Optimized Flux Single-Crystal Growth of the Quantum Spin Liquid Candidate NdTa₇O₁₉ and Other Rare-Earth Heptatantalates, ErTa₇O₁₉ and GdTa₇O₁₉

Single crystals are essential for characterizing a wide range of magnetic states, including exotic ones such as quantum spin liquids. This study reports a flux method for growing single crystals of NdTa₇O₁₉, the first quantum spin liquid candidate on a triangular spin-lattice with dominant Ising-like spin correlations. Purple NdTa₇O₁₉ single crystals with hexagonal morphology were successfully grown by using a K₂Mo₃O₁₀-B₂O₃ flux. With lateral sizes up to 3.5 mm and a thickness up to 2 mm, these are the largest dimensions reported to date. The chemical composition was confirmed by powder and single-crystal X-ray diffraction along with scanning electron microscopy with energy dispersive X-ray spectroscopy. Aiming for an accurate determination of the magnetic anisotropy and its effect on the magnetic properties, NdTa₇O₁₉ crystals were additionally analyzed by magnetic susceptibility, revealing a substantial anisotropy without long-range magnetic ordering down to 2 K. Single crystals of two novel rare-earth heptatantalates, ErTa₇O₁₉ and GdTa₇O₁₉, were also grown, and their magnetic properties investigated. The magnetic anisotropy of ErTa₇O₁₉ closely resembles that of isostructural NdTa₇O₁₉, indicating the possibility of a similar exotic magnetic ground state. In contrast, GdTa₇O₁₉ shows paramagnetic behavior, consistent with previous results obtained for polycrystalline samples.