Anisotropy of the Electric Field Gradient in Two-Dimensional α-MoO3 Investigated by 57Mn(57Fe) Emission Mössbauer Spectroscopy

Abstract

Van der Waals α-MoO3 samples offer a wide range of attractive catalytic, electronic, and optical properties. We present herein an emission Mössbauer spectroscopy (eMS) study of the electric-field gradient (EFG) anisotropy in crystalline free-standing α-MoO3 samples. Although α-MoO3 is a two-dimensional (2D) material, scanning electron microscopy shows that the crystals are 0.5–5-µm thick. The combination of X-ray diffraction and micro-Raman spectroscopy, performed after sample preparation, provided evidence of the phase purity and crystal quality of the samples. The eMS measurements were conducted following the implantation of 57Mn (t1/2 = 1.5 min), which decays to the 57Fe, 14.4 keV Mössbauer state. The eMS spectra of the samples are dominated by a paramagnetic doublet (D1) with an angular dependence, pointing to the Fe2+ probe ions being in a crystalline environment. It is attributed to an asymmetric EFG at the eMS probe site originating from strong in-plane covalent bonds and weak out-of-plane van der Waals interactions in the 2D material. Moreover, a second broad component, D2, can be assigned to Fe3+ defects that are dynamically generated during the online measurements. The results are compared to ab initio simulations and are discussed in terms of the in-plane and out-of-plane interactions in the system.

Temperature Dependence of the Hyperfine Magnetic Field at Fe Sites in Ba-Doped BiFeO3 Thin Films Studied by Emission Mössbauer Spectroscopy

“…The eM spectra were recorded with a parallel plate resonance detector, one electrode of which was made of stainless steel containing enriched 57 Fe. The detector was mounted on a drive unit outside the implantation chamber at an angle of 90 • to the incident Mn beam, corresponding to a gamma emission angle from the sample of θ γ = 60 • relative to the surface normal [17]. Beam intensities of≤ 2 × 10 8 ions/s were sufficient to give spectra of good statistics, thus ensuring single probe ion implantation and obviating clustering effects.…”