Hydrothermal Synthesis and a Composite Crystal Structure of Na6Cu7BiO4(PO4)4[Cl,(OH)]3 as a Candidate for Quantum Spin Liquid

Abstract

A novel sodium bismuth oxo-cuprate phosphate chloride, Na₆Cu₇BiO₄(PO₄)₄[Cl_2.23(OH)_0.77], containing square-kagomé layers of Cu²⁺ has been synthesized by hydrothermal techniques. The compound crystallizes in the tetragonal space group P4/nmm, a = 10.0176(4), c = 10.8545(6), Z = 2, V = 1089.3(1) ų, R₁ = 0.021, wR = 0.053, S = 1.32. Its composite crystal structure includes [O₄Cu₆Bi]⁷⁺ layers, which are formed by the clusters of oxygen-centered tetrahedra [OCu₃Bi]. These positively charged two periodic fragments are intercalated in a negatively charged [CuNa₆Cl₃(PO₄)₄]^7– matrix built by Na-centered polyhedra, PO₄ tetrahedra, and CuO₄Cl pyramids. The composite character of the crystal structure of Na₆Cu₇BiO₄(PO₄)₄[Cl_2.23(OH)_0.77], as well as the way of its self-assembly, are discussed in close connection with the sulfohalite Na₆ClF(SO₄)₂ salt. It is shown that the “host–guest” model of the formation of the tetragonal Na₆Cu₇BiO₄(PO₄)₄[Cl_2.23(OH)_0.77] phase is due to the group–subgroup symmetry relation with the cubic crystal structure of mineral sulfohalite and is also supported by the crystallization condition in excess sodium chloride. The magnetic subsystem of Na₆Cu₇BiO₄(PO₄)₄[Cl_2.23(OH)_0.77] is represented by a dense square-kagomé network of 2Cu1 and 4Cu2 ions, decorated with weakly bonded Cu3 ions. Measurements of magnetization and heat capacity indicate the absence of long-range order up to 2 K, which makes this compound a candidate for a highly demanded spin liquid.