New Lithium\nCopper Borates with BO₃ Triangles:\nLi₆CuB₄O₁₀, Li₃CuB₃O₇, Li₈Cu₇B₁₄O₃₂, and Li₂Cu₉B₁₂O₂₈

Abstract

Crystal structures of three new lithium\ncopper borates, Li₃CuB₃O₇, Li₈Cu₇B₁₄O₃₂, and Li₂Cu₉B₁₂O₂₈, and a new Li₆CuB₄O₁₀ polymorph were solved by single-crystal\nX-ray diffraction.\nIn all of the structures, the boron cations form BO₃ triangles,\nwhich are connected with each other and with copper polyhedra only\nvia corners in Li₆CuB₄O₁₀ and Li₃CuB₃O₇ and via both corners and edges\nin Li₈Cu₇B₁₄O₃₂ and Li₂Cu₉B₁₂O₂₈. The Li₃CuB₃O₇ and Li₈Cu₇B₁₄O₃₂ compounds were synthesized as pure samples\nwith only trace amounts of impurities; hence, their magnetic properties\ncould be investigated and analyzed in terms of underlying magnetic\ncouplings. Other compositions always represented multiphase mixtures.\nLi₃CuB₃O₇ features infinite Cu,O\nchains formed by Cu₂O₆ units consisting of edge-shared\nCuO₄ squares. Together with two apical oxygen atoms with\nlong interatomic Cu–O distances of 2.7–2.8 Å, the\nCu₂O₆ units form chains extended along the <i>a</i> axis. These pseudochains are responsible for strong anisotropic\nthermal expansion behavior. The temperature dependence of the magnetization\nbetween 4 and 380 K for Li₃CuB₃O₇ could be fit well by a spin-dimer model. The magnetic susceptibility\nof Li₈Cu₇B₁₄O₃₂ showed\na more complex temperature dependence, with two different Curie–Weiss\nregimes in the temperature range of 2–380 K.