Strongly correlated electrons in layered perovskite structures have been the birthplace of high-temperature superconductivity, spin liquids, and quantum criticality. Specifically, the cuprate materials with layered structures made of cornersharing square-planar CuO 4 units have been intensely studied due to their Mott insulating ground state, which leads to high-temperature superconductivity upon doping. Identifying new compounds with similar lattice and electronic structures has become a challenge in solid-state chemistry. Here, we report the hydrothermal crystal growth of a new copper tellurite sulfate, Cu 3 (TeO 4 )(SO 4 )•H 2 O, a promising alternative to layered perovskites. The orthorhombic phase (space group Pnma) is made of corrugated layers of corner-sharing CuO 4 square-planar units that are edge-shared with TeO 4 units. The layers are linked by slabs of corner-sharing CuO 4 and SO 4 . Using both the bond valence sum analysis and magnetization data, we find purely Cu 2+ ions within the layers but a mixed valence of Cu 2+ /Cu + between the layers. Cu 3 (TeO 4 )(SO 4 )•H 2 O undergoes an antiferromagnetic transition at T N = 67 K marked by a peak in the magnetic susceptibility. Upon further cooling, a spin-canting transition occurs at T* = 12 K, evidenced by a kink in the heat capacity. The spin-canting transition is explained on the basis of a J 1 −J 2 model of magnetic interactions, which is consistent with the slightly different in-plane superexchange paths. We present Cu 3 (TeO 4 )(SO 4 )•H 2 O as a promising platform for the future doping and strain experiments that could tune the Mott insulating ground state into superconducting or spin liquid states.

Mixed halide chemistry has recently been utilized to tune the intrinsic magnetic properties of transition-metal halides-one of the largest families of magnetic van der Waals materials. Prior studies have shown that the strength of exchange interactions, hence the critical temperature, can be tuned smoothly with halide composition for a given ground state. Here we show that the ground state itself can be altered by a small change of halide composition in FeCl 3−x Br x . Specifically, we find a threefold jump in the Néel temperature and a sign change in the Weiss temperature at x = 0.08 corresponding to only 3% bromine doping. Using neutron scattering, we reveal a change of the ground state from spiral order in FeCl 3 to A-type antiferromagnetic order in FeBr 3 . From first-principles calculations, we show that a delicate balance between nearest and next-nearest neighbor interactions is responsible for such a transition. These results demonstrate how varying the halide composition can tune the competing interactions and change the ground state of a spiral spin liquid system.

Mixed halide chemistry has recently been utilized to tune the intrinsic magnetic properties of transition-metal halides -one of the largest families of magnetic van der Waals materials. Prior studies have shown that the strength of exchange interactions, hence the critical temperature, can be tuned smoothly with halide composition for a given ground-state. Here we show that the groundstate itself can be altered by a small change of halide composition leading to a quantum phase transition in FeCl3−xBrx. Specifically, we find a three-fold jump in the Néel temperature and a sign change in the Weiss temperature at x = 0.08 corresponding to only 3% bromine doping. Using neutron scattering, we reveal a change of the ground-state from spiral order in FeCl3 to Atype antiferromagnetic order in FeBr3. Using first-principles calculations, we show that a delicate balance between nearest and next-nearest neighbor interactions is responsible for such a transition. These results support the proximity of FeCl3 to a spiral spin liquid state, in which competing interactions and nearly degenerate magnetic k-vectors may cause large changes in response to small perturbations.