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Monolayer VSe2, featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic 2D transition-metal dichalcogenides (2D-TMDs). Herein, by means of in situ microscopy and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X-ray and angle-resolved photoemission, and X-ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T-phase and vanadium 3d1 electronic configuration in monolayer VSe2 grown on graphite by molecular-beam epitaxy. Element-specific X-ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe2 as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long-range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic-scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D-TMDs in the search for exotic low-dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim R-143-000-652-112, MOE2016-T2-2-110 National Research Foundation Singapore, NRF Agency for Science, Technology and Research, A*STAR: 1527000016 R-723-000-001-281 Ministry of Science and Higher Education of the Russian Federation: 3.7372.2017/8.9 P.K.J.W. and W.Z. contributed equally to this work. The authors acknowledge financial support from the Singapore Ministry of Education Tier 2 Grants (MOE2016-T2-2-110 and R-143-000-652-112), the National Research Foundation Medium Sized Centre Programme (R-723-000-001-281), and the A*STAR 2D PHAROS Grant (1527000016). D.W.B. acknowledges support from the Ministry of Science and Higher Education of the Russian Federation (Project No. 3.7372.2017/8.9). This work used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under Contract No. DE-AC02-05CH11231, and was partially performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MIUR Italy Progetti Internazionali) project. The authors would also like to acknowledge the Singapore Synchrotron Light Source (SSLS), a National Research Infrastructure under the National Research Foundation Singapore, for providing the facility necessary for conducting the research. The authors appreciate valuable discussions with Yuanping Feng and Jun Zhou, and experimental assistance from Xiaojiang Yu at the SINS beam line of SSLS.

International audience; The crystal structure of single crystalline members of the solid solution series Tl(V1-xCrx)5Se8 (x = 0-1 and Δx = 0.2) was determined and the magnetic and thermoelectric properties of bulk TlV5Se8 were investigated. All compounds crystallize in the pseudo-hollandite-type structure (C2/m) with a nonlinear increase in the unit cell volume due to a simultaneous increase/decrease in the transition metal distances across the edge/face-sharing octahedra. A slight Tl deficiency was found for single crystalline TlV5Se8 and Tl(V0.8Cr0.2)5Se8 as well as for bulk TlV5Se8 according to Rietveld and single crystal structure refinements. The metallic character of bulk TlV5Se8 and its low electrical resistivity compared to poly- and single-crystalline TlCr5Se8 can probably be associated with this probable Tl deficiency. In TlV5Se8, anomalies in the low temperature Seebeck coefficient, i.e. a maximum at T ∼ 14 K and two sign changes at T ∼ 47 K and T ∼ 167 K, were found. Bulk TlV5Se8 is a highly frustrated (f = /TN ≈ 42 K) Curie-Weiss paramagnet ordering antiferromagnetically ( = -1287 K) below TN ≈ 31 K with an effective magnetic moment of μ = 2.68 μB corresponding to V3+. Low-field M(H) measurements revealed the existence of a small ferromagnetic component below TN in bulk TlV5Se8 and, despite the observed spin frustration, no spin glass phase was found in this compound. © 2015 The Royal Society of Chemistry.