# Single Crystal and Magnetic Characterization of Iodobarlowite Cu 4 (OH) 6 FI: A Kagomé Lattice Synthesized from a 2-D Triangular Precursor

Ryan J Evenson, Xiaowen Feng, Miguel I Gonzalez, Andrei Rogalev, Rodolphe Clérac, Daniel G Nocera

► To cite this version:

Ryan J Evenson, Xiaowen Feng, Miguel I Gonzalez, Andrei Rogalev, Rodolphe Clérac, et al.. Single Crystal and Magnetic Characterization of Iodobarlowite Cu 4 (OH) 6 FI: A Kagomé Lattice Synthesized from a 2-D Triangular Precursor. Chemistry of Materials, 2025, 37 (21), pp.8870 - 8878.
10.1021/acs.chemmater.5c01990 . hal-05389177

HAL Id: hal-05389177

https://hal.science/hal-05389177v1

Submitted on 29 Nov 2025

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# Single Crystal and Magnetic Characterization of Iodobarlowite Cu₄(OH)₆FI: A Kagomé Lattice Synthesized from a 2-D Triangular Precursor

Ryan J. Evenson, Xiaowen Feng, Miguel I. Gonzalez, Andrei Rogalev, Rodolphe Clérac,*
and Daniel G. Nocera*

ABSTRACT: A single-step synthetic method to produce single crystals of the barlowite structural family is disclosed with iodobarlowite as a focus. The kagomé lattice of iodobarlowite, Cu₄(OH)₆FI, is furnished by removal of 1/4 of the triangular lattice sites of iodobotallackite, Cu₂(OH)₃I. Single-crystal structures have been solved at 100 and 300 K, both of which show I⁻ vacancies within the lattice with no attendant Cu vacancies. X-ray absorption spectroscopy and diffraction studies reveal only the presence of Cu(II), no F⁻ substitution at the I⁻ site, and no Cu vacancies,

suggesting that the charge balance to accommodate the I⁻ deficiency occurs through loss of protons from the bridging hydroxide ligands during hydrothermal synthesis. Unlike microcrystalline powders, single-crystalline Cu₄(OH)₆FI exhibits a single magnetic transition at 15 K, which contrasts with the large Weiss constant of θ = -163(5) K, highlighting the key role of geometric spin frustration in the kagomé lattice to lower the critical temperature of the magnetically ordered state. Temperature-dependent susceptibility measurements indicate that the low temperature magnetic phase is associated with an uncompensated antiferromagnetic ordered state arising from the competing interactions between the four S = 1/2 Cu(II) spins of the iodobarlowite lattice.

<figure><img src="image_1.png" /></figure>

Inducing magnetic frustration by arranging antiferromagnetically coupled spins on a triangular lattice represents a key strategy toward realizing quantum spin liquids.¹ Materials with S = 1/2 quantum spins fixed onto a kagomé (corner-sharing triangular) lattice exhibit a particularly high degree of geometric frustration due to competing antiferromagnetic interactions between nearest neighbor spin centers.¹⁻³ Of these materials, Zn paratacamite (Figures 1a and 1b), ZnCu₃(OH)₆Cl₂, which is a Zn-substituted synthetic analog of the mineral herbertsmithite (Cu₄(OH)₆Cl₂), is a paradigmatic kagomé lattice⁴,⁵ that has emerged prominently in the search for a quantum spin liquid.⁶ In ZnCu₃(OH)₆Cl₂, the kagomé layers comprise corner-sharing {Cu₃(OH)₃} triangles that are separated by Zn(II) ions with minor Cu(II) ion contamination.⁷ This remarkable material shows no long-range ordering down to 50 mK, despite having strong antiferromagnetic nearest-neighbor superexchange interactions, as evidenced by a Weiss constant (θ) of -314 K.⁸⁻¹¹

Despite intense theoretical interest, experimental studies on the S = 1/2 kagomé lattice have been hampered by the difficulty in synthesizing materials with high purity and

minimal defects. To this end, significant synthetic development has been dedicated to forming new kagomé lattice materials based on promising structural motifs observed in known inorganic solids.²,¹²⁻¹⁴ Our previous work established the synthesis of ZnCu₃(OH)₆Cl₂ by elaborating the chemistry of lindgrenite,¹⁵ a crystalline mineral that consists of tessellated triangular chains of [Cu₃(OH)₂<sup>2+</sup>]<sub>n</sub> separated by MoO₄²⁻ spacers.¹⁶ The process of finding ions that were compatible with the critical {Cu₃(OH)₃} motif shared by both lindgrenite and Zn paratacamite led to the establishment of a procedure capable of producing large single crystals.¹⁷ A key development was to use a starting material that was predisposed to the formation of the desired trimeric {Cu₃(OH)₃} substructure. To this end, the structure of Cu(OH)₂(CO₃) is composed of
<figure><img src="image_2.png" /></figure>

**Figure 1.** Idealized structure of Zn-paratacamite (ZnCu₃(OH)₆Cl₂) viewed (a) along the c-axis and (b) along the a-axis. Idealized structure of barlowite (Cu₄(OH)₆FrB) viewed (c) along the c-axis and (d) along the a-axis. The relative alignment of kagomé planes within one unit cell with respect to the c-axis is highlighted in blue rectangles.

Cu(II) centers bridged by μ-OH⁻ and μ-OCO²⁻ anions. The latter carbonate anion, which donates each of its oxygens to three different Cu(II) centers, is readily displaced, providing a synthetic lever to introduce other ligands at these sites. Specifically, substitution of the carbonate anions by hydroxide results in {Cu₃(OH)₃} triangles arranged in a kagomé lattice to deliver Zn paratacamite.

A related material of interest to herbertsmithite is barlowite, Cu₄(OH)₆FBr (Figure 1c).¹⁸ In herbertsmithite, the interlayer Cl⁻ anions bind to three copper ions on one kagomé layer and engage in hydrogen bonding to three OH⁻ groups on an opposing layer. This asymmetric binding motif leads to an ABC-type horizontal staggering of kagomé layers (Figure 1b). Conversely, in barlowite, F⁻ ions only interact with the OH⁻ groups, while Br⁻ ions coordinate to the copper centers on opposing layers. This ion-packing motif in barlowite establishes perfect axial alignment between neighboring kagomé layers (Figure 1d). Accordingly, significant work has been undertaken to understand the magnetic properties of barlowite as a function of chemical substitution and disorder.¹⁹⁻²¹

While many S = 1/2 kagomé lattice materials have received recent attention,¹³,²²⁻²⁷ barlowite is noteworthy as an isostructural family. The Cl analog of barlowite, Cu₄(OH)₆FCl, exists in nature as the mineral claringbullite.²⁸ Both barlowite and claringbullite, analogous to herbertsmithite, have been substituted with diamagnetic ions to investigate the chemistry of the isolated kagomé layer.²⁹,³⁰ The non-natural material Cu₄(OH)₆FI, which completes a “vertical” homologous halide series, Cu₄(OH)₆FX (X = Cl, Br, or I), has been prepared in microcrystalline form and characterized by high-resolution synchrotron X-ray and neutron powder diffraction.³¹ Detailed and thorough structural analyses revealed positional disorder of the interlayer Cu(II) ions together with compositional disorder of the halide species between the kagomé layers. Curiously, whereas claringbullite and barlowite are known to have a single critical temperature at 15 K, the microcrystalline powders of Cu₄(OH)₆FI exhibited two relatively broad transitions centered at T_C = 7 and 13 K and a Weiss constant of -102

K. As recognized in this and other thorough studies of the lighter barlowite homologues,³² the bulk thermodynamic and magnetic properties of these materials can depend intimately on the synthetic conditions and sample composition. To this end, we sought a reliable synthetic pathway to the barlowite class of materials that would yield isolable single-crystalline samples.

The use of Cu(OH)₂(CO₃) as a synthon for a kagomé lattice, as was the case for Zn paratacamite, requires Cu triangles to form by elimination of carbonate between chains of Cu pairs. We suspected that a more facile pathway for forming the kagomé structure is to use a precursor with preformed Cu₃ triangles bridged by hydroxide and then followed by the removal of specific Cu sites from the lattice as depicted in Scheme 1. To this end, iodinated botallackite (Cu₂(OH)₃I)

<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Conversion of a Triangular Lattice to a Kagomé Lattice through Partial Site Elimination</figcaption></figure>

comprising a layered triangular lattice,³³ appeared to be a good precursor of the barlowite structure. We now report the synthesis of single crystals of Cu₄(OH)₆FI from iodinated botallackite. Unlike microcrystalline powders, crystalline Cu₄(OH)₆FI prepared by our new method exhibits a single magnetic transition at 15 K, which is in contrast to the large Weiss constant of θ = -163(5) K. Temperature-dependent susceptibility measurements are consistent with spontaneous magnetization associated with an uncompensated antiferromagnetic ordered state arising from the competing interactions between the four S = 1/2 Cu(II) spins of the iodobarlowite lattice. Extrapolation of magnetization saturation data indicates that the four Cu(II) spins will fully align above 80 T in agreement with large and competing Cu-Cu antiferromagnetic interactions within the kagomé lattice.

**Synthesis.** Large single crystals of iodobarlowite were prepared from reaction of solid BaF₂, CuI, and Cu₂(OH)₃I under hydrothermal conditions (see Experimental section). Solids were ground together in a mortar and pestle at a x:1:2 (x = 1-10) ratio of BaF₂:CuI:Cu₂(OH)₃I, respectively, diluted with 10 mL of H₂O, sealed in a Teflon pressure vessel, heated at 120 °C for 4 days followed by cooling at 0.1 °C/min to room temperature. In the absence of CuI, the reaction of BaF₂ and Cu₂(OH)₃I yielded powder samples only. The best single crystals of iodobarlowite were obtained for x = 10 and they were separated from excess BaF₂ under an optical microscope.

**Structural Chemistry.** Crystals suitable for X-ray crystallographic analysis were routinely obtained with some runs yielding crystals on the order of a millimeter (Figure S1). Typically, these crystals were found to nucleate on the side of the reaction vessel, while larger cylindrical crystals (~0.6 mm [c-axis] × 0.25 mm [ab-plane]}) were obtained along with residual BaF₂ at the bottom of the vessel. X-ray analysis was
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Overlay of the high- and low-temperature structures obtained from two different single crystals of iodobarlowite. The unit cell of the low-temperature structure is shown in the orange box and that of the high-temperature structure is shown in the green rhombus. The 100 K structure is on the left, and the 300 K structure is on the right, where VESTA is used to highlight atom occupancy for each of the structures. The middle panel of the figure shows the superposition of the two unit cells with the transparency of atoms set at 50% for both structures, thus highlighting atom displacements. The minor displacement of hydroxides from high to low temperature can be observed in the overlap region in the middle of the panel. The overlap region highlights the transition of the 3-fold disorder of the interlayer Cu (33% occupancy in the pie chart) to predominantly single site occupancy (98.85(12)%) at low temperature. The decrease in I⁻ occupancy is due to loss of I⁻ from the lattice, likely balanced by deprotonation of a bridging hydroxide.</figcaption></figure>

performed on two different crystals from the same batch at T = 100 and 300 K. The low temperature 100 K data was collected at ChemMatCARS (APS beamline 15-ID-B). The high temperature 300 K data set was collected on a laboratory diffractometer. Crystal data and structure refinement parameters are listed in Table S1.

Analysis of single-crystal X-ray diffraction data revealed that iodobarlowite adopts the *P6<sub>3</sub>/mmc* space group at 300 K and a lower-symmetry *Pnma* space group at 100 K. The atomic displacement parameter plots of the 300 and 100 K structures are shown in Figure 2. The two single-crystal structures are consistent with powder X-ray diffraction and powder neutron diffraction studies³¹ that show a structural transition from *P6<sub>3</sub>/mmc* to *Pnma* at ~ 274 K. In the higher-symmetry 300 K structure, the interlayer Cu(II) ions are uniformly disordered over a 3-fold axis, while the Cu(II) centers in the kagomé layers are equivalent by 3-fold symmetry with equal Cu–OH distances (1.9647(15) Å) and Cu···Cu separations (3.3469(9) Å). In addition, the I⁻ ions occupy sites that are equidistant to six Cu(II) centers in adjacent kagomé layers with Cu···I distances of 3.0414(6) Å. The structure at 100 K exhibits significantly lower symmetry due to ordering of the Cu(II) centers at the interlayer sites and slight changes in bond distances within the kagomé layers. The interlayer Cu(II) ions

strongly favor one of the three sites (98.85(12)% occupancy), whereas the Cu(II) centers in the kagomé layers are no longer symmetrically equivalent with Cu–OH distances ranging from 1.9132(14) to 1.9957(14) Å and Cu···Cu separations of 3.3185(2), 3.3425(2), and 3.34931(10) Å. The I⁻ ions also shift toward two Cu(II) centers, resulting in Cu···I distances that span 2.9660(3)–3.1192(3) Å. The atom displacements between the 100 and 300 K structures are highlighted by the overlay of these structures shown in Figure S2.

Previous work has shown that the Cu₄(OH)₆FX (X = Cl, Br, or I) series of materials can contain substoichiometric amounts of X⁻ with F⁻ substitution proposed to balance the overall charge and give rise to mixtures of F⁻ and X⁻ at the halide site.³¹ However, in various crystals of iodobarlowite, we do not observe mixed halide occupancy of F⁻ in the I⁻ site but instead see an I⁻ deficiency. For example, refinement of the occupancy of the I⁻ ions in a single-crystal 300 K structure resulted in 70.6(5)% I⁻ occupancy and 92.20(14)% I⁻ occupancy in a structure collected at 100 K. Attempts to model partial substitution of I⁻ by F⁻ failed to give positions where F⁻ could be stabilized by either hydrogen bonding or coordination (the closest possible contacts were found to be Cu···F distances greater than 2.9 Å). Refinement of Cu occupancies showed no indication of Cu vacancies. Moreover, XPS spectra collected
on bulk polycrystalline samples gave no indication of Cu(I); Cu 2p XPS spectra only show the presence of Cu(II) (Figure S3). We believe that the I⁻ deficiency is offset by proton transfer from bridging hydroxide during the hydrothermal synthesis to maintain charge balance as opposed to F⁻ substitution at the I⁻ site, Cu vacancies, or reduction of Cu(II) to Cu(I). Due to variance in I⁻ occupancy between individual single crystals, Rietveld refinement of PXRD data was employed to examine the occupancy of the bulk solid obtained from routine sample preparations. Free refinement of the 300 K single crystal model I⁻ site occupancies against synchrotron PXRD data consistently resulted in an average I⁻ occupancy of 88% (Figure S4). Thus, the average sample of iodobarlowite in this study is best defined as Cu₄(OH)₅.₈₈O₀.₁₂FI₀.₈₈, though for convenience we will use the simplified structural formula of Cu₄(OH)₆FI.

**Magnetism.** Magnetic properties were studied on crushed single crystals of Cu₄(OH)₆FI. Figure 3 shows magnetic dc

<figure><img src="image_5.png" /></figure>

**Figure 3.** Temperature dependence of the molar magnetic susceptibility ($\chi$; normalized per formula unit) between 1.85 and 300 K at 0.1 T and between 200 and 400 K at 1 T shown as a $\chi T$ versus T (red dots) and a $\chi^{-1}$ versus T (blue dots) plots for Cu₄(OH)₆FI. The black solid line is the best fit of the experimental $\chi^{-1}$ versus T data above 160 K to the Curie–Weiss law.

susceptibility ($\chi$) data measured between 1.85 and 400 K as $\chi T$ and $\chi^{-1}$ versus T plots. At 400 K, the $\chi T$ value of 1.3 cm³ K mol⁻¹ is consistent with the presence of four $S = 1/2$ Cu(II) centers ($1.5 \text{ cm}^3 \text{ K mol}^{-1}$ with $g = 2$) with significant antiferromagnetic interactions. Indeed, these antiferromagnetic interactions are highlighted upon lowering the temperature as the $\chi T$ product significantly decreases, attaining a value of 0.59 cm³ K mol⁻¹ at 27 K. Above 160 K, the magnetic susceptibility can be fit to a Curie–Weiss law with $C = 1.76(5) \text{ cm}^3 \text{ K mol}^{-1}$ and $\theta = -163(5) \text{ K}$ (Figure 3), supporting the presence of large and dominating nearest-neighbor antiferromagnetic interactions among $S = 1/2$ Cu(II) centers ($g = 2.17(5)$). At low temperatures, the Curie–Weiss law deviates from linearity. Noting that the Curie–Weiss law is a mean-field approximation, deviation of the $1/\chi$ vs T plot from strict linearity is consistent with a canted antiferromagnetic or ferrimagnetic ground state, as has been long-established.³⁴ Deviations from linearity arise naturally due to competing exchange interactions, sublattice canting, or magnetic anisotropy, as is common to spin frustrated lattices. Indeed, below 27 K, uncompensated Cu(II) spins lead to a sharp increase of the $\chi T$ product in agreement with a magnetic phase transition.

This magnetic phase transition is clearly detected at a critical temperature ($T_C$) of 15 K by dc and ac magnetic susceptibilities as shown in Figure 4. The presence of an

<figure><img src="image_6.png" /></figure>

**Figure 4.** Temperature (T) dependence of the (a) in-phase ($\chi'$) and (b) out-of-phase ($\chi''$) components of the ac susceptibility (with a 3 Oe ac field and without a dc field; normalized per formula unit) between 1.85 and 20 K at indicated ac frequencies (inset: expanded view between 12.5 and 16 K) for Cu₄(OH)₆FI. (c) Temperature (T) dependence of the dc susceptibility at 12 Oe between 1.85 and 20 K in field-cooled (FC) and zero-field cooled (ZFC) modes (at 0.15 K/min) for Cu₄(OH)₆FI. Solid lines are guides for the eye.

out-of-phase ac susceptibility peak at the transition and a clear bifurcation of the field cooled (FC) and zero-field cooled (ZFC) dc susceptibility data at low dc field demonstrate the presence of a low temperature magnetic phase, which possesses a spontaneous magnetization (i.e., thus excluding an antiferromagnetic ground state). Considering to the presence of four $S = 1/2$ Cu(II) spins in dominating antiferromagnetic interactions in Cu₄(OH)₆FI, the three-dimensional ordered magnetic state is most likely an uncompensated antiferromagnetic state. Sharp and intense peaks are observed for both in-phase ($\chi'$) and out-of-phase ($\chi''$) components of the ac susceptibility below 15 K (Figures 4a and 4b) with weak ac frequency dependence as expected for well-ordered and crystalline materials. The frequency dependence of the ac susceptibility of less than 0.1 K (between 1 and 1500 Hz) at the critical temperature of 15 K is in line with dynamics of magnetic domains as observed in ordered ferromagnetic, ferrimagnetic or canted antiferromagnetic states, and as has been discussed,³⁵ is inconsistent with spin-glass behavior. The vanishing $\chi'$ and $\chi''$ values below the ac peak clearly denote the high purity of the sample as well as a complete blocking of the magnetization in agreement with the bifurcation of the ZFC/FC data (Figure 4c) at 15 K and the presence of M vs H hysteresis loops ($H_C = 8$ mT at 1.85 K), which disappear above 15 K (Figure 5). These experimental results confirm the presence of slow dynamics of the uncompensated magnet-
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Field (H) dependence of the magnetization (M; normalized per formula unit) for Cu₄(OH)₆FI at indicated temperatures between (a) 0 and 9 T and (b) -0.1 and 0.1 T. Solid lines are guides for the eye.</figcaption></figure>

ization induced by the intrinsic magnetic viscosity of the domain walls in the ordered state.

As shown in Figure 5a, the magnetization of Cu₄(OH)₆FI at 1.85 K shows no sign of saturation at high dc field, even at 9 T. Instead, a linear variation of the magnetization is observed above 2 T, attaining 0.78 µB at 9 T, which is less than 20% of the expected magnetization saturation value for four S = 1/2 Cu(II) spins (4.3 µB with g = 2.17(5)). The extrapolation of this linear variation above the experimental field window indicates that the magnetization of the material could be fully saturated with an applied dc field between 80 and 100 T. The observed behavior is relatively similar to that of barlowite,³⁶ which exhibits magnetic properties that are consistent with canted antiferromagnetism originating from intralayer Dzyaloshinskii-Moriya interactions between the Cu(II) spins in the kagomé lattice. From the M versus H data at low fields (±0.1 T, Figure 5b), the remnant magnetization, M<sub>rem</sub>, was estimated as a function of the temperature below 20 K (Figure 6). In agreement with the ac and dc magnetic susceptibilities (Figure 4), M<sub>rem</sub> vanishes at a critical temperature of 15 K. Below T<sub>C</sub>, the temperature dependence of M<sub>rem</sub> can be fitted to an empirical Bloch law with a standard T<sup>3/2</sup> dependence approaching zero Kelvin (in agreement with the relevance of the spin-waves) and a critical exponent of 0.52(2) close to the expected mean field value of 0.5,<sup>37</sup> i.e. demonstrating the presence of three-dimensional magnetic order in this system. The small value of M<sub>rem</sub>(0 K), 0.32(1) µB, reflects the presence of an uncompensated antiferromagnetic ordered state arising

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Temperature dependence of the remnant magnetization, M<sub>rem</sub>, determined from the M versus H data between 1.85 and 20 K shown in Figure 5b. The solid red line is the best fit to the mean-field Bloch law, M<sub>rem</sub> = M<sub>rem</sub>(0 K)[1 - (T/T<sub>c</sub>)<sup>3/2</sup>]<sup>β</sup>. Considering the data between 1.85 and 15 K (open symbols), the final set of parameters is M<sub>rem</sub>(0 K) = 0.32(1) µB, T<sub>C</sub> = 15.0(1) K, and β = 0.52(2) with χ<sup>2</sup>(GoF) = 0.0010.</figcaption></figure>

from the competing antiferromagnetic interactions between the four S = 1/2 Cu(II) spins of the iodobarlowite lattice.

To explore the possibility of electronic contributions of the I⁻ ion to the kagomé sublattice, X-ray absorption near edge structure (XANES) spectra of iodobarlowite (Cu₄(OH)₆FI), barlowite (Cu₄(OH)₆FBr), relevant starting materials, and standards were acquired at beamline ID12 of the ESRF at ambient temperatures (Figure S5). XANES spectra at the K-edges of Cu and Br, and L₁-edge of iodine were recorded. Unfortunately, L₁-edge iodine spectra could not be obtained in a reliable manner due to a deleterious spectral contamination by coincident diffraction Bragg peaks. The rising Cu K-edge shifts significantly in the case of hybridization between ligand and metal orbitals in anhydrous CuBr₂; this is not seen in the case of the iodobarlowite, barlowite, and botallackites, which exhibit similar spectra. Br and Cu K-edge data make clear that signatures of Cu-Br covalency are observed only for the anhydrous CuBr₂ standard. From this observation and the high similarity of the botallackite and barlowite Cu K-edge spectra, it can be inferred that neither halide is directly bonding with Cu(II) orbitals in iodobarlowite, barlowite or the corresponding Cu(II) site in botallackites, despite occupying its coordination shell in both compounds.

The kagomé lattice occupies a prominent overlook on the landscape of quantum spin liquid materials. Conversion of triangular lattices to kagomé lattices through removal of 1/4 of the triangular lattice sites of botallackite offers a general strategy for the synthesis of barlowite materials, as demonstrated here for the smooth conversion of iodinated botallackite to iodobarlowite. Recognizing the significance of fluoride in stabilizing kagomé interlayers in barlowite via hydrogen bonding to adjacent kagomé planes, our synthesis emphasizes the use of BaF₂ as an additional F⁻ source. The low solubility of BaF₂ ensures the slow release of F⁻ into the hydrothermal reaction mixture, thus aiding in the sustained growth of large crystals of iodobarlowite. In the course of developing this synthesis, other fluoride sources with smaller divalent cations (e.g., Sr, Ca) were tested and found to promote the decomposition of the starting material with little or no product formation. BaF₂ supports crystal formation by serving as a slow-release source of F⁻ without Ba²⁺ structurally incorporating into the lattice. Iodobarlowite was formed
throughout the reaction mixture, but the highest quality crystals from the BaF₂ synthesis were observed to form on the walls of the reaction vessel, spatially separated from the fluoride source and solid starting materials.

Structural analysis of samples grown by our methods indicates I⁻ deficiency within the iodobarlowite lattice. Refinement of the occupancy of the I⁻ ions on routine samples suggests an average I⁻ occupancy of 88%. XPS and XANES spectra indicate that the charge balance that must accompany I⁻ deficiency is not accompanied by Cu(II) reduction, as spectra indicate the absence of Cu(I). Also, there is no indication of the loss of Cu ions from the kagomé planes in X-ray structural refinements. Thus, the charge balance accompanying I⁻ deficiency is likely due to formal loss of HI from the lattice, with the proton supplied from the bridging hydroxide within the kagomé plane.

Magnetically, single crystalline iodobarlowite retains a strong resemblance to its halide congeners, Cu₄(OH)₆FX (X = Cl and Br) with the distinction that Cu₄(OH)₆FI possesses a lower Curie-Weiss temperature despite ordering at approximately the same temperature of 15 K (vide supra). The magnetic structure of stoichiometric barlowite and iodobarlowite has been directly interrogated previously by careful neutron scattering of deuterated powder samples and core refinement of high and low temperature data sets.³¹ Large moments are observed on the Cu interlayer sites, which cant slightly along the c-axis of the Pnma cell to produce a net ferromagnetic moment coplanar with the displacement vector of the interlayer Cu from the ideal Wyckoff 2d position in P6₃/mmc. The orientations and magnitudes of the moments from the powder neutron scattering structure are consistent with the observed uncompensated antiferromagnetism of the I⁻-deficient single crystalline material studied in this work, where a low temperature ordered state at 15 K is preceded by confinement of the disordered interlayer Cu to occupy of a single lattice site as observed in the 100 K crystal structure.

We note, based on molecular compounds, I⁻ appears to have little impact on the in-plane magnetism. {Cu₃O₃} triangles have been assembled within multidentate hexapyridyl ligand platforms.³⁸,³⁹ As shown in Figure 7, the X⁻ (X = Br, I) ion

<figure><img src="image_9.png" /><figcaption>**Figure 7.** Molecular complexes of {Cu₃O₃} triangles residing in hexapyridyl ligand architectures: (a) from ref 38 where a Br⁻ caps the {Cu₃O₃} triangular face as in barlowite and (b) from ref 39 where a Br⁻ or I⁻ caps the {Cu₃O₃} face as in barlowite and iodobarlowite, respectively. (c) For comparison, the highlighted {Cu₃O₃}I{Cu₃O₃} framework of iodobarlowite, as structurally determined in this study.</figcaption></figure>

resides centrally above {Cu₃O₃} molecular triangles in much the same way as it does above the {Cu₃(OH)₃} triangles of barlowite. The magnetism of the molecular {Cu₃O₃} complexes is governed by the Cu–O–Cu angle, consistent with structural arguments based on the Goodenough-Kanamori rules. Changing from I⁻ to Br⁻ appears to minimally perturb magnetic exchange, underscoring the robustness of the magnetic framework against halide substitution in both the molecular and solid state congeners. Indeed, consistent with this contention is the observation of T~C~ = 15 K for barlowite and iodobarlowite, as observed here for single crystals of the compound.

The barlowite family of compounds, Cu₄(OH)₆FX, are intriguing as neighboring kagomé layers are perfectly aligned owing to the interlayer F⁻ ions to specifically hydrogen bond to hydroxides and X⁻ ions to coordinate Cu of opposing layers. A new synthesis for the barlowite structural family is reported here that is derived from a {Cu₃(OH)₃} triangular framework. With {Cu₃(OH)₃} triangles established, removal of 1/4 of the triangular lattice sites leads directly to the kagomé lattice. We have exploited this synthetic method to deliver Cu₄(OH)₆FI, which was the member of the barlowite halide series of compounds that had escaped single crystal preparation. Crystalline Cu₄(OH)₆FI exhibits a single magnetic transition at 15 K, which is in line with the T~C~ of Cu₄(OH)₆FCl and Cu₄(OH)₆FBr, indicating that the role of halide in the barlowite series is strictly structural in nature and is not involved with the magnetism of barlowite materials. The large single crystals afforded by the new synthetic method should be beneficial to the application of neutron scattering and magnetometrical techniques for deciphering the controlling parameters of the correlated nuclear and electronic phases of the barlowite structural family and related materials.

**Sample Preparation.** Water (Type 1) was distilled and deionized with a EMD Millipore filtering system. Besides Cu₂(OH)₃I, all other reagents were obtained from commercial vendors and used without further purification: BaF₂ (99.99+% Ba, Strem), CuI/Cu₂(OH)₃I mixture was synthesized by a literature method⁴⁰ employing Cu(acetate)₂ monohydrate (Strem, 98+% ) and KI (Sigma-Aldrich, 99%). The 1:2 CuI:Cu₂(OH)₃I ratio was determined by Rietveld refinement. Hydrothermal reactions were carried out under autogenous pressure in 20 mL Teflon-lined pressure vessels purchased from Parr Instrument Co. Fisher Isotemp programmable ovens with forced-air circulation were used to obtain the desired temperature profiles for the hydrothermal reactions.

**Iodobarlowite.** In a typical reaction, a 20 mL liner was charged with 133 mg (0.43 mmol, 2 equiv) of Cu₂(OH)₃I, 1.16 g (6.62 mmol, 15 equiv) of BaF₂, and 42 mg of CuI (0.22 mmol, 1 equiv) and 10 mL of water, capped and placed into a steel hydrothermal bomb. The hydrothermal vessel was heated to 120 °C at a rate of 5 °C/min, maintained at this temperature for 4 days, and cooled to room temperature at a rate of 0.1 °C/min. Small single crystals of iodobarlowite lined the rim of the vessel while masses of large single crystals covered the BaF₂ substrate at the bottom of the reaction vessel. The crystals were then washed with deionized water, isolated by vacuum filtration, dried under house vacuum, and stored in air. The composition of the product mixture was assessed through combination of powder diffraction and single crystal structure determination. Samples of crystals used for magnetic measurements were mechanically separated under an optical microscope.
**X-ray Structure Determinations.** X-ray diffraction analysis of Cu₄(OH)₆FI₀.₉₂ (100 K structure) was performed on a single crystal (Figure S1) coated with Paratone-N oil and mounted on a glass fiber. During the experiments, crystals were frozen at 100 K using an Oxford Cryosystems Cryostream. Data were collected at ChemMatCARS at the Advanced Photon Source at Argonne National Laboratory using synchrotron radiation (λ = 0.49594 Å) on a Huber three-circle diffractometer equipped with a Pilatus 1M(CdTe) detector. The crystal was found to be a three-component nonmerohedric twin based on the diffraction pattern. The program CELL_NOW¹¹ was used to determine the orientation matrices. The second and third domains were found to be related to the first domain by 180° rotations around the real axes [-1 1 0] (second domain) and [1 1 0] (third domain). Raw data for all three components were integrated and corrected for Lorentz and polarization effects using Bruker AXS SAINT software¹² and corrected for absorption using TWINABS.⁴³

X-ray diffraction analysis of Cu₄(OH)₆FI₀.₇₀ (300 K structure) was performed on a single crystal coated in Paratone-N oil mounted on a MiTeGen loop. During the experiment, the crystal was kept at 300 K using an Oxford Cryosystems Cryostream. Data were collected at the Harvard Department of Chemistry and Chemical Biology X-ray Laboratory using Mo Kα radiation on a Bruker D8 Venture diffractometer equipped with a Bruker PHOTON100 CMOS detector. Raw data were integrated and corrected using Bruker AXS SAINT⁴² software and corrected for absorption using SADABS.⁴⁴

Both the 100 and 300 K structures were solved by intrinsic phasing using SHELXT⁴⁵ and refined using SHELXL⁴⁶ operated in the OLEX2⁴⁷ interface. No significant crystal decay was observed during data collection. Space group assignments were determined by examination of systematic absences, E-statistics, and successive refinement of the structures. Thermal parameters were refined anisotropically for all non-hydrogen atoms. Hydrogen atoms were located in the electron density difference map and refined using a riding model for all structures. Disorder of the interlayer copper ions in both structures required displacement parameter restraints. Additional crystallographic data are provided in Table S1, and overlaid atomic displacement parameter plots for the crystal structures at 100 and 300 K are shown in Figure S2.

**Powder X-ray Diffraction.** PXRD and total scattering samples were sent to the Advanced Photon Source at Argonne National Laboratory as part of a 17-BM/11-ID-B mail-in proposal. Data were collected at λ = 0.45211 Å. The aim of this measurement and refinement was to estimate the I⁻ fraction across polycrystalline bulk sample; to this end, the scattered intensity was modeled with two phases in which the histogram scale, lattice parameters, phase fraction, crystallite size, and microstrain were refined, in addition to the occupancy of the I⁻ site in iodobarlowite. No attempt was made to account for distortions caused by stoichiometry variation between crystals, which can be inferred to contribute to peak structure not captured in the applied model. These parameters were generally refined in the order listed, with several refinements being undertaken to check for robustness. Instrumental peak shape parameters were imported from a separate, prior refinement of a LaB₆ calibration data set included with the 17-BM measurements. For the background treatment, the “auto-background” feature of GSAS-II (version 5766) was engaged at the default setting (arpls, log(lambda) = 5.25). The results of the refinement are plotted as logᵢ₀(intensity) to emphasize the good qualitative fit obtained, shown in Figure S4. A best fit to the PXRD data was obtained for an I⁻ occupancy of 0.8745. The crystal structure parameters used to generate Figure S4 are listed in Table S2.

**Magnetic Measurements.** Magnetic susceptibility measurements were performed on Quantum Design SQUID MPMS-XL magneto-meter and PPMS-9 susceptometer housed at the Centre de Recherche Paul Pascal at temperatures between 1.8 and 400 K and in dc magnetic fields ranging from -9 to +9 T. The ac magnetic susceptibility measurements were performed in an oscillating ac field of 3 Oe with frequencies between 1 and 1500 Hz in zero dc field. The reported measurements were carried out on crushed samples of larger single-crystals of Cu₄(OH)₆FI (18.90 mg) introduced in a sealed polypropylene bag (3 × 0.5 × 0.02 cm, 16.84 mg). Prior to the

experiments, the field-dependent magnetization was measured at 100 K to exclude the presence of bulk ferromagnetic impurities. In fact, common paramagnetic or diamagnetic materials should exhibit a perfectly linear dependence of the magnetization at 100 K that extrapolates to zero at zero dc field; the samples appeared to be free of any ferromagnetic impurities. The magnetic susceptibilities were corrected for the sample holder and the intrinsic diamagnetic (−0.5 × MW × 10⁻⁶ cm³/mol) contributions.

**X-ray Photoelectron Spectroscopy (XPS).** XPS spectra were collected on a Thermo Fischer Nexsa XPS housed at the Center for Nanoscale Systems at Harvard University. Powders were mounted on a sample plate in air, using carbon tape for adhesion and conductivity. Samples were then pumped down to a minimum 10⁻⁸ Torr before being admitted to the sample chamber and irradiated with monochromated Al Kα X-rays. Photoelectrons were collected in the binding energy range corresponding to the Cu 2p states.

**X-ray absorption spectroscopy (XAS).** X-ray absorption near edge structure (XANES) spectra at the Cu and Br K-edges were collected on polycrystalline samples of iodobarlowite and selected standards at beamline ID12 of the ESRF by means of measuring the total fluorescence yield at room temperature and zero applied field. The detector is a passivated implanted planar silicon (PIPS) photodiode that features an oblong hole through which the incident X-rays pass and X-ray fluorescence photons are collected in a “backscattering” geometry over a wide solid angle. The spectra were normalized but not corrected for reabsorption effects.

## Corresponding Authors

Rodolphe Clérac – Univ. of Bordeaux, CNRS, CRPP, UMR 5031, F-33600 Pessac, France; ● orcid.org/0000-0001-5429-7418; Email: rodolphe.clerac@u-bordeaux.fr

Daniel G. Nocera – Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States; ● orcid.org/0000-0001-5055-320X; Email: dnocera@fas.harvard.edu

## Authors

Ryan J. Evenson – Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States

Xiaowen Feng – Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States

Miguel I. Gonzalez – Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States; ● orcid.org/0000-0003-4250-9035

Andrei Rogalev – ESRF–The European Synchrotron, BP220, F-38043 Grenoble, France
R.J.E., X.W.F., R.C., and D.G.N. designed the project. M.I.G. and R.J.E. collected and analyzed X-ray diffraction single crystal and powder data. R.C. undertook and analyzed the magnetometry data, also obtaining the XAS data with A.R. R.J.E. carried out the synthesis and computations. R.J.E., R.C., and D.G.N. analyzed the magnetic data. The manuscript was compiled by D.G.N., R.C., and R.J.E. with input from all authors.

## Funding

This work was supported by the Solar Photochemistry Program of the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences of the U.S. Department of Energy DE-SC0017619. NSF's Chem-MatCARS, Sector 15 at the Advanced Photon Source, Argonne National Laboratory, is supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under grant number NSF/CHE-1834750. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The mail-in program at Beamline 11-ID-B and 17-BM contributed to the data. This work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. ECCS-2025158. CNS is part of Harvard University. This work was also supported by the University of Bordeaux, the Région Nouvelle Aquitaine, Quantum Matter Bordeaux, the Centre National de la Recherche Scientifique (CNRS) and the European Synchrotron (ESRF).

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Supporting Information
Table of Contents

<table><tr><td>A. Single Crystal X-ray Data</td><td>S3</td></tr><tr><td>Figure S1. Single crystal of iodobarlowite</td><td>S3</td></tr><tr><td>Figure S2. Atomic displacement parameter plots of 100 and 300 K structures</td><td>S4</td></tr><tr><td>Table S1. Crystal data and structure refinement of iodobarlowite</td><td>S5</td></tr><tr><td>B. X-ray Photoelectron Spectroscopy</td><td>S6</td></tr><tr><td>Figure S3. Cu 2p XPS spectra for kagomé minerals and selected Cu(II) standards.</td><td>S6</td></tr><tr><td>C. Powder X-ray Diffraction</td><td>S7</td></tr><tr><td>Figure S4. Synchrotron PXRD of iodobarlowite crystals</td><td>S7</td></tr><tr><td>Table S2. Structural parameters used to model PXRD</td><td>S8</td></tr><tr><td>D. XAS Spectroscopy</td><td>S9</td></tr><tr><td>Figure S5. XANES spectra of iodobarlowite and related compounds</td><td>S10</td></tr><tr><td>Figure S6. Calculated XANES spectra for I and Br botallackite and barlowite</td><td>S11</td></tr><tr><td>E. References</td><td>S12</td></tr></table>
A. Single Crystal X-ray Data

X-ray diffraction analysis of Cu₄(OH)₆FI₀.₉₂ was performed on a single crystal (Figure S1) coated with Paratone-N oil and mounted on a glass fiber. During the experiments, crystals were frozen at 100 K using an Oxford Cryosystems Cryostream. Data were collected at ChemMatCARS at the Advanced Photon Source at Argonne National Laboratory using synchrotron radiation (λ = 0.49594 Å) on a Huber three-circle diffractometer equipped with a Pilatus 1M(CdTe) detector. The crystal was found to be a three-component non-merohedric twin based on the diffraction pattern. The program CELL_NOW¹ was used to determine the orientation matrices. The second and third domains were found to be related to the first domain by 180° rotations around the real axes [-1 1 0] (second domain) and [1 1 0] (third domain). Raw data for all three components were integrated and corrected for Lorentz and polarization effects using Bruker AXS SAINT software² and corrected for absorption using TWINABS.³ Space group assignments were determined by examination of systematic absences, E-statistics, and successive refinement of the structures. Structures were solved by intrinsic phasing using SHELXT⁴ and refined using SHELXL⁵ operated in the OLEX2⁶ interface. No significant crystal decay was observed during data collection. Thermal parameters were refined anisotropically for all non-hydrogen atoms. Hydrogen atoms were located in the electron density difference map and refined using a riding model for all structures. Disorder of the interlayer copper ions required displacement parameter restraints. Additional crystallographic data are provided in Table S1, and atomic displacement parameter plots for the crystal structures at 100 and 300 K are shown in Figure S2.

<figure><img src="image_10.png" /><figcaption>Figure S1. Iodobarlowite single crystal under an optical microscope. The crystal pictured is roughly 0.6 mm by 0.25 mm.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 2.** Atomic displacement parameter plots of the low-temperature Pnma (100 K, left) and high-temperature P63/mmc (300 K, right) phases with atomic displacement parameters rendered at the 80% probability level. De-symmetrization occurs at the interlayer Cu site and along the columns of F⁻ sites parallel to the c-axis.</figcaption></figure>
<table><caption>**Table S1.** Crystal data and structure refinement for Cu₄(OH)₆FI₀.₉₂ and Cu₄(OH)₆FI₀.₇₀.</caption><thead><tr><th>Empirical formula</th><th>Cu<sub>4</sub>O<sub>6</sub>H<sub>6</sub>FI<sub>0.92</sub><br>(100 K structure)</th><th>Cu<sub>4</sub>O<sub>6</sub>H<sub>6</sub>FI<sub>0.70</sub><br>(300 K structure)</th></tr></thead><tbody><tr><td>Radiation</td><td>Synchrotron</td><td>Mo Kα</td></tr><tr><td>λ (Å)</td><td>0.49594</td><td>0.71073</td></tr><tr><td>T (K)</td><td>100(2)</td><td>300(2)</td></tr><tr><td>Crystal system</td><td>Orthorhombic</td><td>Hexagonal</td></tr><tr><td>Space group</td><td><i>Pnma</i></td><td><i>P</i>6<sub>3</sub>/<i>mmc</i></td></tr><tr><td><i>a</i> (Å)</td><td>11.5154(3)</td><td>6.6938(18)</td></tr><tr><td><i>b</i> (Å)</td><td>9.4308(2)</td><td>6.6938(18)</td></tr><tr><td><i>c</i> (Å)</td><td>6.6902(2)</td><td>9.395(3)</td></tr><tr><td>α, β, γ (°)</td><td>90</td><td>90, 90, 120</td></tr><tr><td><i>V</i> (Å<sup>3</sup>)</td><td>726.55(3)</td><td>364.6(2)</td></tr><tr><td><i>Z</i></td><td>4</td><td>2</td></tr><tr><td>2Θ range for data collection (°)</td><td>4.934 to 48.842</td><td>7.030 to 71.162</td></tr><tr><td><i>R</i><sub>int</sub></td><td>0.0422</td><td>0.0307</td></tr><tr><td>Completeness to 2Θ</td><td>100.0%<br>(2Θ = 34.622°)</td><td>100.0%<br>(2Θ = 50.484°)</td></tr><tr><td>Data / Restraints / Parameters</td><td>2760 / 3 / 76</td><td>352 / 1 / 23</td></tr><tr><td>Goodness of Fit on F<sup>2</sup></td><td>1.121</td><td>1.146</td></tr><tr><td><i>R</i><sub>1</sub><sup>a</sup></td><td>0.0171</td><td>0.0273</td></tr><tr><td>w<i>R</i><sub>2</sub><sup>b</sup> (<i>I</i> > 2σ(<i>I</i>))</td><td>0.0459</td><td>0.0926</td></tr><tr><td><i>R</i><sub>1</sub><sup>a</sup> (all data)</td><td>0.0177</td><td>0.0284</td></tr><tr><td>w<i>R</i><sub>2</sub><sup>b</sup> (all data)</td><td>0.0461</td><td>0.0932</td></tr><tr><td>Largest diff. peak and hole (e Å<sup>-3</sup>)</td><td>1.162 and -1.009</td><td>0.676 and -0.600</td></tr></tbody></table>

$$ ^{a}R_{1} = \sum_{||F_{o}| - ||F_{c}| / \sum_{||F_{o}|} ||.} ^{b}wR_{2} = \{\sum[w(F_{o}^{2} - F_{c}^{2})] / \sum[w(F_{o}^{2})]\}^{1/2}. $$
## B. X-ray Photoelectron Spectroscopy (XPS)

Spectra were collected on a Thermo Scientific Nexsa XPS system at the Center for Nanoscale Systems at Harvard University. Powders were mounted on a sample plate in air, using carbon tape for adhesion and conductivity. Samples were then pumped down to a minimum 10⁻⁸ torr before being admitted to the sample chamber. XPS spectra at the Cu2p edge were obtained summing 10 scans to produce the plots in Figure S2. A Cu(I) standard (CuI) clearly shows a sharp peak below 933 eV, well-resolved and several eV less than the corresponding feature in Cu(II) standards of CuCl₂ and Cu(II) in the comparable kagomé lattices of herbertsmithite (Zn paratacamite) and barlowite, which have the same Cu(II) coordination geometry of iodobarlowite. Comparison of these XPS spectra demonstrates the absence of Cu(I) in iodobarlowite, as a means of obtaining charge balance in I⁻-deficient iodobarlowite.

<figure><img src="image_12.png" /><figcaption>**Figure S3.** Cu 2p XPS spectra for kagomé minerals (including iodobarlowite Cu₄(OH)₆Fl₀.₈八) and selected Cu(II) standards.</figcaption></figure>
### C. Powder X-ray Diffraction

PXRD and total scattering samples were sent to the Advanced Photon Source at Argonne National Laboratory as part of a 17-BM/11-ID-B mail-in proposal. Data were collected at $\lambda = 0.45211$ Å. The aim of this measurement and refinement were to estimate the I fraction across polycrystalline bulk samples. No attempt was made to account for distortions caused by stoichiometry variation between crystals, which can be inferred to contribute peak structure not captured in the applied model. To this end, the scattered intensity was modeled with two phases in which the histogram scale, lattice parameters, phase fraction, crystallite size, and microstrain were refined, in addition to the occupancy of the I⁻ site in iodobarlowite. These parameters were generally refined in the order of listed, with several refinements being undertaken to check the refinement for robustness. Instrumental peak shape parameters were imported from a separate, prior refinement of a LaB₆ calibration dataset included with the 17-BM measurements. For the background treatment, the relatively new “auto-background” feature of GSAS-II (version 5766) was engaged at the default setting (arpls, log(lambda) = 5.25). The results of the refinement are plotted as log₁₀(intensity) to emphasize the good qualitative fit obtained, shown in Figure S4. A best fit to the PXRD data was obtained for an I⁻ occupancy of 0.8745. The crystal structure parameters used to generate Figure S4 are listed in Table S2.

<figure><img src="image_13.png" /><figcaption>**Figure S4.** Synchrotron PXRD data collected for a ground batch of iodobarlowite single crystals and barium fluoride starting material.</figcaption></figure>
<caption>**Table S2.** Structural parameters used to model Figure S4.</caption>

Iodobarlowite crystal parameters

a,b = 6.69013 Å, c = 9.44074 Å
phase fraction = 0.154
size = 10 µm
µ-strain = 5509.4

<table><thead><tr><th>Atom</th><th>Site sym.</th><th>x</th><th>y</th><th>z</th><th>Occupancy</th><th>Uiso or aniso</th></tr></thead><tbody><tr><td>I1</td><td>-6m2(100)</td><td>0.66667</td><td>0.33333</td><td>0.75</td><td>0.8745</td><td>aniso</td></tr><tr><td>Cu1</td><td>2/m(110)</td><td>0.5</td><td>0.5</td><td>0.5</td><td>1.0</td><td>aniso</td></tr><tr><td>Cu2</td><td>mm2(+−0)</td><td>0.6296</td><td>0.3704</td><td>0.25</td><td>0.3333</td><td>aniso</td></tr><tr><td>O1</td><td>m(100)</td><td>0.2022</td><td>0.4044</td><td>0.5923</td><td>1.0</td><td>aniso</td></tr><tr><td>H1</td><td>1</td><td>0.139</td><td>0.279</td><td>0.631</td><td>1.0</td><td>0.019</td></tr><tr><td>F1</td><td>-6m2(100)</td><td>0.0</td><td>0.0</td><td>0.75</td><td>1.0</td><td>aniso</td></tr></tbody></table>

Barium fluoride crystal parameters

a,b,c = 6.19847 Å
phase fraction = 1
size = 0.4681 µm*
µ-strain = 1629.5

<table><thead><tr><th>Atom</th><th>Site sym.</th><th>x</th><th>y</th><th>z</th><th>Occupancy</th><th>Uiso</th></tr></thead><tbody><tr><td>Ba1</td><td>m3m</td><td>0.0</td><td>0.0</td><td>0.0</td><td>1.0</td><td>0.008</td></tr><tr><td>F1</td><td>-43m</td><td>0.25</td><td>0.25</td><td>0.25</td><td>1.0</td><td>0.004</td></tr></tbody></table>

Anisotropic displacement parameters – not refined, used as obtained from single crystal structure determination.

<table><thead><tr><th>Atom</th><th>U11</th><th>U22</th><th>U33</th><th>U12</th><th>U13</th><th>U23</th></tr></thead><tbody><tr><td>I1</td><td>0.0157</td><td>0.0157</td><td>0.0105</td><td>0.0079</td><td>0.0</td><td>0.0</td></tr><tr><td>Cu1</td><td>0.0104</td><td>0.0104</td><td>0.0217</td><td>0.0055</td><td>0.0018</td><td>-0.0018</td></tr><tr><td>Cu2</td><td>0.0107</td><td>0.0107</td><td>0.0101</td><td>0.0031</td><td>0.0</td><td>0.0</td></tr><tr><td>O1</td><td>0.0117</td><td>0.0108</td><td>0.0157</td><td>0.0054</td><td>0.0008</td><td>0.0015</td></tr><tr><td>F1</td><td>0.016</td><td>0.016</td><td>0.033</td><td>0.008</td><td>0.0</td><td>0.0</td></tr></tbody></table>

<footnote>* samples were ground by hand with mortar and pestle prior to loading into Kapton capillaries for data collection</footnote>
D. X-ray Absorption Spectroscopy (XAS)

X-ray absorption near edge structure (XANES) spectra were collected at the ID12
beamline of the ESRF using total fluorescence yield at room temperature. The spectra
were normalized but not corrected for re-absorption effects. Scans at the Cu and Br K-
edge were taken to judge the relative influence of the coordinating X⁻ ion upon the
kagomé planes and check for signs of Cu⁺ compensation for I⁻ vacancies.

Figure S5 shows the XANES spectra for Cu⁺ and Cu²⁺ halide standards as well as barlowite and iodobarlowite. XANES spectra of botallackite (Cu₂(OH)₃X, X = Br and I) were also recorded, as Cu resides in the same coordination geometry in barlowite and botallackite. Seen across all compounds measured, there are three kinds of Cu states visible, corresponding to Cu⁺, Cu²⁺, and Cu²⁺ with strong hybridization with the halide states—the last case being observed only in the case of anhydrous CuBr₂. These shared features represent states accessible to the promoted 1s electrons as a direct consequence of the intimate electronic relationship in between metal and halide in CuBr₂. Every other compound measured is free of these hybridization features (including CuCl₂, as that sample is hydrated—possessing discrete Cu(OH)₂Cl₂ molecules rather than polymeric CuCl₂ planes analogous to CuBr₂). In light of the extremely strong correspondence between the I/Br botallackite and barlowite spectra that lack any sign of metal-halide hybridization, the ionic nature of the copper-halide relationship in the geometrically frustrated lamellar compounds becomes evident. In both botallackite and barlowite, the first rising edge feature is more intense in the Br⁻ congener and the second more intense in the I⁻ congener. All four compounds share two isosbestic points on either side of the white line, demonstrating the purity of the samples with respect to the copper phase.

FDMNES<sup>7,8</sup> calculations were undertaken to assign spectral features to specific Cu sites in botallackite and barlowite (Figure S6). By symmetry, there are two distinct Cu sites in botallackite and three in the low temperature barlowite structure, two kagomé and the interlayer. These sites are marked in the legend of the respective plot, with sites 1/2 in barlowite corresponding to the kagomé plane Cu. The geometry of each site is summarized in Fig SN and table SN.

Cu site 1 in botallackite possesses the geometry of the kagomé sites in barlowite. The greatest difference between the calculated XANES for botallackite can be attributed to changes in the intensity distribution of Cu site 2, which possesses 5-fold OH⁻ distorted square pyramidal coordination. Whereas the halide isn't directly bound to Cu, each OH⁻ interacts more directly, and as such, the lateral strain upon the C<sub>s</sub> symmetry Cu site 2 produces more pronounced changes in the spectra than is experienced at the C<sub>i</sub> symmetry, kagomé-like site 1 under the steric influence of the different halides.
<figure><img src="image_14.png" /><figcaption>Figure S5. Room temperature XANES spectra collected at ID12. Small Bragg peaks contaminate the iodobarlowite spectra at ~9035 and 9070 eV in the CU K-edge scan, significantly above the edge and not intense enough to interfere with observation of strong near-edge structural similarity to the bromide congener.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Figure S6. Calculated XANES spectra for I and Br botallackite and barlowite. While the absolute energy values are not meaningful, the band shapes of the calculated curves (blue and mustard) agree well with the rising edge of barlowite and botallackite (shown in the lower left panel of Figure S5).</figcaption></figure>
E. References

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5. Sheldrick, G. M. SHELXL (University of Göttingen, Germany, 2014), University of Göttingen, Germany.

6. Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: A Complete Structure Solution, Refinement and Analysis Program. *J. Appl. Cryst.* **2009**, 42, 339–341.

7. Bunău, O.; Joly, Y. Self-Consistent Aspects of X-Ray Absorption Calculations. *J. Phys. Condens. Matter* **2009**, 21, 345501.

8. Guda, S. A.; Guda, A. A.; Soldatov, M. A.; Lomachenko, K. A.; Bugaev, A. L.; Lamberti, C.; Gawelda, W.; Bressler, C.; Smolentsev, G.; Soldatov, A. V.; Joly, Y. Optimized Finite Difference Method for the Full-Potential XANES Simulations: Application to Molecular Adsorption Geometries in MOFs and Metal–Ligand Intersystem Crossing Transients. *J. Chem. Theory Comput.* **2015**, 11, 4512–4521.