Spin-glass behavior and vacancy order in van der Waals layered β-MoCl4

Two-dimensional β-MoCl4 is an attractive material from the perspectives of magnetism in 4d transition metal compounds, geometrically frustrated lattices, and magnetic van der Waals layered materials, but the magnetism in this compound has not been particularly well studied to date. Here the magnetic properties and crystal structure of MoCl4 are revisited, and results of ac and dc magnetic measurements and single crystal x-ray diffraction are reported. Crystals grow as well-formed and easily cleaved hexagonal plates that are unstable in air. The revised structural model comprises CdCl2-type layers with 50% Mo vacancies distributed over the sites of the triangular cation net. Interestingly, a structural ambiguity regarding the vacancy distribution is identified in the analysis of the diffraction data. The orbital moment is not expected to be quenched in this 4d2 compound. Accordingly, magnetization measurements indicate an effective moment that is about 20% lower than the spin-only value. The magnetic data reveal an anomaly near 5 K, below which a divergence of field-cooled and zero-field-cooled dc magnetization, a slow relaxation of thermoremanent magnetization, and enhanced frequency dependence of ac magnetization are observed. Thus, β-MoCl4 represents an uncommon example of a cleavable spin-glass system.