High-Resolution In-Situ Synchrotron X-Ray Studies of Inorganic Perovskite CsPbBr3 : New Symmetry Assignments and Structural Phase Transitions.

Perovskite photovoltaic ABX3 systems are being studied due to their high energy-conversion efficiencies with current emphasis placed on pure inorganic systems. In this work, synchrotron single-crystal diffraction measurements combined with second harmonic generation measurements reveal the absence of inversion symmetry below room temperature in CsPbBr3 . Local structural analysis by pair distribution function and X-ray absorption fine structure methods are performed to ascertain the local ordering, atomic pair correlations, and phase evolution in a broad range of temperatures. The currently accepted space group assignments for CsPbBr3 are found to be incorrect in a manner that profoundly impacts physical properties. New assignments are obtained for the bulk structure: I m 3 ¯ (above ≈410 K), P21 /m (between ≈300 K and ≈410 K), and the polar group Pm (below ≈300 K), respectively. The newly observed structural distortions exist in the bulk structure consistent with the expectation of previous photoluminescence and Raman measurements. High-pressure measurements reveal multiple low-pressure phases, one of which exists as a metastable phase at ambient pressure. This work should help guide research in the perovskite photovoltaic community to better control the structure under operational conditions and further improve transport and optical properties.

Intertwined density waves in a metallic nickelate.

Nickelates are a rich class of materials, ranging from insulating magnets to superconductors. But for stoichiometric materials, insulating behavior is the norm, as for most late transition metal oxides. Notable exceptions are the 3D perovskite LaNiO3, an unconventional paramagnetic metal, and the layered Ruddlesden-Popper phases R4Ni3O10, (R = La, Pr, Nd). The latter are particularly intriguing because they exhibit an unusual metal-to-metal transition. Here, we demonstrate that this transition results from an incommensurate density wave with both charge and magnetic character that lies closer in its behavior to the metallic density wave seen in chromium metal than the insulating stripes typically found in single-layer nickelates like La2-xSrxNiO4. We identify these intertwined density waves as being Fermi surface-driven, revealing a novel ordering mechanism in this nickelate that reflects a coupling among charge, spin, and lattice degrees of freedom that differs not only from the single-layer materials, but from the 3D perovskites as well.

Materializing rival ground states in the barlowite family of kagome magnets: quantum spin liquid, spin ordered, and valence bond crystal states.

The spin- 1 2 kagome antiferromagnet is considered an ideal host for a quantum spin liquid (QSL) ground state. We find that when the bonds of the kagome lattice are modulated with a periodic pattern, new quantum ground states emerge. Newly synthesized crystalline barlowite (Cu4(OH)6FBr) and Zn-substituted barlowite demonstrate the delicate interplay between singlet states and spin order on the spin- 1 2 kagome lattice. Comprehensive structural measurements demonstrate that our new variant of barlowite maintains hexagonal symmetry at low temperatures with an arrangement of distorted and undistorted kagome triangles, for which numerical simulations predict a pinwheel valence bond crystal (VBC) state instead of a QSL. The presence of interlayer spins eventually leads to an interesting pinwheel q = 0 magnetic order. Partially Zn-substituted barlowite (Cu3.44Zn0.56(OH)6FBr) has an ideal kagome lattice and shows QSL behavior, indicating a surprising robustness of the QSL against interlayer impurities. The magnetic susceptibility is similar to that of herbertsmithite, even though the Cu2+ impurities are above the percolation threshold for the interlayer lattice and they couple more strongly to the nearest kagome moment. This system is a unique playground displaying QSL, VBC, and spin order, furthering our understanding of these highly competitive quantum states.

Pressure-Thresholded Response in Cylindrically Shocked Cyclotrimethylene Trinitramine (RDX).

We demonstrate a strongly thresholded response in cyclotrimethylene trinitramine (RDX) when it is cylindrically shocked using a novel waveguide geometry. Using ultrafast single-shot multi-frame imaging, we demonstrate that <100 µm diameter single crystals of RDX embedded in a polymer host deform along preferential planes for >100 ns after the shock first arrives in the crystal. We use in situ imaging and time-resolved photoemission to demonstrate that short-lived chemistry occurs with complex deformation pathways. Using scanning electron microscopy and ultra-small-angle X-ray scattering, we demonstrate that the shock-induced dynamics leave behind porous crystals, with pore shapes and sizes that change significantly with shock pressure. A threshold pressure of ∼12 GPa at the center of convergence separated the single-mode planar crystal deformations from the chemistry-coupled multi-plane dynamics at higher pressures. Our observations indicate preferential directions for deformation in our cylindrically shocked system, despite the applied stress along many different crystallographic planes.

III-V Clathrate Semiconductors with Outstanding Hole Mobility: Cs8In27Sb19 and A8Ga27Sb19 (A = Cs, Rb).

Three novel unconventional clathrates with unprecedented III-V semiconducting frameworks have been synthesized: Cs8In27Sb19, Cs8Ga27Sb19, and Rb8Ga27Sb19. These clathrates represent the first examples of tetrel-free clathrates that are completely composed of main group elements. All title compounds crystallize in an ordered superstructure of clathrate-I in the Ia3̅ space group (No. 206; Z = 8). In the clathrate framework, a full ordering of {Ga or In} and Sb is observed by a combination of high-resolution synchrotron single-crystal and powder X-ray diffraction techniques. Density functional theory (DFT) calculations show that all three clathrates are energetically stable with relaxed lattice constants matching the experimental data. Due to the complexity of the crystal structure composed of heavy elements, the reported clathrates exhibit ultralow thermal conductivities of less than 1 W·m-1·K-1 at room temperature. All compounds are predicted and experimentally confirmed to be narrow-bandgap p-type semiconductors with high Seebeck thermopower values, up to 250 µV·K-1 at 300 K for Cs8In27Sb19. The latter compound shows carrier concentrations and mobilities, 1.42 × 1015 cm-3 and 880 cm2 ·V-1·s-1, which are on par with the values for parent binary InSb, one of the best electronic semiconductors. The high hole carrier mobility is uncommon for complex bulk materials and a highly desirable trait, opening ways to design semiconducting materials based on tunable III-V clathrates.

Site-Specific Structure at Multiple Length Scales in Kagome Quantum Spin Liquid Candidates.

Realizing a quantum spin liquid (QSL) ground state in a real material is a leading issue in condensed matter physics research. In this pursuit, it is crucial to fully characterize the structure and influence of defects, as these can significantly affect the fragile QSL physics. Here, we perform a variety of cutting-edge synchrotron X-ray scattering and spectroscopy techniques, and we advance new methodologies for site-specific diffraction and L-edge Zn absorption spectroscopy. The experimental results along with our first-principles calculations address outstanding questions about the local and long-range structures of the two leading kagome QSL candidates, Zn-substituted barlowite (Cu3Zn x Cu1-x (OH)6FBr) and herbertsmithite (Cu3Zn(OH)6Cl2). On all length scales probed, there is no evidence that Zn substitutes onto the kagome layers, thereby preserving the QSL physics of the kagome lattice. Our calculations show that antisite disorder is not energetically favorable and is even less favorable in Zn-barlowite compared to herbertsmithite. Site-specific X-ray diffraction measurements of Zn-barlowite reveal that Cu2+ and Zn2+ selectively occupy distinct interlayer sites, in contrast to herbertsmithite. Using the first measured Zn L-edge inelastic X-ray absorption spectra combined with calculations, we discover a systematic correlation between the loss of inversion symmetry from pseudo-octahedral (herbertsmithite) to trigonal prismatic coordination (Zn-barlowite) with the emergence of a new peak. Overall, our measurements suggest that Zn-barlowite has structural advantages over herbertsmithite that make its magnetic properties closer to an ideal QSL candidate: its kagome layers are highly resistant to nonmagnetic defects while the interlayers can accommodate a higher amount of Zn substitution.

Crystal structures and phase relationships in magnetostrictive Tb1-x Dy x Co2 system.

The crystal structures and phase relationships of Tb1-x Dy x Co2 alloys with 0 ⩽ x ⩽ 1 were investigated by synchrotron-based high-resolution x-ray powder diffraction. Three different crystal structures are observed in the system: all the compositions show cubic structure with space group [Formula: see text] at temperatures above the Curie temperature T C; the Tb-rich side sample shows a rhombohedral structure with space group [Formula: see text] and the Dy-rich side sample has a tetragonal [Formula: see text] space group. In situ measurements on the intermediate compound Tb0.3Dy0.7Co2 show a rhombohedral to tetragonal structural transition, and the two phases coexist from 99 K to 111 K, where the so-called magnetic morphotropic phase boundary (MPB) is found. The coexisting phases are believed to induce the anomalous magnetostrictive effect in the MPB regime.

Exposing the inadequacy of redox formalisms by resolving redox inequivalence within isovalent clusters.

In this report we examine a family of trinuclear iron complexes by multiple-wavelength, anomalous diffraction (MAD) to explore the redox load distribution within cluster materials by the free refinement of atomic scattering factors. Several effects were explored that can impact atomic scattering factors within clusters, including 1) metal atom primary coordination sphere, 2) M-M bonding, and 3) redox delocalization in formally mixed-valent species. Complexes were investigated which vary from highly symmetric to fully asymmetric by 57Fe Mössbauer and X-ray diffraction to explore the relationship between MAD-derived data and the data available from these widely used characterization techniques. The compounds examined include the all-ferrous clusters [ n Bu4N][(tbsL)Fe3(µ3-Cl)] (1) ([tbsL]6- = [1,3,5-C6H9(NC6H4-o-NSi t BuMe2)3]6-]), (tbsL)Fe3(py) (2), [K(C222)]2[(tbsL)Fe3(µ3-NPh)] (4) (C222 = 2,2,2-cryptand), and the mixed-valent (tbsL)Fe3(µ3-NPh) (3). Redox delocalization in mixed-valent 3 was explored with cyclic voltammetry (CV), zero-field 57Fe Mössbauer, near-infrared (NIR) spectroscopy, and X-ray crystallography techniques. We find that the MAD results show an excellent correspondence to 57Fe Mössbauer data; yet also can distinguish between subtle changes in local coordination geometries where Mössbauer cannot. Differences within aggregate oxidation levels are evident by systematic shifts of scattering factor envelopes to increasingly higher energies. However, distinguishing local oxidation levels in iso- or mixed-valent materials can be dramatically obscured by the degree of covalent intracore bonding. MAD-derived atomic scattering factor data emphasize in-edge features that are often difficult to analyze by X-ray absorption near edge spectroscopy (XANES). Thus, relative oxidation levels within the cluster were most reliably ascertained from comparing the entire envelope of the atomic scattering factor data.

Ligand-Based Control of Single-Site vs. Multi-Site Reactivity by a Trichromium Cluster.

The trichromium cluster (tbs L)Cr3 (thf) ([tbs L]6- =[1,3,5-C6 H9 (NC6 H4 -o-NSit BuMe2 )3 ]6- ) exhibits steric- and solvation-controlled reactivity with organic azides to form three distinct products: reaction of (tbs L)Cr3 (thf) with benzyl azide forms a symmetrized bridging imido complex (tbs L)Cr3 (µ3 -NBn); reaction with mesityl azide in benzene affords a terminally bound imido complex (tbs L)Cr3 (µ1 -NMes); whereas the reaction with mesityl azide in THF leads to terminal N-atom excision from the azide to yield the nitride complex (tbs L)Cr3 (µ3 -N). The reactivity of this complex demonstrates the ability of the cluster-templating ligand to produce a well-defined polynuclear transition metal cluster that can access distinct single-site and cooperative reactivity controlled by either substrate steric demands or reaction media.

Electron Cartography in Clusters.

Deconvoluting the atom-specific electron density within polynuclear systems remains a challenge. A multiple-wavelength anomalous diffraction study on four clusters that share the same [Co6 Se8 ] core was performed. Two cluster types were designed, one having a symmetric ligand sphere and the other having an asymmetric ligand sphere. It was found that in the neutral, asymmetric, CO-bound cluster, the Co-CO site is more highly oxidized than the other five Co atoms; when an electron is removed, the hole is distributed among the Se atoms. In the neutral, symmetric cluster, the Co atoms divide by electron population into two sets of three, each set being meridional; upon removal of an electron, the hole is distributed among all the Co atoms. This ligand-dependent tuning of the electron/hole distribution relates directly to the performance of clusters in biological and synthetic systems.

η2-SO2 Linkage Photoisomer of an Osmium Coordination Complex.

We report the discovery of an η2-SO2 linkage photoisomer in the osmium pentaammine coordination complex, [Os(NH3)5(SO2)][Os(NH3)5(HSO3)]Cl4 (1). Its dark- and light-induced crystal structures are determined via synchrotron X-ray crystallography, at 100 K, where the photoinduced state is metastable in a single crystal that has been stimulated by 505 nm light for 2.5 h. The SO2 photoisomer in the [Os(NH3)5(SO2)]2+ cation contrasts starkly with the photoinactivity of the HSO3 ligand in its companion [Os(NH3)5(HSO3)]+ cation within the crystallographic asymmetric unit of this single crystal. Panchromatic optical absorption characteristics of this single crystal are revealed in both dark- and light-induced states, using concerted absorption spectroscopy and optical microscopy. Its absorption halves across most of its visible spectrum, upon exposure to 505 nm light. The SO2 ligand seems to be responsible for this photoinduced bleaching effect, judging from a comparison of the dark- and light-induced crystal structures of 1. The SO2 photoisomerism is found to be thermally reversible, and so 1 presents a rare example of an osmium-based solid-state optical switch. Such switching in an osmium complex is significant because bottom-row transition metals stand to offer linkage photoisomerism with the greatest photoconversion levels and thermal stability. The demonstration of η2-SO2 bonding in this complex also represents a fundamental contribution to osmium coordination chemistry.