Single crystal growth of Cu4(OH)6BrF and universal behavior in quantum spin liquid candidates synthetic barlowite and herbertsmithite

TL;DR

This study reports the scalable synthesis and detailed structural analysis of Cu4(OH)6BrF, a quantum spin liquid candidate, revealing universal magnetic behavior in copper kagome lattices even with structural distortions.

Contribution

The paper provides the first scalable synthesis of single crystals of Cu4(OH)6BrF and clarifies its structure, demonstrating universal magnetic behavior in copper kagome materials.

Findings

Resolved the structure as orthorhombic Cmcm with ordered interlayer Cu2+ ions.

Found that Zn substitution maintains orthorhombic symmetry and similar magnetic properties.

Showed universal behavior in magnetic susceptibility and specific heat across different copper kagome compounds.

Abstract

Synthetic barlowite, Cu4(OH)6BrF, has emerged as a new quantum spin liquid (QSL) host, containing kagome layers of S=1/2 Cu2+ ions separated by interlayer Cu2+ ions. Similar to synthetic herbertsmithite, ZnCu3(OH)6Cl2, it has been reported that Zn2+ substitution for the interlayer Cu2+ induces a QSL ground state. Here we report a scalable synthesis of single crystals of Cu4(OH)6BrF. Through x-ray, neutron, and electron diffraction measurements coupled with magic angle spinning 19F and 1H NMR spectroscopy, we resolve the previously reported positional disorder of the interlayer Cu2+ ions and find that the structure is best described in the orthorhombic space group, Cmcm, with lattice parameters a = 6.665(13) A, b = 11.521(2) A, c = 9.256(18) A and an ordered arrangement of interlayer Cu2+ ions. Infrared spectroscopy measurements of the O-H and F-H stretching frequencies demonstrate that the orthorhombic symmetry persists upon substitution of Zn2+ for Cu2+. Specific heat and magnetic susceptibility measurements of Zn-substituted barlowite, ZnxCu4-x(OH)6BrF, reveal striking similarities with the behavior of ZnxCu4-x(OH)6Cl2. These parallels imply universal behavior of copper kagome lattices even in the presence of small symmetry breaking distortions. Thus synthetic barlowite demonstrates universality of the physics of synthetic Cu2+ kagome minerals and furthers the development of real QSL states.