FeMoO₄ Revisited: Crosslike 90° Noncollinear Antiferromagnetic Structure Caused by Dzyaloshinskii–Moriya Interaction

Abstract

The ground state of Fe2+ (S = 2) in α- and β-FeMoO4 is investigated by experiments including X-ray diffraction, Raman scattering, and 57Fe–Mössbauer spectroscopy below 300 K and evaluated by theoretical modeling. Both modifications crystallize in the space group C2/m with the same set of Wyckoff positions. The structural feature of α- and β-FeMoO4 is a tetramer of the so-called butterfly motif. Two iron-sites (Fe2) form an antiferromagnetically coupled dimer whereas two Fe1 establish an antiferromagnetic intertetramer coupling. The effective magnetic exchange of the two magnetic sublattices is based on dominating Dzyaloshinskii–Moriya interaction due to the rare situation of canceling Heisenberg exchange interactions. According to our investigations, the ground states of the two polymorphs differ in terms of their Fe-site specific electric field gradients V ii . Contrary to the α-phase, a degenerate set of V zz and V yy for both iron sites in β-FeMoO4 is extracted from density functional theory calculations. In the vicinity of the phase transition (β → α), the degeneracy of the β-phase is lifted. Correspondingly, we observe a softening of the ν­(Mo–O) phonon modes. Detailed Mössbauer spectra confirm the crosslike 90° antiferromagnetic structure for both modifications and solve the origin of the longstanding issue of disparate quadrupole splittings in α- and β-FeMoO4.

Role of Fe–O–M Bond in Controlling the Electroactive Species Generation from the FeMO4 (M: Mo and W) Electro(pre)catalyst during OER

“…The diffraction peaks can be indexed with the standard diffraction pattern of monoclinic FeMoO 4 (JCPDS 22-0628) which furthermore ascertains the phase purity of the sample. 15 The lattice parameters, obtained from the crystal data (JCPDS 22-0628), confirmed the space group P2/m, the cell dimensions of a: 10.09 Å, b: 9.22 Å, and c: 6.86 Å, and the cell volume of 638 Å 3 . 23 The unit cell of the monoclinic β-FeMoO 4 consisted of four distorted six-coordinated iron and molybdenum units connected to each other through sharing one or more faces (Figure 1b).…”

“…These bimetallic oxides possess such an electronic structure that the surface adsorption–desorption kinetics of the reaction intermediate for these ternary oxides would be optimum . Among the transition-metal molybdates, FeMoO 4 nanostructured materials are widely used in batteries and as an electrocatalyst for nitrogen fixation and largely for selective organic oxidation . Recently, in situ grown FeMoO 4 and other transition-metal molybdates CoMoO 4 and NiMoO 4 have also been studied as anode materials for OER .…”

“…12 Among the transition-metal molybdates, FeMoO 4 nanostructured materials are widely used in batteries 13 and as an electrocatalyst for nitrogen fixation 14 and largely for selective organic oxidation. 15 Recently, in situ grown FeMoO 4 and other transition-metal molybdates CoMoO 4 and NiMoO 4 have also been studied as anode materials for OER. 16 ex situ spectroscopic study with the vertically grown NiMoO 4 nanorods on nickel foam (NF) provides evidence in support of the formation of NiO(OH) as an active phase.…”

Reactivity of FeMoO4 in CsCl Fluxes and Formation of the Salt-Inclusion Type of Compounds

“…FeMoO 4 (mainly α-polymorph with up to 15% β-phase) was synthesized via a metallothermic reduction by reacting iron powder (99.9% ChemPur, fine chemicals), Fe 2 O 3 (99.9% ChemPur, fine chemicals), and MoO 3 (99.9+ % ChemPur, fine chemicals) in evacuated ( p ≈ 2–4·10 –2 mbar) silica glass ampules at 800 °C according to ref . CsCl (≥98% Sigma) and BaCl 2 ·2H 2 O (99.9+ % Alfa Aesar) were dried for 12–48 h at 250 °C.…”

“…42,43 In the presence of Fe The aim of this work is to elucidate the reactions taking place in a combination of the two well-known flux media, namely CsCl and alkaline-metal molybdates. We investigated the reactivity of FeMoO 4 50 in CsCl fluxes and address the following aspects: (i) coexisting product phases, (ii) Fe II (solv) as a probe for oxidation processes, and (iii) reactions of [MoO 4 ] 2− (solv) in alkaline-metal chlorides. Furthermore, we present insights into the formation and structural data of a mixed substitution series of the salt-inclusion type of dimolybdates, A 2 Mo 2 O 7 •ACl (A = K, Rb, Cs).…”