Crystallographic and Magnetic Properties of Solid Solutions of the Phosphides M2P, M = Cr, Mn, Fe, Co, and Ni

Abstract

Several phosphides with the chemical formula M2P crystallize in a hexagonal structure: Mn2P, Fe2P, and Ni2P. The structure of Co2P is orthorhombic. These two structures are related via an identical elementary subcell consisting of a tetrahedral-site and a pyramidal-site M-atom pair. This investigation shows that solid solutions between two hexagonal end members, such as Fe2P or Ni2P with Mn2P, may exhibit orthorhombic structures at intermediate compositions. Fe2P shows complete solid solubility with Ni2P. Curiously, hexagonal and orthorhombic symmetries alternate with decreasing number of 3d electrons, Ni2P(hex) -Co2P(ortho) -Fe2P(hex) -FeMnP(ortho) -Mn2P(hex). Lattice parameter variations with composition and Mössbauer studies reveal atomic ordering in the mixed systems, Mn and Cr substituting for pyramidal-site iron in FeMnP and FeCrP while Ni and Co substitute preferentially for tetrahedralsite iron in FeCoP and FeNiP. While neither Co2P nor Mn2P are ferromagnetic, intermediate phases are, the Curie temperature and magnetization reaching the maximum values 310°C and 3.03 μB/molecule for MnCoP. Metamagnetism appears for a range of compositions about (Mn0.7Co0.3)2P. The Co2P-Fe2P system is also ferromagnetic with a maximum Curie temperature near FeCoP. This system shows a sharp discontinuity in magnetization, but only a small discontinuity in Curie temperature, across the phase transition. Sensitivity to stoichiometry in the magnetization of Fe2-εP is attributed to an electron/atom ratio near that for the appearance of metamagnetism.

Tunable dimensional crossover and magnetocrystalline anisotropy in Fe2P -based alloys

“…Of particular interest is the magnetocrystalline anisotropy (MCA) of Fe 2 P, which, at 2.3 MJ/m 3 , is record high at zero temperature for systems without heavy elements [10,11]. Although ferromagnetism in Fe 2 P vanishes through a first-order phase transition at T C = 216 K, this temperature can be greatly increased by alloying with Si, Ni, Co, and other easily available elements [1,2,[12][13][14]. The origin of such unusual T C sensitivity to alloying is not understood.…”

“…A uniform mesh of 12 × 12 × 20 points provided sufficient accuracy for the Brillouin zone integration. At x < 0.2 the (Fe 1−x Co x ) 2 P alloy has a hexagonal structure with lattice constants that are almost independent of x [2]. Therefore, we used the experimental values for pure Fe 2 P (a = 5.8675Å, c = 3.4581Å) [2] at all concentrations in these alloys, and the internal coordinates from Ref.…”

“…1.051401 Transition-metal pnictide alloys based on Fe 2 P have attracted considerable attention due to the unusual sensitivity of their magnetic properties to temperature, pressure, external magnetic field, and alloying [1][2][3][4][5], as well as their possible magnetocaloric applications [1], and they have been extensively studied theoretically [6][7][8][9]. Of particular interest is the magnetocrystalline anisotropy (MCA) of Fe 2 P, which, at 2.3 MJ/m 3 , is record high at zero temperature for systems without heavy elements [10,11].…”

First-order ferromagnetic transition in single-crystalline (Mn,Fe)2(P,Si)

“…Among those, the Fe 2 P-based magnetocaloric materials have a rich and long history that dates back to the 1960s. [1][2][3][4] A great step towards a better understanding of this material was later made, thanks to the synthesis of high purity Fe 2 P single crystals. It is now well accepted that Fe 2 P is a ferromagnet with the easy direction along the c axis and that it shows a firstorder ferromagnetic transition at T C % 216 K. However, a rare peculiarity is that the crystal symmetry does not change across the phase transition, as the space group remains P 62m.…”

Magnetic and Transport Properties of (Fe1-xNix)2P in View of KKR-CPA Results

“…The T2D hexagonal systems (space group P62m), where T represent 3d (Mn, Fe, Co, Ni) or 4d (Ru, Rh) transition metal elements, and D are metalloids (Si, P, As), exhibit complex behaviour of some measured quantities. The structural, magnetic and transport properties were investigated by various experimental techniques (neutron diffraction measurements, Mössbauer effect, specific heat, electrical resistivity, magnetization measurements) [1,2], as well as by the (475) band theory methods [3][4][5]. In general, T2P compounds form solid solutions with the same crystal structure.…”