Crystalline water intercalation into the Kitaev honeycomb cobaltate Na${}_2$Co${}_2$TeO${}_6$

Abstract

We herein report the successful intercalation of water molecules into the layered honeycomb lattice of Na${}_2$Co${}_2$TeO${}_6$, a Kitaev-candidate compound, to obtain the hydrated phase Na${}_2$Co${}_2$TeO${}_6$$\cdot$$y$H${}_2$O ($y\sim$ 2.4). Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and Rietveld refinements indicate that crystalline water resides between the cobalt-based honeycomb layers. This insertion of neutral molecules significantly alters the crystal structure, increasing the interlayer spacing and modifying the local bonding environment. Magnetization measurements reveal an antiferromagnetic transition at $T_N\sim 17.2$ K, accompanied by a discernible weak ferromagnetic component. The application of moderate magnetic fields induces a spin-flop reorientation at ${\mu }_{0}H\sim 5.7$ T. The $\lambda$-type anomaly and long-range order persist up to 9 T, showing the reconfiguration of the ground state as opposed to its suppression. Heat-capacity analysis reveals the full $2R\ln 2$ magnetic entropy expected for two $J_{\mathrm{eff}}=1/2$ moments per formula unit, confirming the pseudospin description. These findings demonstrate that water intercalation is a robust strategy for tuning the magnetic properties of honeycomb lattice materials. Overall, this study highlights neutral-molecule insertion as a promising route toward the discovery and engineering of quantum magnets based on layered transition metal oxides.

13 pages, 10 figures, accepted in Physical Review Materials

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