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

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.We acknowledge the financial support received from the Federal Ministry of Education and Research (BMBF) through grants 05K16PGA, 05K16SI1, and 05K19SI1 ‘eMIL’ and ‘eMMA’. We acknowledge the support of the European Union’s Horizon 2020 Framework research and innovation program under grant agreement no. 654002 (ENSAR2) given to the ISOLDE experiment IS611 ‘Study of molybdenum oxide by means of Perturbed Angular Correlations and Mössbauer spectroscopy’. We further acknowledge Koichi Momma and Fujio Izumi, the creators of VESTA Version 3, for providing the license under Copyright (C) 2006–2021, Koichi Momma, and Fujio Izumi. We thank the Ministry of Economy and Competitiveness Consolider—Ingenio Project CSD2009 0013 ‘IMAGINE’ Spain, and Banco Santander-UCM, project PR87/19-22613. We also acknowledge Österreichische Forschungsförderungsgesellschaft funded projects Competence Headquarters Program E2-Spattertech, Austria, Project: FFGP13222004 and the Austrian Science Fund (FWF), Project: P31423. We are grateful for the support from the Icelandic University Research Fund. K. Bharuth-Ram, H. Masenda, and D. Naidoo acknowledge support from the South African National Research Foundation and the Department of Science and Innovation within the SA-CERN programme. H. Masenda also acknowledges support from the Alexander von Humboldt (AvH) Foundation. I. Unzueta acknowledges the support of Ministry of Economy and Competitiveness (MINECO/FEDER) for grant Nº RTI2018-094683-B-C55. J. N. Gonçalves acknowledges support from by CICECO-Aveiro Institute of Materials (POCI-01-0145-FEDER-007679)—FCT reference (UID/CTM/50011/2013)