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, K_2Cu_2TeO_6, K_2Cu_2TeO_6 · H_2O, and K_6Cu_9Te_4O_24 · 2 H_2O, contain Cu^2+ within CuO_4 square planar plaquettes and TeO_6 octahedra ordering to form structural honeycomb layers isolated by interlayer K^+ ions and H_2O molecules. We find the synthesized structures display varying tilt sequences of the CuO_4 plaquettes, leading to distinct Cu^2+ magnetic motifs on the structural honeycomb lattice and a range of effective magnetic dimensionalities. We find that K_2Cu_2TeO_6 · H_2O does not order and displays alternating chain Heisenberg antiferromagnetic (AFM) behavior, while K_2Cu_2TeO_6 and K_6Cu_9Te_4O_24 · 2 H_2O order antiferromagnetically (T_N = 100 K and T_N = 6.5 K respectively). The previously known phase, K_2CuTeO_4(OH)_2 · H_2O, we find contains structurally and magnetically one-dimensional CuO_4 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.