Disorder-Induced Structural Complexity in the Barlowite Family of S = 1/2 Kagomé Magnets

Abstract

We present a comprehensive structural and magnetic characterization of the barlowite family of S = 1/2 kagomé magnets, Cu4(OH)6FX, where X = Cl, Br, or I. Through high-resolution synchrotron X-ray and neutron powder diffraction measurements, we reveal two sources of structural complexity within this family of materials, namely, compositional disorder of the halide species that occupy sites in between the kagomé layers and the positional disorder of the interlayer Cu2+ ions that persists well into the Pnma structural ground state. We demonstrate that understanding these inherent structural disorders is key as they correlate with the degree of partial order in the magnetic ground states of these quantum frustrated magnets.

Magnetically ordered and kagome quantum spin liquid states in the Zn-doped claringbullite series

“…The magnetic structure found for Cu 4 in this work is similar to that reported for a different polycrystalline sample [16], 024416-2 I).…”

“…This could be intrinsic to our ZnCu 3 sample; however, we note that the hysteresis loop opens below 6 K, which corresponds to the ordering temperature of clinoatacamite [23]. Although no impurity phase was observed in the D2B neutron diffraction data, previous work has reported impurities of polymorphs of clinoatacamite [16] or herbertsmithite [24] in both Cu 4 and ZnCu 3 . The observed spontaneous moment at 2 K could be attributed to a 0.4% clinoatacamite impurity [25], which would not be visible in our powder diffraction data.…”

“…Subtraction of data taken at T = 25 K, just above the magnetic phase transition at T N = 17 K, show more than a dozen magnetic peaks that correspond to an antiferromagnetic order with propagation vector k = 0 in the orthorhombic space group (see Supplemental Material [19]). Refinements of the different irreducible representations (IRs) obtained by representation analysis [27] gave a good fit only for the IR 7 , in Kovalev's notation, which corresponds to the magnetic space group Pn m a, previously found for both barlowite [16,28] and claringbullite [16]. The ordered magnetic moments are found to be reduced from the fully ordered 1μ B expected for S = 1/2, which we relate to quantum fluctuations, with values of 0.26 μ B , 0.37 μ B , and 0.52 μ B for the Cu1 site at (0, 0, 0), Cu2 at (0.247, 0.496, 0.249), and Cu3 at (0.191, 1/4, 0.051), respectively.…”

“…The interlayer Cu, which is disordered over three equivalent positions in this structure, orders slightly below room temperature, leading to an orthorhombic distortion and a doubling of the crystallographic unit cell [15,16]. The magnetic interactions in claringbullite are predominantly antiferromagnetic, as seen from the Weiss temperature θ W = −135 K, and magnetic order is established below T N = 17 K [16][17][18]. Zinc-doped claringbullite (ZnCu 3 ), on the other hand, remains in the P6 3 /mmc space group down to the lowest temperatures, as will be shown below.…”

High-Pressure Induced Continuous Structural Evolution of Kagome Antiferromagnet MgMn3(OH)6Cl2: A Structural Analogue to Quantum Spin Liquid Herbertsmithite

“…As the temperature continues to decrease, a second antiferromagnetic transition is observed at ∼8 K (see insets in Figures a and b). This dual magnetic phase-transition phenomenon, reminiscent of barlowite, − suggests a potential structural phase transition in MgMn 3 (OH) 6 Cl 2 at low temperatures. The first antiferromagnetic transition occurs within the R 3̅ m structure, where the crystal structure undergoes a phase transition with decreasing temperature.…”