Single crystals of the van der Waals layered $5d$ transition-metal compound ${\mathrm{Os}}_{0.55}{\mathrm{Cl}}_2$ were grown and characterized by x-ray diffraction, magnetization and heat-capacity measurements, and atomic resolution electron microscopy. The crystals are stable in air and easily cleaved. The structure is derived from the ${\mathrm{CdCl}}_2$ structure type, with triangular layers of transition metal sites coordinated by edge-sharing octahedra of Cl and separated by a van der Waals gap. On average, only 55% of the metal sites are occupied by Os, and evidence for short- and long-ranged vacancy orders is observed by diffraction and real-space imaging. Magnetization data indicate magnetocrystalline anisotropy due to spin-orbit coupling, antiferromagnetic correlations, and no sign of magnetic order or spin freezing down to 0.4 K. Heat-capacity measurements in applied magnetic fields show only a broad, field-dependent anomaly. The magnetic susceptibility and heat capacity obey power laws at low temperature and low field with exponents close to 0.5. The power law behaviors of the low-temperature heat capacity and magnetic susceptibility suggest gapless magnetic fluctuations prevent spin freezing or ordering in ${\mathrm{Os}}_{0.55}{\mathrm{Cl}}_2$. Divergence of the magnetic Gruneisen parameter indicates nearness to a magnetic quantum critical point. Similarities to behaviors of spin-liquid materials are noted, and in total the results suggest ${\mathrm{Os}}_{0.55}{\mathrm{Cl}}_2$ may be an example of a quantum spin liquid in the limit of strong chemical disorder.

• Received 15 April 2019

DOI:https://doi.org/10.1103/PhysRevB.99.214402