Hydroflux-Controlled Growth of Magnetic K-Cu-Te-O(H) Phases

Innovative synthetic approaches can yield new phases containing novel structural and magnetic motifs. In this work, we show the synthesis and magnetic characterization of three new and one previously reported layered phase in the K-Cu-Te-O(H) phase space using a tunable hydroflux technique. The hydroflux, with a roughly equal molar ratio of water and alkali hydroxide, is a highly oxidizing, low melting solvent which can be used to isolate metastable phases unattainable through traditional solid state or flux techniques. The newly synthesized phases, K₂Cu₂TeO₆, K₂Cu₂TeO₆ · H₂O, and K₆Cu₉Te₄O₂₄ · 2 H₂O, contain Cu²⁺ within CuO₄ square planar plaquettes and TeO₆ octahedra ordering to form structural honeycomb layers isolated by interlayer K⁺ ions and H₂O molecules. We find the synthesized structures display varying tilt sequences of the CuO₄ plaquettes, leading to distinct Cu²⁺ magnetic motifs on the structural honeycomb lattice and a range of effective magnetic dimensionalities. We find that K₂Cu₂TeO₆ · H₂O does not order and displays alternating chain Heisenberg antiferromagnetic (AFM) behavior, while K₂Cu₂TeO₆ and K₆Cu₉Te₄O₂₄ · 2 H₂O order antiferromagnetically (TN = 100 K and TN = 6.5 K respectively). The previously known phase, K₂CuTeO₄(OH)₂ · H₂O, we find contains structurally and magnetically one-dimensional CuO₄ plaquettes leading to uniform chain Heisenberg AFM behavior and shows no magnetic order down to T = 0.4 K. We discuss and highlight the usefulness of the hydroflux technique in novel syntheses and the interesting magnetic motifs that arise in these particular phases.