, § Yuemei Zhang, ‡ Matthew J. Kramer, †,∥ Paul C. Canfield, †,§ and Gordon J. Miller †,‡ The Ames Laboratory, U.S. Department of Energy, Iowa State University, Ames, Iowa 50011, United States Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, United States Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, United States S Supporting Information P ermanent magnets are exceptionally critical for many environmentally friendly, energy harvesting technologies in our energy demanding modern world. 1,2 In particular, rare earth (RE) based permanent magnets such as Nd 2 Fe 14 B 23−5 and SmCo 56,7 are extensively used for energy conversion purposes, mainly in high-power generators and motors found in wind turbines and electric vehicles, due to their very high energy product (BH max ) and lightweight. In particular, demand for RE-based permanent magnets has been growing exponen- tially in recent years. Therefore, to ease reliance on RE-based permanent magnets, development of low cost alternative materials that have high energy products and high Curie temperatures are critical for future sustainability. Mn-based 8 intermetallic compounds are examples gaining ground as an effective alternative, especially the ferromagnetic, low temper- ature (LT) BiMn phase adopting the NiAs-type structure and exhibiting a large uniaxial magnetic anisotropy (K = 2.2 × 10 7 erg cm −3 at 500 K). At temperatures exceeding 300 K, LT- BiMn shows remarkably high coercivity, which is even larger than that for Nd 2 Fe 14 B 2 magnets, making LT-BiMn suitable for high temperature applications. 9−12 However, at 633 K ferromagnetic LT-BiMn transforms to a paramagnetic, high- temperature phase (HTP), 13,14 which, upon rapid quenching, results in a ferromagnetic phase that shows an interesting magneto-optical property 15−18 applicable for magneto-optical memory devices. There are two structural transitions reported for LT-BiMn: one at ca. 100 K where spin reorientation occurs; 19 and another above its Curie or decomposition temperature (633 K). Theoretical calculations suggested that partial replacement of Mn by other transition metals could stabilize its structure in the hexagonal NiAs-type, which is essential for retaining the magnetic properties as well as increasing the magnetic anisotropy, 20,21 but experimental results reveal a change in crystal structure upon doping. 22 Herein, we report a new phase Bi 21.2(1) (Mn 1−x Co x ) 20 (x ∼ 0.15) that was discovered during systematic substitution of 3d and 4d 22 transition metals for Mn in LT-BiMn (NiAs-type) as a theoretically predicted strategy to increase magnetic anisotropy and stabilization of NiAs-type structure at elevated temperature. Crystals of a new metastable Co-doped BiMn phase were grown using Bi as a self-flux at 280 °C. 23 Figure 1 shows the soft and highly fibrous nature of these crystals, which split into submicron-size strands upon applied pressure. According to © 2016 American Chemical Society Figure 1. SEM micrographs of Bi 21.2(1) (Mn 1−x Co x ) 20 crystals showing fibrous morphology. DSC, the Co-doped BiMn phase decomposes endothermically on heating around 168 °C, with no evidence of formation of a new phase during cooling (see Supporting Information, Figure S2). This clearly indicates the metastable nature of the new phase. On the contrary, typical high-temperature reaction conditions of a sample with nominal composition of Mn 43 Co 7 Bi 50 resulted in the hexagonal NiAs-type structure (a = 4.2907(1) A, c = 6.1199(3) A), 24 which decomposes around 355 °C (see Supporting Information, Figure S4) upon heating. Single crystal X-ray diffraction revealed that the new phase crystallizes in orthorhombic symmetry, space group Imma, and the refined composition is Bi 21.2(1) (Mn 1−x Co x ) 20 (x ∼ 0.15). However, the positions of Co atoms in the crystal structure could not be established precisely due to the similar X-ray scattering contrasts of Mn and Co. Thus, wavelength-dispersive X-ray spectroscopy was employed to estimate the amount of Co to be 7(1) at.% in the structure. To evaluate any possible chemical ordering of Co in the structure, we have calculated total energies for two different compositions, each with three different coloring schemes, using VASP. The calculated energy differences among these various coloring models were found to be negligible (11−64 meV/cell or 0.26−1.52 meV/atom) (see Supporting Information, Figure S9), indicating low probability for chemical ordering of Co atoms in the structure under these synthetic conditions. Therefore, we can conclude that Co atoms are essentially randomly mixed with Mn atoms at the 3d metal positions. Received: October 21, 2016 Revised: November 15, 2016 Published: November 15, 2016 DOI: 10.1021/acs.chemmater.6b04505 Chem. Mater. 2016, 28, 8484−8488