Single-Crystal Growthand Unusual Nearly IsotropicMagnetism of Two-Dimensional K2Co2(HPO3)2(C2O4)(H2O)2 with a Triangular Lattice of Spin Dimers

We report the hydrothermal growth of millimeter-sized single crystals of the two-dimensional bilayer triangular-lattice antiferromagnet K2Co2(HPO3)2(C2O4)(H2O)2 using a methanol/H2O (v/v, 4:1) solvent. Magnetic susceptibility exhibits only weak anisotropy: data for μ0H//ab and μ0H//c nearly coincide above ∼14 K and differ only modestly below it. The Curie–Weiss fits yield effective magnetic moments of 5.37(1) μB and 5.40(1) μB for μ0H//ab and μ0H//c, respectively, with negative Weiss temperatures of −35 K and −33 K, confirming dominant antiferromagnetic interactions. High-field magnetization curves for both orientations converge above ∼18 T to a common saturation value of ∼2.2 μB per Co2+, matching the powder data. This unusual, nearly isotropic behavior is attributed to the weakly distorted oxygen octahedra and averaging over symmetry-equivalent Co sites with different anisotropy axes. 31P NMR measurements on aligned single crystals at 1.7 K reveal a spectral change around 6 T and line splittings above 9 T, consistent with the powder-derived phase diagram. Our results demonstrate that rational synthesis combined with single-crystal investigations provides comprehensive insights into the unique dimer-based triangular lattice, establishing K2Co2(HPO3)2(C2O4)(H2O)2 as a valuable platform for studying field-induced quantum phase transitions in frustrated bilayer magnets.