Hydrothermal Synthesis and a Composite Crystal Structure of NaCuBiO(PO)[Cl,(OH)] as a Candidate for Quantum Spin Liquid.

A novel sodium bismuth oxo-cuprate phosphate chloride, NaCuBiO(PO)[Cl(OH)], containing square-kagomé layers of Cu has been synthesized by hydrothermal techniques. The compound crystallizes in the tetragonal space group 4/, = 10.0176(4), = 10.8545(6), = 2, = 1089.3(1) Å, = 0.021, w = 0.053, = 1.32. Its composite crystal structure includes [OCuBi] layers, which are formed by the clusters of oxygen-centered tetrahedra [OCuBi]. These positively charged two periodic fragments are intercalated in a negatively charged [CuNaCl(PO)] matrix built by Na-centered polyhedra, PO tetrahedra, and CuOCl pyramids. The composite character of the crystal structure of NaCuBiO(PO)[Cl(OH)], as well as the way of its self-assembly, are discussed in close connection with the sulfohalite NaClF(SO) salt. It is shown that the "host-guest" model of the formation of the tetragonal NaCuBiO(PO)[Cl(OH)] 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 NaCuBiO(PO)[Cl(OH)] 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.