Nuclear and Magnetic Structures of the Frustrated Quantum Antiferromagnet Barlowite, Cu$_{4}$(OH)$_{6}$FBr

TL;DR

This study determines the nuclear and magnetic structures of barlowite using neutron diffraction, revealing an orthorhombic space group and a magnetic structure similar to related quantum spin liquid materials.

Contribution

The paper provides the first comprehensive neutron diffraction analysis confirming the orthorhombic Pnma structure and magnetic space group Pn'm'a' of barlowite at low temperatures.

Findings

Nuclear structure is orthorhombic Pnma.

Magnetic structure is described by Pn'm'a' space group.

Magnetic structure explains ferromagnetic component observed in measurements.

Abstract

Barlowite, Cu$_{4}$(OH)$_{6}$FBr, has attracted much attention as the parent compound of a new series of quantum spin liquid candidates, Zn$_{x}$Cu$_{4-x}$(OH)$_{6}$FBr. While it is known to undergo a magnetic phase transition to a long-range ordered state at $T_{N} = 15$ K, there is still no consensus over either its nuclear or magnetic structures. Here, we use comprehensive powder neutron diffraction studies on deuterated samples of barlowite to demonstrate that the only space group consistent with the observed nuclear and magnetic diffraction at low-temperatures is the orthorhombic $Pnma$ space group. We furthermore conclude that the magnetic intensity at $T < T_{N}$ is correctly described by the $Pn^\prime m^\prime a$ magnetic space group, which crucially allows the ferromagnetic component observed in previous single-crystal and powder magnetisation measurements. As such, the magnetic structure of barlowite resembles that of the related material clinoatacamite, Cu$_{4}$(OH)$_{6}$Cl$_{2}$, the parent compound of the well-known quantum spin liquid candidate hebertsmithite, ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$.