Vibronic Structure in Room Temperature Photoluminescence of the Halide Perovskite Cs3Bi2Br9

We report a study on the optical properties of the layered polymorph of vacancy-ordered triple perovskite Cs3Bi2Br9. The electronic structure, determined from density functional theory calculations, shows the top of the valence band and bottom of the conduction band minima are, unusually, dominated by Bi s and p states, respectively. This produces a sharp exciton peak in the absorption spectra with a binding energy that was approximated to be 940 meV, which is substantially stronger than values found in other halide perovskites and, instead, more closely reflects values seen in alkali halide crystals. This large binding energy is indicative of a strongly localized character and results in a highly structured emission at room temperature as the exciton couples to vibrations in the lattice

Ising-like antiferromagnetism on the octahedral sublattice of a cobalt-containing garnet and the potential for quantum criticality

In this contribution, we report that CaY2Co2Ge3O12 exhibits an unusual anisotropic and chainlike antiferromagnetic arrangement of spins despite crystallizing in the highly symmetric garnet structure. Using low-temperature powder neutron diffraction and symmetry analysis, we identify a magnetic structure consisting of chainlike motifs oriented along the body diagonals of the cubic unit cell with moments pointing parallel to the chain direction due to the strong Ising character of the Co ions. Antiferromagnetic order sets in below 6 K and exhibits both temperature- and field-induced magnetic transitions at high fields. Combining the results, we present a magnetic phase diagram that suggests CaY2Co2Ge3O12 undergoes a quantum phase transition at low temperatures and moderate fields

Transition Metal Migration Can Facilitate Ionic Diffusion in Defect Garnet-Based Intercalation Electrodes

The importance of metal migration during multielectron redox activity has been characterized, revealing a competing demand to satisfy bonding requirements and local strains in structures upon alkali intercalation. The local structural evolution required to accommodate intercalation in Y2(MoO4)3 and Al2(MoO4)3 has been contrasted by operando characterization methods, including X-ray absorption spectroscopy and diffraction, along with nuclear magnetic resonance measurements. Computational modeling further rationalized behavioral differences. The local structure of Y2(MoO4)3 was maintained upon lithiation, while the structure of Al2(MoO4)3 underwent substantial local atomic rearrangements as the more ionic character of the bonds in Al2(MoO4)3 allowed Al to mix off its starting octahedral position to accommodate strain during cycling. However, this mixing was prevented in the more covalent Y2(MoO4)3, which accommodated strain through rotational motion of polyhedral subunits. Knowing that an increased ionic character can facilitate the diffusion of redox-inactive metals when cycling multielectron electrodes offers a powerful design principle when identifying next-generation intercalation hosts

Lithium-ion conductivity in Li6Y(BO3)3: a thermally and electrochemically robust solid electrolyte

The development of new frameworks for solid electrolytes exhibiting fast Li-ion diffusion is critical for enabling new energy storage technologies. Here, we present a combined experimental and computational investigation into the ionic conductivity of Li6Y(BO3)3, a new class of solid electrolytes with a pseudo-layered structure. Temperature-dependent impedance spectroscopy shows the pristine material exhibits an ionic conductivity of 2.2 × 10-3 S cm-1 around 400 °C, despite the fact that density functional theory calculations point to multiple remarkably low-energy diffusion pathways. Our calculations indicate small energy barriers for lithium interstitials to diffuse along one-dimensional channels oriented in the c-direction, and also for lithium vacancies diffusing within ac planes. This coexistence of diffusion mechanisms indicates that Li6Y(BO3)3 is an extremely versatile host for exploring and understanding mechanisms for lithium-ion conductivity. We also find no evidence for reactivity with moisture in the atmosphere and that the material appears electrochemically stable when in direct contact with metallic lithium. This robust stability, alongside ionic conductivity that can be manipulated through appropriate aliovalent substitution, make Li6Y(BO3)3 an exceptionally promising new class of solid electrolyte

Metallic Conductivity in a Two-Dimensional Cobalt Dithiolene Metal-Organic Framework

Two-dimensional (2D) metal–organic frameworks (MOFs) have received a great deal of attention due to their relatively high charge carrier mobility and low resistivity. Here we report on the temperature-dependent charge transport properties of a 2D cobalt 2,3,6,7,10,11-triphenylenehexathiolate framework. Variable temperature resistivity studies reveal a transition from a semiconducting to a metallic phase with decreasing temperature, which is unprecedented in MOFs. We find this transition to be highly dependent on the film thickness and the amount of solvent trapped in the pores, with density functional theory calculations of the electronic-structure supporting the complex metallic conductivity of the material. These results identify the first experimentally observed MOF that exhibits band-like metallic conductivity

Electrochemical Oxidative Fluorination of an Oxide Perovskite

We report on the electrochemical fluorination of the A-site vacant perovskite ReO3 using high-temperature solid-state cells as well as room-temperature liquid electrolytes. Using galvanostatic oxidation and electrochemical impedance spectroscopy, we find that ReO3 can be oxidized by approximately 0.5 equiv of electrons when in contact with fluoride-rich electrolytes. Results from our density functional theory calculations clearly rule out the most intuitive mechanism for charge compensation, whereby F-ions would simply insert onto the A-site of the perovskite structure. Operando X-ray diffraction, neutron total scattering measurements, X-ray spectroscopy, and solid-state 19F NMR with magic-angle spinning were, therefore, used to explore the mechanism by which fluoride ions react with the ReO3 electrode during oxidation. Taken together, our results indicate that a complex structural transformation occurs following fluorination to stabilize the resulting material. While we find that this process of fluorinating ReO3 appears to be only partially reversible, this work demonstrates a practical electrolyte and cell design that can be used to evaluate the mobility of small anions like fluoride that is robust at room temperature and opens new opportunities for exploring the electrochemical fluorination of many new materials

Correlated Polyhedral Rotations in the Absence of Polarons during Electrochemical Insertion of Lithium in ReO₃

Understanding the structural transformations that materials undergo during (de)insertion of Li ions is crucial for designing high-performance intercalation hosts as these deformations can lead to significant capacity fade. Herein, we present a study of the metallic defect perovskite ReO₃ to determine whether these distortions are driven by polaronic charge transport (i.e., the electrons and ions moving through the lattice in a coupled way) due to the semiconducting nature of most oxide hosts. Employing numerous techniques, including electrochemical probes, operando X-ray diffraction, X-ray photoelectron spectroscopy, and density functional theory calculations, we find that the cubic structure of ReO₃ experiences multiple phase changes involving the correlated twisting of rigid octahedral subunits upon lithiation. This results in exceptionally poor long-term cyclability due to large strains upon lithiation, even though metallic character is maintained throughout. This suggests that phase transformations during alkali ion intercalation are the result of local strains in the lattice and not exclusively due to polaron migration

Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations

Recently, there has been an explosive growth in research based on hybrid lead–halide perovskites for photovoltaics owing to rapid improvements in efficiency. The advent of these materials for solar applications has led to widespread interest in understanding the key enabling properties of these materials. This has resulted in renewed interest in related compounds and a search for materials that may replicate the defect-tolerant properties and long lifetimes of the hybrid lead-halide perovskites. Given the rapid pace of development of the field, the rises in efficiencies of these systems have outpaced the more basic understanding of these materials. Measuring or calculating the basic properties, such as crystal/electronic structure and composition, can be challenging because some of these materials have anisotropic structures, and/or are composed of both heavy metal cations and volatile, mobile, light elements. Some consequences are beam damage during characterization, composition change under vacuum, or compound effects, such as the alteration of the electronic structure through the influence of the substrate. These effects make it challenging to understand the basic properties integral to optoelectronic operation. Compounding these difficulties is the rapid pace with which the field progresses. This has created an ongoing need to continually evaluate best practices with respect to characterization and calculations, as well as to identify inconsistencies in reported values to determine if those inconsistencies are rooted in characterization methodology or materials synthesis. This article describes the difficulties in characterizing hybrid lead–halide perovskites and new materials and how these challenges may be overcome. The topic was discussed at a seminar at the 2015 Materials Research Society Fall Meeting & Exhibit. This article highlights the lessons learned from the seminar and the insights of some of the attendees, with reference to both recent literature and controlled experiments to illustrate the challenges discussed. The focus in this article is on crystallography, composition measurements, photoemission spectroscopy, and calculations on perovskites and new, related absorbers. We suggest how the reporting of the important artifacts could be streamlined between groups to ensure reproducibility as the field progresses

Anionic order and band gap engineering in vacancy ordered triple perovskites

We demonstrate that the optical absorption of the vacancy-ordered triple perovskite, Cs3Bi2Br9, can be significantly red-shifted by substituting Br with I while maintaining the layered structural topology. We also present evidence that Br ions prefer to occupy the bridging halide position within the layers in order to minimize strain within the lattice that results from the incorporation of the significantly larger iodide anions into the lattice. These results not only quantify the upper limit for I content in the layered polymorph, but also establish the minimum band gap obtainable from these Bi-based phases

Revisiting the charge compensation mechanisms in LiNi0.8Co0.2-: YAlyO2 systems

Oxygen participation, arising from increased transition metal-oxygen covalency during delithiation, is considered essential for the description of charge compensation in conventional layered oxides. The advent of high-resolution mapping of the O K-edge resonant inelastic X-ray scattering (RIXS) provides an opportunity to revisit the onset and extent of oxygen participation. Combining RIXS with an array of structural and electronic probes for the family of Ni-rich LiNi0.8Co0.2-yAlyO2cathodes, we identify common charge compensation regimes that are assigned to formal transition metal redox (<4.25 V) and oxygen participation through covalency (>4.25 V). From O K-edge RIXS maps, we find the emergence of a sharp RIXS feature in these systems when approaching full delithiation, which has previously been associated with lattice oxidized oxygen in alkali-rich systems. The lack of transition metal redox signatures and strong covalency at these high degrees of delithiation suggest this RIXS feature is similarly attributed to lattice oxygen charge compensation as in the alkali-rich systems. The RIXS feature's evolution with state of charge in conventional layered oxides is evidence that this feature reflects the depopulation of occupied O 2p states associated with oxygen participation