Manipulation of the Structure and Optoelectronic Properties through Bromine Inclusion in a Layered Lead Bromide Perovskite.

One of the great advantages of organic-inorganic metal halides is that their structures and properties are highly tuneable and this is important when optimizing materials for photovoltaics or other optoelectronic devices. One of the most common and effective ways of tuning the electronic structure is through anion substitution. Here, we report the inclusion of bromine into the layered perovskite [H3N(CH2)6NH3]PbBr4 to form [H3N(CH2)6NH3]PbBr4·Br2, which contains molecular bromine (Br2) intercalated between the layers of corner-sharing PbBr6 octahedra. Bromine intercalation in [H3N(CH2)6NH3]PbBr4·Br2 results in a decrease in the band gap of 0.85 eV and induces a structural transition from a Ruddlesden-Popper-like to Dion-Jacobson-like phase, while also changing the conformation of the amine. Electronic structure calculations show that Br2 intercalation is accompanied by the formation of a new band in the electronic structure and a significant decrease in the effective masses of around two orders of magnitude. This is backed up by our resistivity measurements that show that [H3N(CH2)6NH3]PbBr4·Br2 has a resistivity value of one order of magnitude lower than [H3N(CH2)6NH3]PbBr4, suggesting that bromine inclusion significantly increases the mobility and/or carrier concentration in the material. This work highlights the possibility of using molecular inclusion as an alternative tool to tune the electronic properties of layered organic-inorganic perovskites, while also being the first example of molecular bromine inclusion in a layered lead halide perovskite. By using a combination of crystallography and computation, we show that the key to this manipulation of the electronic structure is the formation of halogen bonds between the Br2 and Br in the [PbBr4]∞ layers, which is likely to have important effects in a range of organic-inorganic metal halides.

Syntheses and crystal structures of three novel oxalate coordination compounds: Rb2Co(C2O4)2·4H2O, Rb2CoCl2(C2O4) and K2Li2Cu(C2O4)3·2H2O.

Single crystals of three novel transition-metal oxalates, dirubidium di-aqua-dioxalatocobalt(II) dihydrate or dirubidium cobalt(II) bis-(oxalate) tetra-hydrate, Rb2[Co(C2O4)2(H2O)2]·2H2O, (I), catena-poly[dirubidium [[di-chlorido-cobalt(II)]-µ-oxalato]] or dirubidium cobalt(II) dichloride oxalate, {Rb2[CoCl2(C2O4)]} n , (II), and poly[dipotassium [tri-µ-oxalato-copper(II)dilithium] dihydrate] or dipotassium dilithium copper(II) tris-(oxalate) dihydrate, {K2[Li2Cu(C2O4)3]·2H2O} n , (III), have been grown under hydro-thermal conditions and their crystal structures determined using single-crystal X-ray diffraction. The structure of (I) exhibits isolated octa-hedral [Co(C2O4)2(H2O)2] units, whereas (II) consists of trans chains of Co2+ ions bridged by bidentate oxalato ligands and (III) displays a novel tri-periodic network of Li+ and Cu2+ ions linked by oxalato bridging ligands.

K2Fe(C2O4)2: An Oxalate Cathode for Li/Na-Ion Batteries Exhibiting a Combination of Multielectron Cation and Anion Redox.

The development of multielectron redox-active cathode materials is a top priority for achieving high energy density with long cycle life in the next-generation secondary battery applications. Triggering anion redox activity is regarded as a promising strategy to enhance the energy density of polyanionic cathodes for Li/Na-ion batteries. Herein, K2Fe(C2O4)2 is shown to be a promising new cathode material that combines metal redox activity with oxalate anion (C2O4 2-) redox. This compound reveals specific discharge capacities of 116 and 60 mAh g-1 for sodium-ion batterie (NIB) and lithium-ion batterie (LIB) cathode applications, respectively, at a rate of 10 mA g-1, with excellent cycling stability. The experimental results are complemented by density functional theory (DFT) calculations of the average atomic charges.

Exploiting anion and cation redox chemistry in lithium-rich perovskite oxalate: a novel next-generation Li/Na-ion battery electrode.

The fundamental understanding of electrochemical reaction kinetics for lithium/sodium-ion batteries (LIBs & NIBs) is a significant criterion for advancing new-generation electrode materials. Herein, we demonstrate a novel lithium-rich perovskite oxalate KLi3Fe(C2O4)3 (KLFC) cathode with the combination of cation and anion redox delivering discharge capacities of 86 and 99 mA h g-1 after 100 cycles for a LIB and NIB, respectively, with good cyclability. Experimental Raman spectroscopy analysis combined with DFT calculations of charged/discharged samples illustrate the oxalate anion redox activity. Further, first-principles calculations of the partial density of states and Bader charges analysis have also characterised the redox behaviour and charge transfer during the potassium extraction processes.

Perovskite-derived structure modulation in the iron sulfate family.

We report the first example of a perovskite sulfate [Na3(H2O)]Fe(SO4)3. Further structure modulation, by dimensional reduction or ligand extension, has resulted in two related layered perovskite-like compounds Na6Fe(SO4)4 and Na12Fe3(SO4)6F8. Taken together, these results open up a more general strategy for the future design of more complex perovskite-related materials.

Polarity and Ferromagnetism in Two-Dimensional Hybrid Copper Perovskites with Chlorinated Aromatic Spacers.

Two-dimensional (2D) organic-inorganic hybrid copper halide perovskites have drawn tremendous attention as promising multifunctional materials. Herein, by incorporating ortho-, meta-, and para-chlorine substitutions in the benzylamine structure, we first report the influence of positional isomerism on the crystal structures of chlorobenzylammonium copper(II) chloride perovskites A2CuCl4. 2D polar ferromagnets (3-ClbaH)2CuCl4 and (4-ClbaH)2CuCl4 (ClbaH+ = chlorobenzylammonium) are successfully obtained. They both adopt a polar monoclinic space group Cc at room temperature, displaying significant differences in crystal structures. In contrast, (2-ClbaH)2CuCl4 adopts a centrosymmetric space group P 21/ c at room temperature. This associated structural evolution successfully enhances the physical properties of the two polar compounds with high thermal stability, discernible second harmonic generation (SHG) signals, ferromagnetism, and narrow optical band gaps. These findings demonstrate that the introduction of chlorine atoms into the interlayer organic species is a powerful tool to tune crystal symmetries and physical properties, and this inspires further exploration of designing high-performance multifunctional copper-based materials.

Polar Ferromagnet Induced by Fluorine Positioning in Isomeric Layered Copper Halide Perovskites.

We present the influence of positional isomerism on the crystal structure of fluorobenzylammonium copper(II) chloride perovskites A2CuCl4 by incorporating ortho-, meta-, and para-fluorine substitution in the benzylamine structure. Two-dimensional (2D) polar ferromagnet (3-FbaH)2CuCl4 (3-FbaH+ = 3-fluorobenzylammonium) is successfully obtained, which crystallizes in a polar orthorhombic space group Pca21 at room temperature. In contrast, both (2-FbaH)2CuCl4 (2-FbaH+ = 2-fluorobenzylammonium) and (4-FbaH)2CuCl4 (4-FbaH+ = 4-fluorobenzylammonium) crystallize in centrosymmetric space groups P21/c and Pnma at room temperature, respectively, displaying significant differences in crystal structures. These differences indicate that the position of the fluorine atom is a driver for the polar behavior in (3-FbaH)2CuCl4. Preliminary magnetic measurements confirm that these three perovskites possess dominant ferromagnetic interactions within the inorganic [CuCl4]∞ layers. Therefore, (3-FbaH)2CuCl4 is a polar ferromagnet, with potential as a type I multiferroic. This work is expected to promote further development of high-performance 2D copper(II) halide perovskite multiferroic materials.

Structural chemistry of layered lead halide perovskites containing single octahedral layers.

We present a comprehensive review of the structural chemistry of hybrid lead halides of stoichiometry APbX 4, A 2PbX4 or A A'PbX 4, where A and A' are organic ammonium cations and X = Cl, Br or I. These compounds may be considered as layered perovskites, containing isolated, infinite layers of corner-sharing PbX 4 octahedra separated by the organic species. First, over 250 crystal structures were extracted from the CCDC and classified in terms of unit-cell metrics and crystal symmetry. Symmetry mode analysis was then used to identify the nature of key structural distortions of the [PbX 4]∞ layers. Two generic types of distortion are prevalent in this family: tilting of the octahedral units and shifts of the inorganic layers relative to each other. Although the octahedral tilting modes are well known in the crystallography of purely inorganic perovskites, the additional layer-shift modes are shown to enormously enrich the structural options available in layered hybrid perovskites. Some examples and trends are discussed in more detail in order to show how the nature of the interlayer organic species can influence the overall structural architecture; although the main aim of the paper is to encourage workers in the field to make use of the systematic crystallographic methods used here to further understand and rationalize their own compounds, and perhaps to be able to design-in particular structural features in future work.

Structural Features in Some Layered Hybrid Copper Chloride Perovskites: ACuCl4 or A2CuCl4.

We present three new hybrid copper(II) chloride layered perovskites of generic composition ACuCl4 or A2CuCl4, which exhibit three distinct structure types. (m-PdH2)CuCl4 (m-PdH22+ = protonated m-phenylenediamine) adopts a Dion-Jacobson (DJ)-like layered perovskite structure type and exhibits a very large axial thermal contraction effect upon heating, as revealed via variable-temperature synchrotron X-ray powder diffraction (SXRD). This can be attributed to the contraction of an interlayer block, via a slight repositioning of the m-PdH22+ moiety. (3-AbaH)2CuCl4 (3-AbaH+ = protonated 3-aminobenzoic acid) and (4-AbaH)2CuCl4 (4-AbaH+ = protonated 4-aminobenzoic acid) possess the same generic formula as Ruddlesden-Popper (RP) layered perovskites, A2BX4, but adopt different structures. (4-AbaH)2CuCl4 adopts a near-staggered structure type, whereas (3-AbaH)2CuCl4 adopts a near-eclipsed structure type, which resembles the DJ rather than the RP family. (3-AbaH)2CuCl4 also displays static disorder of the [CuCl4]∞ layers. The crystal structures of each are discussed in terms of the differing nature of the templating molecular species, and these are compared to related layered perovskites. Preliminary magnetic measurements are reported, suggesting dominant ferromagnetic interactions.

Tilting and twisting in a novel perovzalate, K3NaMn(C2O4)3.

In the title compound, the oxalate ligand simultaneously bridges both Mn-centred and Na-centred octahedra to produce a unique 'doubly-interpenetrated' perovskite-like lattice with an unconventional octahedral tilt system. In turn, the coordination requirements of the oxalate ligand lead to a rare 'twisted' conformation.

Synthesis, Structure, and Electrochemical Properties of Some Cobalt Oxalates.

Transition-metal oxalates have wide applications in magnetics, photoemission, electrochemistry, etc. Herein, using hydrothermal reactions, five cobalt(II) oxalates, Na2Co2(C2O4)3·2H2O (I), Na2Co(C2O4)2·8H2O (II), KLi3Co(C2O4)3 (III), Li4Co(C2O4)3 (IV), and (NH4)2Co2(C2O4)F4 (V) have been synthesized, and their structures are determined from single-crystal X-ray diffraction or Rietveld refinement of powder X-ray diffraction data. Notably, IV and V are identified for the first time. The structures of these cobalt oxalates are versatile, covering 0D, 1D, 2D, and 3D frameworks, while the coordination environments of Co2+ centers are uniquely distorted octahedra. As representative examples, I and III are investigated as cathode materials for secondary batteries. Both exhibited electrochemical activity despite large cell polarization. The present study enriches the transition-metal oxalate family and provides new options for energy storage materials.

Structural variations in (001)-oriented layered lead halide perovskites, templated by 1,2,4-triazolium.

A new hybrid lead(ii) halide perovskite, (TzH)2PbCl4, ([TzH+] = 1,2,4-triazolium), adopts a (001)-oriented layered perovskite structure, which can be considered as derived from the n = 1 Ruddlesden-Popper (RP) type. Variable temperature single crystal X-ray diffraction reveals a structural phase transition in the region 125 K < T < 173 K between a high temperature, high symmetry polymorph, space group Cmcm, and a low temperature, low symmetry chiral polymorph, space group P212121, which has a tripled unit cell volume. UV-Vis spectra suggest a band gap of 3.30 eV for (TzH)2PbCl4. A second polymorph of the bromide analogue, (TzH)2PbBr4-II, is also reported, and structural relationships between all three variants are discussed.

Variable dimensionality in 'hollow' hybrid tin iodide perovskites.

Two 'hollow' B-site deficient perovskites, (TzH)11(H3PO2)Sn6I23 and (TzH)3Sn2I7 (TzH+ = 1,2,4-triazolium, H3PO2 = hypohosphorous acid), have been prepared. (TzH)11(H3PO2)Sn6I23 is the first example of a 2D layered structure of this type. Leaving the same reaction mixture for an extended time also affords the 3D derivative (TzH)3Sn2I7.

Structural diversity of lead halide chain compounds, APbX3, templated by isomeric molecular cations.

Three new 1D chain structure type hybrid organic-inorganic lead(ii) halides are presented: IQPbBr3, QPbBr3 and QPbI3, templated by large organic cations, isoquinolinium ([IQ+] = protonated isoquinoline) and its isomer quinolonium ([Q+] = protonated quinoline). All three compounds possess the same generic formula as cubic perovskite, ABX3, but adopt different structures. IQPbBr3 adopts a 1D face-sharing single chain hexagonal perovskite structure type, and the other two, QPbBr3 and QPbI3, adopt a non-perovskite structure which is built from 1D edge-sharing octahedral double chains. Crystal structures and preliminary photophysical properties are discussed. Two of them have lower bandgaps than the other reported materials with the same structure type, indicating the value of further exploratory studies for these types of materials.

Structural Diversity in Layered Hybrid Perovskites, A2PbBr4 or AA'PbBr4, Templated by Small Disc-Shaped Amines.

We present three new hybrid layered lead(II) bromide perovskites of generic composition A2PbBr4 or AA'PbBr4 that exhibit three distinct structure types. [TzH]2PbBr4 ([TzH+] = 1,2,4-triazolium) adopts a (001)-oriented layer structure and [AaH]2PbBr4, ([AaH+] = acetamidinium) adopts a (110)-oriented type, whereas [ImH][TzH]PbBr4, ([ImH+] = imidazolium) adopts a rare (110)-oriented structure with enhanced corrugation (i.e., 3 × 3 type). The crystal structures of each are discussed in terms of the differing nature of the templating molecular species. Photoluminescent spectra for each are reported and the behaviors discussed in relation to the different structure of each composition.

Unprecedented tin iodide perovskite-like structures featuring ordering of organic moieties.

Two unique hybrid tin iodides, with generic compositions A0.5A'0.5SnI3 and A1.5A'0.5SnI4 have been prepared. Each shows ordering of the two organic moieties (A and A') on distinct crystallographic sites, leading to novel 3D and 2D structure types, respectively.

Perovzalates: a family of perovskite-related oxalates.

A family of hybrid Perovskite-oxalates ("Perovzalates") of general composition AILi3MII(C2O4)3 (A = K+, Rb+, Cs+; M = Fe2+, Co2+, Ni2+) are presented. All eight new compounds are isostructural with the previously reported examples NH4Li3Fe(C2O4)3 and KLi3Fe(C2O4)3, crystallising in the rhombohedral space group R3[combining macron]c, with a∼11.3-11.6 Å, c∼14.8-15.2 Å. In contrast to other families of "hybrid perovskites" such as the formates, these compounds can be regarded as closer structural relatives to inorganic (oxide) perovskites, in the sense that they contain direct linkages of the octahedral sites via bridging oxygen atoms (of the oxalate groups). It is of note, therefore, that monoatomic cations as large as Cs+ can be incorporated into the perovskite-like A sites of this structure type, which is not feasible in traditional ABO3 perovskites; indeed CsLi3Ni(C2O4)3 appears to exhibit the 'mostly tightly bound' 12-coordinate Cs+ ion in an oxide environment, according to a bond valence analysis.

Structure-directing effects in (110)-layered hybrid perovskites containing two distinct organic moieties.

The hybrid perovskites (ImH)(GuH)PbBr4 and (TzH)(GuH)PbBr4 (ImH+ = imidazolium, GuH+ = guanidinium, TzH+ = 1,2,4-triazolium) both adopt (110)-oriented layer structures. However, the GuH+ cation adopts differing crystallographic sites in the two structures (intra-layer versus inter-layer); this is discussed in terms of the sizes of the organic cations and their hydrogen-bonding preferences.

An Electronically Driven Improper Ferroelectric: Tungsten Bronzes as Microstructural Analogs for the Hexagonal Manganites.

Since the observation that the properties of ferroic domain walls (DWs) can differ significantly from the bulk materials in which they are formed, it has been realized that domain wall engineering offers exciting new opportunities for nanoelectronics and nanodevice architectures. Here, a novel improper ferroelectric, CsNbW2 O9 , with the hexagonal tungsten bronze structure, is reported. Powder neutron diffraction and symmetry mode analysis indicate that the improper transition (TC = 1100 K) involves unit cell tripling, reminiscent of the hexagonal rare earth manganites. However, in contrast to the manganites, the symmetry breaking in CsNbW2 O9 is electronically driven (i.e., purely displacive) via the second-order Jahn-Teller effect in contrast to the geometrically driven tilt mechanism of the manganites. Nevertheless CsNbW2 O9 displays the same kinds of domain microstructure as those found in the manganites, such as the characteristic six-domain "cloverleaf" vertices and DW sections with polar discontinuities. The discovery of a completely new material system, with domain patterns already known to generate interesting functionality in the manganites, is important for the emerging field of DW nanoelectronics.

An oxalate cathode for lithium ion batteries with combined cationic and polyanionic redox.

The growing demand for advanced lithium-ion batteries calls for the continued development of high-performance positive electrode materials. Polyoxyanion compounds are receiving considerable interest as alternative cathodes to conventional oxides due to their advantages in cost, safety and environmental friendliness. However, polyanionic cathodes reported so far rely heavily upon transition-metal redox reactions for lithium transfer. Here we show a polyanionic insertion material, Li2Fe(C2O4)2, in which in addition to iron redox activity, the oxalate group itself also shows redox behavior enabling reversible charge/discharge and high capacity without gas evolution. The current study gives oxalate a role as a family of cathode materials and suggests a direction for the identification and design of electrode materials with polyanionic frameworks.