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 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.

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.

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.

Materialization of a Geometrically Frustrated Magnet in a Hybrid Coordination Framework: A Study of the Iron(II) Oxalate Fluoride Framework, KFe(C2O4)F.

Here we discuss magnetic hybrid coordination frameworks in relation to the realization of new geometrically frustrated magnets. In particular, we present the nuclear and magnetic structures of one such system-the Fe2+-based oxalate fluoride framework KFe(C2O4)F-through analysis of the powder neutron diffraction and muon spectroscopy data. KFe(C2O4)F retains an orthorhombic Cmc21 structure upon cooling to 2 K composed of quasi-one-dimensional iron fluoride chains connected to a distorted triangular network via oxalate anions. Previous magnetometry measurements of KFe(C2O4)F indicate that it is a strongly interacting system with a Weiss constant θ ≈ -300 K that undergoes a magnetic ordering transition at TN ≈ 20 K, yielding a frustration index, f = |θ|/TN ≈ 15, reflective of high-spin frustration. We determine the nature of this frustrated antiferromagnetic ordering below TN and show that the resulting magnetic structure is best described by a model in the Cmc'21' magnetic space group.

NaFe3(HPO3)2((H,F)PO2OH)6: A Potential Cathode Material and a Novel Ferrimagnet.

A novel iron fluorophosphite, NaFe3(HPO3)2((H,F)PO2OH)6, was synthesized by a dry low-temperature synthesis route. The phase was shown to be electrochemically active for reversible insertion of Na(+) ions, with an average discharge voltage of 2.5 V and an experimental capacity at low rates of up to 90 mAhg(-1). Simple synthesis, low-cost materials, excellent capacity retention, and efficiency suggest this class of material is competitive with similar oxyanion-based compounds as a cathode material for Na batteries. The characterization of physical properties by means of magnetization, specific heat, and electron spin resonance measurements confirms the presence of two magnetically nonequivalent Fe(3+) sites. The compound orders magnetically at TC ≈ 9.4 K into a state with spontaneous magnetization.

Extending the Family of V(4+) S=(1/2) Kagome Antiferromagnets.

The ionothermal synthesis, structure, and magnetic susceptibility of a novel inorganic-organic hybrid material, imidazolium vanadium(III,IV) oxyfluoride [C3 H5 N2 ][V9 O6 F24 (H2 O)2 ] (ImVOF) are presented. The structure consists of inorganic vanadium oxyfluoride slabs with kagome layers of V(4+) S=${{ 1/2 }}$ ions separated by a mixed valence layer. These inorganic slabs are intercalated with imidazolium cations. Quinuclidinium (Q) and pyrazinium (Pyz) cations can also be incorporated into the hybrid structure type to give QVOF and PyzVOF analogues, respectively. The highly frustrated topology of the inorganic slabs, along with the quantum nature of the magnetism associated with V(4+) , means that these materials are excellent candidates to host exotic magnetic ground states, such as the highly sought quantum spin liquid. Magnetic susceptibility measurements of all samples suggest an absence of conventional long-range magnetic order down to 2 K despite considerable antiferromagnetic exchange.

Unusual phase behavior in the piezoelectric perovskite system, Li(x)Na(1-x)NbO3.

The system Li(x)Na(1-x)NbO3 has been studied by using a combination of X-ray and neutron powder diffraction and (23)Na solid-state NMR spectroscopy. For x = 0.05 we confirm a single polar orthorhombic phase. For 0.08 ≤ x ≤ 0.20 phase mixtures of this orthorhombic phase, together with a rhombohedral phase, isostructural with the low-temperature ferroelectric polymorph of NaNbO3, are observed. The relative fractions of these two phases are shown to be critically dependent on synthetic conditions: the rhombohedral phase is favored by higher annealing temperatures and rapid cooling. We also observe that the orthorhombic phase transforms slowly to the rhombohedral phase on standing in air at ambient temperature. For 0.25 ≤ x ≤ 0.90 two rhombohedral phases coexist, one Na-rich and the other Li-rich. In this region the phase behavior is independent of reaction conditions.

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.

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.

New twists on the perovskite theme: crystal structures of the elusive phases R and S of NaNbO3.

The crystal structure of NaNbO(3) has been studied in detail in the temperature regime 360 < T < 520 °C using a combination of high-resolution neutron and synchrotron X-ray powder diffraction, supported by first-principles calculations. A systematic symmetry-mode analysis is used to determine the presence of the key active distortion modes that, in turn, provides a small and an unambiguous set of trial structural models. A unique model for Phase S (480 < T < 510 °C) is elucidated, having a 2 × 2 × 4 superlattice of the aristotype perovskite structure, space group Pmmn. This unusual and unique structure features a novel example of a compound octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 < T < 470 °C) are determined, each having a 2 × 2 × 6 superlattice and differing only in the nature of the complex tilt system along the 'long' axis. It is impossible to identify a definitive model from the present study, although reasons for preferring one over the other are discussed. Some of the possible pitfalls in determining such complex, pseudosymmetric crystal structures from powder diffraction data are also highlighted.

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.

93Nb NMR and DFT investigation of the polymorphs of NaNbO3.

Sodium niobate (NaNbO(3)) has a particularly complex phase diagram, with a series of phase transitions as a function of temperature and pressure, and even at room temperature a number of different structural variations have been suggested. Recent work has demonstrated that bulk powders of NaNbO(3), prepared using a variety of synthetic approaches, contain a mixture of perovskite phases; the commonly reported Pbcm phase and a second, polar phase tentatively identified as belonging to space group P2(1)ma. The two phases exhibit very similar (23)Na MAS NMR spectra, although high-resolution MQMAS spectra were able to distinguish between them. Here, we investigate whether different perovskite polymorphs can be distinguished and/or identified using a variety of (93)Nb NMR methods, including MAS, MQMAS and wideline experiments. We compare the experimental results obtained for these more common perovskite materials to those for the metastable ilmenite polymorph of NaNbO(3). Our experimental results are supported by first-principles calculations of NMR parameters using a planewave pseudopotential approach. The calculated NMR parameters appear very different for each of the phases investigated, but high forces on the atoms indicate many of the structural models derived from diffraction require optimisation of the atomic coordinates. After geometry optimisation, most of these perovskite phases exhibit very similar NMR parameters, in contrast to recent work where it was suggested that (93)Nb provides a useful tool for distinguishing NaNbO(3) polymorphs. Finally, we consider the origin of the quadrupolar coupling in these materials, and its dependence on the deviation from ideality of the NbO(6) octahedra.

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.

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.

The polar phase of NaNbO(3): a combined study by powder diffraction, solid-state NMR, and first-principles calculations.

A polar phase of NaNbO(3) has been successfully synthesized using sol-gel techniques. Detailed characterization of this phase has been undertaken using high-resolution powder diffraction (X-ray and neutron) and (23)Na multiple-quantum (MQ) MAS NMR, supported by second harmonic generation measurements and density functional theory calculations. Samples of NaNbO(3) were also synthesized using conventional solid-state methods and were observed to routinely comprise of a mixture of two different polymorphs of NaNbO(3), namely, the well-known orthorhombic phase (space group Pbcm) and the current polar phase, the relative quantities of which vary considerably depending upon precise reaction conditions. Our studies show that each of these two polymorphs of NaNbO(3) contains two crystallographically distinct Na sites. This is consistent with assignment of the polar phase to the orthorhombic space group P2(1)ma, although peak broadenings in the diffraction data suggest a subtle monoclinic distortion. Using carefully monitored molten salt techniques, it was possible to eradicate the polar polymorph and synthesize the pure Pbcm phase.

A new chain structure: catena-poly[4,4'-(ethane-1,2-diyl)dipyridinium bis[[aquadifluoridooxidovanadate]-mu-fluorido]].

The title compound, {(C(12)H(12)N(2))[V(2)F(6)O(2)(H(2)O)(2)]}(n), features a novel extended-chain moiety, [VOF(2)F(2/2)(H(2)O)](n), comprising trans vertex-connected VOF(4)(H(2)O) octahedra. The octahedra themselves show the characteristic distortion due to the off-centering of the V(4+) ion, such that a short terminal V=O bond and an elongated trans V-OH(2) bond are present. Hydrogen bonding from the water molecules to terminal F atoms in adjacent chains generates associated chain dimers, which are loosely linked into sheets via additional hydrogen bonding involving the organic moieties. Structural relationships with previously described vanadium oxyfluoride species are briefly discussed.