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 paper, 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, ${\mathrm{K}}₂{\mathrm{Cu}}₂{\mathrm{TeO}}₆, {\mathrm{K}}₂{\mathrm{Cu}}₂{\mathrm{TeO}}₆\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}₂\mathrm{O}$, and ${\mathrm{K}}₆{\mathrm{Cu}}₉{\mathrm{Te}}₄{\mathrm{O}}₂₄\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}₂\mathrm{O}$, contain ${\mathrm{Cu}}²⁺$ within ${\mathrm{CuO}}₄$ square planar plaquettes and ${\mathrm{TeO}}₆$ octahedra ordering to form structural honeycomb layers isolated by interlayer ${\mathrm{K}}⁺$ ions and ${\mathrm{H}}₂\mathrm{O}$ molecules. We find the synthesized structures display varying tilt sequences of the ${\mathrm{CuO}}₄$ plaquettes, leading to distinct ${\mathrm{Cu}}²⁺$ magnetic motifs on the structural honeycomb lattice and a range of effective magnetic dimensionalities. We find that ${\mathrm{K}}₂{\mathrm{Cu}}₂{\mathrm{TeO}}₆\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}₂\mathrm{O}$ does not order and displays alternating chain Heisenberg antiferromagnetic (AFM) behavior, while ${\mathrm{K}}₂{\mathrm{Cu}}₂{\mathrm{TeO}}₆$ and ${\mathrm{K}}₆{\mathrm{Cu}}₉{\mathrm{Te}}₄{\mathrm{O}}₂₄\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}₂\mathrm{O}$ order antiferromagnetically (${T}_N=100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_N=6.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, respectively). The previously known phase, ${\mathrm{K}}₂{\mathrm{CuTeO}}₄{(\text{OH})}₂\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}₂\mathrm{O}$, we find contains structurally and magnetically one-dimensional ${\mathrm{CuO}}₄$ plaquettes leading to uniform chain Heisenberg AFM behavior and shows no magnetic order down to $T=0.4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. We discuss and highlight the usefulness of the hydroflux technique in novel syntheses and the interesting magnetic motifs that arise in these particular phases.