Although Cu2+ is ubiquitous, the relativistic destabilization of the 5d orbitals makes the isoelectronic Au2+ exceedingly rare, typically stabilized only through Au–Au bonding or by using redox non-innocent ligands. Here we report the perovskite Cs4AuIIAuIII2Cl12, an extended solid with mononuclear Au2+ sites, which is stable to ambient conditions and characterized by single-crystal X-ray diffraction. The 2+ oxidation state of Au was assigned using 197Au Mössbauer spectroscopy, electron paramagnetic resonance, and magnetic susceptibility measurements, with comparison to paramagnetic and diamagnetic analogues with Cu2+ and Pd2+, respectively, as well as to density functional theory calculations. This gold perovskite offers an opportunity to study the optical and electronic transport of the uncommon Au2+/3+ mixed-valence state and the characteristics of the elusive Au2+ ion coordinated to simple ligands. Compared with the perovskite Cs2AuIAuIIICl6, which has been studied since the 1920s, Cs4AuIIAuIII2Cl12 exhibits a 0.7 eV reduction in optical absorption onset and a 103-fold increase in electronic conductivity.
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All the data underlying the findings of this study are available in this article and its Supplementary Information. The crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre under deposition numbers CCDC 2164336 (Cu_Cl @ 300 K), 2164337 (Au_Cl @ 300 K), 2164338 (Pd_Cl @ 300 K), 2164339 (Au_Cl @ 100 K), and 2164340 (Au_Cl @ 400 K). These data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. Raw images from single-crystal X-ray diffraction collections are available from the corresponding author upon reasonable request. Source data are provided with this paper.
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This research was funded by the US National Science Foundation (DMR2102306). K.P.L. was supported by the Center for Molecular Analysis and Design (CMAD) at Stanford University and the Stanford Department of Chemistry William S. Johnson award. C.R.D was supported by a CMAD fellowship. A.J.H. was supported by the Anne T. and Robert M. Bass Stanford Graduate Fellowship. All calculations were supported by the Theory FWP at the Lawrence Berkeley National Laboratory, funded by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under contract no. DE-AC02-05CH11231. Computations were performed at the National Energy Research Scientific Computing Center, a US Department of Energy Office of Science User Facility located at Lawrence Berkeley National Laboratory, also operated under contract no. DE-AC02-05CH11231. We thank S. J. Teat at Lawrence Berkeley National Laboratory for assistance with crystallography, A. Sica and J. Caram at the University of California Los Angeles for low-energy UV–vis–NIR spectroscopy measurements, and M. Brueggemeyer at Stanford University for additional assistance with EPR measurements. Single-crystal XRD studies were performed at beamline 12.2.1 at the Advanced Light Source at Lawrence Berkeley National Laboratory. Additional single-crystal XRD studies were performed at the Stanford Nanocharacterization Laboratory and differential scanning calorimetry studies were performed at the Soft and Hybrid Materials Facility, both part of the Stanford Nano Shared Facilities, supported by the National Science Foundation under award ECCS-1542152. XAS studies used beamlines 4-1 and 4-3 at Stanford Synchrotron Radiation Lightsource at the SLAC National Accelerator Laboratory. X-ray PDF studies used beamline 11-ID-B at the Advanced Photon Source, a US Department of Energy Office of Science User Facility operated for the Department of Energy Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. Financial support for D.H.R. was provided by Fonds Quebecois de la Recherche sur la Nature et les Technologies and the Natural Sciences and Engineering Research Council Canada. The 197Au Mössbauer source was prepared in the McMaster Nuclear Reactor, Hamilton Ontario, and the assistance of R. Pasuta is gratefully acknowledged. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.
The authors declare no competing interests.
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a, A 3D representation where turquoise and light green spheres represent Cs and Cl atoms, respectively, and gold and grey octahedra represent AuIIICl6 and PdIICl6, respectively, and the viewing direction is shown by the coordinate system in panel (b). b,c, 2D representations of the crystal structure of Pd_Cl where the short, equatorial bonds of the axially elongated AuIIICl6 and PdIICl6 octahedra are represented by gold and grey lines, respectively. Equatorial bonds perpendicular to the plane are not drawn. The numbers represent the stacking sequence of the layers along the a axis for (b) and the c axis for (c). The structure is isostructral with that of Au_Cl. Note: the Au and Pd atoms are partially disordered with respect to one another in the crystal structure, though they still exhibit clear preferences for particular crystallographic sites, which are shown in this figure.
a, A 3D representation where turquoise and light green spheres represent Cs and Cl atoms, respectively, and gold and green octahedra represent AuIIICl6 and CuIICl6, respectively, and the viewing direction is shown by the coordinate system in panel (b). b,c, 2D representations of the crystal structure of Cu_Cl where the short, equatorial bonds of the axially elongated AuIIICl6 and CuIICl6 octahedra are represented by gold and green lines, respectively. Equatorial bonds perpendicular to the plane are not drawn. The numbers represent the stacking sequence of the layers along the a axis for (b) and the c axis for (c). The structure is similar to that of Au_Cl, but with a different ordering of the Au3+/B2+ and vacancies with respect to one another.
a–d, Data are plotted in k space (b–d) and Fourier transformed to real space (a), all with a k weight of 2. c,d, EXAFS data at the Au L3 edge for Au_Cl and Pd_Cl and fitted Au2+–Cl component for Au_Cl. The text at the top of each plot denotes the number of independent photoelectron threshold energies (E0) used for the fit to obtain the Au2+–Cl component. The dashed line is placed at y = 0 to examine the zero crossing of each wave: the zero crossings for Au_Cl are at lower values of k than for Pd_Cl, particularly in the low-k range ( ~ 4 to 8 k), consistent with the presence of the Au2+–Cl component that is offset from the Au3+–Cl component due to the difference in bond length and/or photoelectron threshold energy. See Methods and Supplementary Information for a detailed discussion.
a–f, Data are plotted in k space (b,d,f) and Fourier transformed to real space (a,c,e), all with a k weight of 2. The text at the top of each plot denotes the parameters used for the fit. The best fit is shown in panels (c,d), which incorporates both Au2+ and Au3+ with one independent photoelectron threshold energy. See Methods and Supplementary Information for a detailed discussion and Supplementary Table 7 for fitting results.
a, Overlayed spectra of Au_Cl and Cs2AuIAuIIICl6, with the red ellipses highlighting the differences between the two spectra that correspond to the replacement of Au1+ with Au2+. b, The best fit to the spectrum of Cs2AuIAuIIICl6. c, The best fit to the spectrum of Au_Cl. The Au3+ doublets are unresolved due to their small values of quadrupole splitting. The observed data in (b) and (c) are plotted as vertical bars representing the measurement uncertainty (1σ). The results confirm the oxidation-state assignments of Au in Au_Cl. See Methods and Supplementary Information for details about the fitting, Extended Data Table 1 for fitting results, and Supplementary Table 9 for literature values.
a, Heat capacity normalized at high temperatures ( ~ 200–275 K). b, A magnified region of the same data presented in (a). The scaling factors denoted in the legends in (a,b) are probably due to inaccuracies in the measured sample masses and are scaled with respect to Au_Cl. c, The heat capacity in excess of that of Pd_Cl. The vertical bars represent the measurement uncertainty (1σ). d, Entropy (ΔS) as extracted from the heat capacity in excess of that of Pd_Cl. The dashed horizontal line at Rln(2) represents the expected excess entropy for a magnetic centre with spin S = 1/2. e, Heat capacity of Au_Cl with no applied magnetic field as compared to that in an applied magnetic field of 9 T, showing no meaningful change. f, Heat capacity of single crystals and a pressed pellet of Au_Cl powder. The scaling factor is an estimate due to the difficulty in obtaining an accurate mass of the crystals used for the measurement. The error bars in (a,b,e,f) are smaller than the points as drawn and were therefore not included in the figures. The results show an intriguing excess of magnetic entropy for Au_Cl beyond the expected value for a system with S = 1/2.
Magnetic susceptibility (χ) from DC magnetization data with χ versus temperature at a single applied field (μ0H) and with a diamagnetic correction χ0 = 2.25 × 10−3 emu ∙ (mol F.U.)−1, where F.U. = formula unit and 1 emu ∙ (mol Oe)−1 = (4π)10−6 m3 ∙ mol−1. The inset shows the inverse susceptibility data with a Curie–Weiss fit (red line) from 50 to 200 K. The results indicate weak antiferromagnetic interactions. See Methods and Supplementary Information for details about the fitting and Supplementary Table 10 for fitting results.
Extended Data Fig. 9 UV–vis–NIR absorbance spectra of crystals pulverized into a mull with BaSO4 and converted from diffuse reflectance spectra using the Kubelka–Munk transformation.
a,b,d Spectra normalized to the highest point at or below 4.5 eV. c, Spectra normalized to align near the first absorption peak. The same spectra are shown in (a) and (b) with two different energy axis ranges for clarity. The two spectra in (c) were collected with two different spectrophotometers to acquire a larger energy range. The low-energy absorption onset for Au_Cl is probably due to a low-energy Au2+/3+ intervalence charge transfer. See Methods and Supplementary Information for a detailed discussion.
Supplementary discussion, Figs. 1–27 and Tables 1–15.
Crystallographic data for Cu_Cl at 300 K; CCDC reference 2164336.
Crystallographic data for Au_Cl at 300 K; CCDC reference 2164337.
Crystallographic data for Pd_Cl at 300 K; CCDC reference 2164338.
Crystallographic data for Au_Cl at 100 K; CCDC reference 2164339.
Crystallographic data for Au_Cl at 400 K; CCDC reference 2164340.
Experimental structural coordinates for Au_Cl used for DFT calculations.
Structural coordinates for Au_Cl from HSE relaxation used for DFT calculations.
Structural coordinates for Au_Cl from Perdew, Burke and Enzerhof relaxation used for DFT calculations.
Structural coordinates for ‘pristine’ Au_Cl used for DFT calculations.
Supplementary source data.
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Lindquist, K.P., Eghdami, A., Deschene, C.R. et al. Stabilizing Au2+ in a mixed-valence 3D halide perovskite. Nat. Chem. 15, 1780–1786 (2023). https://doi.org/10.1038/s41557-023-01305-y