Using pressure to unravel the structure-dynamic-disorder relationship in metal halide perovskites.

The exceptional optoelectronic properties of metal halide perovskites (MHPs) are presumed to arise, at least in part, from the peculiar interplay between the inorganic metal-halide sublattice and the atomic or molecular cations enclosed in the cage voids. The latter can exhibit a roto-translative dynamics, which is shown here to be at the origin of the structural behavior of MHPs as a function of temperature, pressure and composition. The application of high hydrostatic pressure allows for unraveling the nature of the interaction between both sublattices, characterized by the simultaneous action of hydrogen bonding and steric hindrance. In particular, we find that under the conditions of unleashed cation dynamics, the key factor that determines the structural stability of MHPs is the repulsive steric interaction rather than hydrogen bonding. Taking as example the results from pressure and temperature-dependent photoluminescence and Raman experiments on MAPbBr[Formula: see text] but also considering the pertinent MHP literature, we provide a general picture about the relationship between the crystal structure and the presence or absence of cationic dynamic disorder. The reason for the structural sequences observed in MHPs with increasing temperature, pressure, A-site cation size or decreasing halide ionic radius is found principally in the strengthening of the dynamic steric interaction with the increase of the dynamic disorder. In this way, we have deepened our fundamental understanding of MHPs; knowledge that could be coined to improve performance in future optoelectronic devices based on this promising class of semiconductors.

Photoelectrochemical Performance of Strontium Titanium Oxynitride Photo-Activated with Cobalt Phosphate Nanoparticles for Oxidation of Alkaline Water.

Photoelectrochemical (PEC) solar water splitting is favourable for transforming solar energy into sustainable hydrogen fuel using semiconductor electrodes. Perovskite-type oxynitrides are attractive photocatalysts for this application due to their visible light absorption features and stability. Herein, strontium titanium oxynitride (STON) containing anion vacancies of SrTi(O,N)3-δ was prepared via solid phase synthesis and assembled as a photoelectrode by electrophoretic deposition, and their morphological and optical properties and PEC performance for alkaline water oxidation are investigated. Further, cobalt-phosphate (CoPi)-based co-catalyst was photo-deposited over the surface of the STON electrode to boost the PEC efficiency. A photocurrent density of ~138 µA/cm at 1.25 V versus RHE was achieved for CoPi/STON electrodes in presence of a sulfite hole scavenger which is approximately a four-fold enhancement compared to the pristine electrode. The observed PEC enrichment is mainly due to the improved kinetics of oxygen evolution because of the CoPi co-catalyst and the reduced surface recombination of the photogenerated carriers. Moreover, the CoPi modification over perovskite-type oxynitrides provides a new dimension for developing efficient and highly stable photoanodes in solar-assisted water-splitting reactions.

Structure and electrochemical activity of nickel aluminium fluoride nanosheets during urea electro-oxidation in an alkaline solution.

An electrocatalyst of potassium nickel aluminium hexafluoride (KNiAlF6) nanosheets has been prepared using solid-phase synthesis at 900 °C. X-ray diffraction, scanning electron microscopy, and conductivity studies confirmed the formation of KNiAlF6 nanosheets having a cubic defect pyrochlore structure with an average thickness of 60-70 nm and conductivity of 1.297 × 103 S m-1. The electrochemical catalytic activity of the KNiAlF6 nanosheets was investigated for urea oxidation in alkaline solution. The results show that the KNiAlF6 nanosheets exhibit a mass activity of ∼395 mA cm-2 mg-1 at 1.65 V vs. HRE, a reaction activation energy of 4.02 kJ mol-1, Tafel slope of 22 mV dec-1 and an oxidation onset potential of ∼1.35 V vs. HRE which is a significant enhancement for urea oxidation when compared with both bulk Ni(OH)2 and nickel hydroxide-based catalysts published in the literature. Chronoamperometry and impedance analysis of the KNiAlF6 nanosheets reveal lower charge transfer resistance and long-term stability during the prolonged urea electro-oxidation process, particularly at 60 °C. After an extended urea electrolysis process, the structure and morphology of the KNiAlF6 nanosheets were significantly changed due to partial transformation to Ni(OH)2 but the electrochemical activity was sustained. The enhanced electrochemical surface area and the replacement of nickel in the lattice by aluminium make KNiAlF6 nanosheets highly active electrocatalysts for urea oxidation in alkaline solution.

Equal Footing of Thermal Expansion and Electron-Phonon Interaction in the Temperature Dependence of Lead Halide Perovskite Band Gaps.

Lead halide perovskites, which are causing a paradigm shift in photovoltaics, exhibit an atypical temperature dependence of the fundamental gap: it decreases in energy with decreasing temperature. Reports ascribe such a behavior to a strong electron-phonon renormalization of the gap, neglecting contributions from thermal expansion. However, high-pressure experiments performed on the archetypal perovskite MAPbI3 (MA stands for methylammonium) yield a negative pressure coefficient for the gap of the tetragonal room-temperature phase, which speaks against the assumption of negligible thermal expansion effects. Here we show that for MAPbI3 the temperature-induced gap renormalization due to electron-phonon interaction can only account for about 40% of the total energy shift, thus implying thermal expansion to be more if not as important as electron-phonon coupling. Furthermore, this result possesses general validity, holding also for the tetragonal or cubic phase, stable at ambient conditions, of most halide perovskite counterparts.

Synthesis, structure and optoelectronic properties of hybrid iodobismuthate & iodoantimonate semiconducting materials.

The syntheses and structures of five iodobismuthate and nine iodoantimonate hybrid materials are presented. The 1,3-dimethyl-2-oxo-2,3-dihydropyrimidinium cation has been used to template the isostructural, one-dimensional [C6H9N2O][SbI4] (I) and [C6H9N2O][BiI4] (II), while 4-methylmorpholinium templates the compounds, [C5H12NO]4[Sb6I22] (III) and [C5H12NO]4[Bi4I16] (IV), both containing isolated iodopnictogenide cluster anions. Five iodoantimonate compounds, templated by piperazinium cation derivatives, have been synthesised; [C4H12N2][SbI4]2·4H2O (V), [C5H14N2][SbI4]2·3H2O (VI), two polymorphs of [C6H16N2][SbI4]2·2H2O (VII and VIII) and [C6H16N2][Sb4I16]0.5·H2O (IX), mainly adopting structures closely related to previously published 1D iodobismuthate hybrid materials. 1-Ethyl-2-methylbenzimidazolium cations, formed in situ in the reaction medium, template the isostructural 1D structures of [C10H13N2][SbI4] (X) and [C10H13N2][BiI4] (XI). 1,4-Diazabicyclo[2.2.2]octandiium (DABCOH2)2+ di-cations are shown to template a hydrated iodoantimonate structure [C6H14N2]2[Sb4I16]·2H2O (XII) containing [Sb4I16]4- complex anions and a mixed phase of two iodobismuthate materials; one phase contains, uniquely, a mixture of the complex anions [Bi2I10]4- and [BiI6]3- in the form [C6H14N2]10[Bi2I10]2[BiI6]4·(H2O)8 (XIII), and the second, [C6H14N2]2[Bi4I16]·2H2O (XIV), contains discrete [Bi4I16]4- clusters. The stability and thermal decomposition routes of these phases have been determined using thermogravimetric analysis. UV-vis spectroscopy has been used to determine band gap energy estimates which are related, for a range of iodobismuthate and iodoantimonate materials, to their structural features and potential optoelectronic applications.

Zinc Tantalum Oxynitride (ZnTaO2N) Photoanode Modified with Cobalt Phosphate Layers for the Photoelectrochemical Oxidation of Alkali Water.

Photoanodes fabricated by the electrophoretic deposition of a thermally prepared zinc tantalum oxynitride (ZnTaO2N) catalyst onto indium tin oxide (ITO) substrates show photoactivation for the oxygen evolution reaction (OER) in alkaline solutions. The photoactivity of the OER is further boosted by the photodeposition of cobalt phosphate (CoPi) layers onto the surface of the ZnTaO2N photoanodes. Structural, morphological, and photoelectrochemical (PEC) properties of the modified ZnTaO2N photoanodes are studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet visible (UV-Vis) diffuse reflectance spectroscopy, and electrochemical techniques. The presence of the CoPi layer significantly improved the PEC performance of water oxidation in an alkaline sulphate solution. The photocurrent-voltage behavior of the CoPi-modified ZnTaO2N anodes was improved, with the influence being more prominent at lower oxidation potentials. A stable photocurrent density of about 2.3 mA·cm-2 at 1.23 V vs. RHE was attained upon visible light illumination. Relative to the ZnTaO2N photoanodes, an almost three-fold photocurrent increase was achieved at the CoPi/ZnTaO2N photoelectrode. Perovskite-based oxynitrides are modified using an oxygen-evolution co-catalyst of CoPi, and provide a new dimension for enhancing the photoactivity of oxygen evolution in solar-assisted water-splitting reactions.

Synthesis and structure of pseudo-three dimensional hybrid iodobismuthate semiconductors.

The synthesis, structures and semiconducting properties of three isostructural, piperazinium-cation based iodobismuthates, [NH2(CH2)4NH2][BiI4]2·4H2O, [CH3NH(CH2)4NH2][BiI4]2·3H2O and [CH3NH(CH2)4HNCH3][BiI4]2·2H2O, are reported. The materials have pseudo-three dimensional structures consisting of infinite chains formed from edge/face sharing [BiI6] octahedra with short interchain II interactions of <3.8 Å. The materials have band gaps of ∼1.9-2.0 eV and show variable optoelectronic properties based on the degree of methylation of the templating piperazinium ring-based organic species and the accordingly associated level of solvation in the structure.

Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites.

We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations. Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm-1. Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamic effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.

Synthesis and structural characterisation of transition metal fluoride sulfates.

Eleven first row transition metal fluoride sulfates synthesised in hydrofluorothermal conditions have been structurally characterised by single crystal X-ray diffraction and exhibit a wide variety of structural motifs. The polyionic structures containing Ti, V, Mn and Fe vary from discrete polyhedral units in Na4TiF4(SO4)2 and [N2C10H12] TiF4SO4, through one dimensional chains in (K2FeF3SO4, Li3FeF2(SO4)2·H2O, Li1.87Ti1.13O0.39F1.61(SO4)2, [N2C10H12]TiF2(SO4)2, [N2C6H16]Fe(SO4)2F and [N2C6H16]V(SO4)2F), and to two dimensional layers in ([N2C6H16](2+)Mn2F2(SO4)2, Na2VF3SO4 and Na3CrF2(SO4)2).

Experimental and theoretical optical properties of methylammonium lead halide perovskites.

The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3](+) cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS).

Structural diversity in hybrid organic-inorganic lead iodide materials.

The structural chemistry of hybrid organic-inorganic lead iodide materials has become of increasing significance for energy applications since the discovery and development of perovskite solar cells based on methylammonium lead iodide. Seven new hybrid lead iodide compounds have been synthesized and structurally characterized using single-crystal X-ray diffraction. The lead iodide units in materials templated with bipyridyl, 1,2-bis(4-pyridyl)ethane, 1,2-di(4-pyridyl)ethylene and imidazole adopt one-dimensional chain structures, while crystallization from solutions containing piperazinium cations generates a salt containing isolated [PbI6](4-) octahedral anions. Templating with 4-chlorobenzylammonium lead iodide adopts the well known two-dimensional layered perovskite structure with vertex shared sheets of composition [PbI4](2-) separated by double layers of organic cations. The relationships between the various structures determined, their compositions, stability and hydrogen bonding between the protonated amine and the iodide ions of the PbI6 octahedra are described.

Gallium fluoroarsenates.

Six new phases in the gallium-fluoride-arsenate system have been synthesised hydrofluorothermally using a fluoride-rich medium and "HAsF6" (HF : AsF5) as a reactant. RbGaF3(H2AsO4), KGaF(H2AsO4) and [piperazine-H2]2[Ga2F8(HAsO4)]·H2O have one dimensional structures, [DABCO-H2]2[Ga4F7O2H(AsO4)2]·4H2O consists of two dimensionally connected polyhedral layers, while GaF(AsO3[OH,F])2 and (NH4)3Ga4F9(AsO4)2 both have three-dimensionally connected polyhedral frameworks.

Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352 K.

The methylammonium cation in [CH3NH3]PbI3 demonstrates increasing positional disorder on heating from 100 K to 352 K. In the tetragonal phase, stable between 165 K and 327 K, the cation is disordered over four sites directed toward the faces of the distorted cubic [PbI3](-) framework and migrates towards the cavity centre with increasing temperature.

Hydrogeniridate(VI) anion and the geometries of tetrahedral oxo-anions.

The compound KHIrO4, potassium hydrogentetraoxidoiridate(VI)(1-), crystallizes in a Scheelite-type structure containing discrete, slightly flattened, [Ir(O3OH)](-) tetrahedra--the first observation of a group 9 element in the 6+ oxidation state as an oxoanion.

Iron fluorophosphates.

18 new iron fluorophosphates and a chlorofluorophosphate have been synthesised hydrothermally in a fluoride-rich medium, using FeF2, FeF3, Fe, HPF6. HCl, monovalent metal fluorides as reactants and amines as templating agents. Products have been fully structurally characterised using single crystal X-ray diffraction, and the stability of some compounds investigated using thermogravimetric analysis. Reaction in fluoride-rich conditions produce ribbon-like, layer and framework structures containing new and unusual structural motifs based on the linking of Fe(O,F)6, PO3F, and PO2(OH,F)2 polyhedra. Structures exhibiting inter-layer spaces and channels are frequently lined by terminal fluoride anions of the PO3F, PO2(OH,F)2 and Fe(O,F)6 polyhedra.

Direct hydrofluorothermal synthesis of sodium transition metal fluorosulfates as possible Na-ion battery cathode materials.

Synthesis under hydrofluorothermal conditions produces new iron, manganese and vanadium fluorosulfates; these include the isomorphous Na(3)M(III)F(2)(SO(4))(2), M = V, Mn, Fe, with a sheet-like structure formed of linked M(III)F(2)O(4) and SO(4) polyhedra separated by sodium ions which may be extracted electrochemically at potentials of >3.5 V.

Manganese(III) fluorophosphate frameworks.

Nine new manganese(III) fluorophosphates have been synthesised hydrothermally in a fluoride-rich medium, through the use of MnF(3), HPF(6) and monovalent metal fluorides as reactants. Products have been structurally characterised using single crystal X-ray diffraction. Reaction in fluoride-rich conditions produces chain, layer and three dimensional framework structures containing new and unusual structural features based on the linking of PO(3)F, PO(2)(OH,F)(2) and Mn(O(6-n),F(n)) octahedra, with n averaging 2.8 over the family of compounds. The Mn(III), d(4), oxidation state is stabilised under these reaction conditions and products frequently show Jahn-Teller distorted Mn(O,F)(6) units with axially elongated Mn-O or, less commonly, Mn-F distances. Structures exhibiting inter-layer spaces and channels frequently have these lined by terminal fluoride anions of the PO(3)F, PO(2)(OH,F)(2) and Mn(O,F)(6) octahedra.

Complex anion inclusion compounds: flexible anion-exchange materials.

Copper chloropyrophosphate frameworks have been synthesised with a wide variety of complex inorganic anions trapped in a large, flexible, one-dimensional pore, with anions including chloride, bromide, phosphate and the complex metal halo-anions PtCl(4)(2-), PdBr(4)(2-), CuCl(4)(2-) and AuCl(4)(-).

Lightweight nanoporous metal hydroxide-rich zeotypes.

Nanoporous materials have important industrial applications as molecular sieves, catalysts and in gas separation and storage. They are normally produced as moderately dense silicates (SiO(2)) and aluminosilicates making their specific capacities for the uptake and storage of gases, such as hydrogen, relatively low. Here we report the synthesis and characterization of lightweight, nanoporous structures formed from the metal hydroxide Be(OH)(2) in combination with relatively low levels of framework phosphate or arsenate. Three new zeotype structures are described, constructed mainly of Be(OH)(4) tetrahedra bridged through hydroxide into three-membered rings; these units link together to produce several previously unknown zeotype cage types and some of the most structurally complex, nanoporous materials ever discovered. These materials have very low densities between 1.12 and 1.37 g cm(-3) and theoretical porosities of 63-68% of their total volume thereby yielding very high total specific pore volumes of up to 0.60 cm(3) g(-1).

Cobalt(II) fluorophosphate frameworks.

Nine new cobalt fluorophosphates frameworks have been synthesised hydrothermally in a fluoride-rich medium, through the use of CoF(3) and HPF(6) as reactants in combination with Group I/II metal hydroxides and organic cations as templating species. Products have been structurally characterised using single crystal X-ray diffraction. Reaction in fluoride-rich conditions produces chain, layer and three dimensional framework structures containing new and unusual structural features based on the linking of PO(3)F, PO(2)(OH,F)(2) and Co(O,F)(n) polyhedra (n = 5-6) including µ(2), µ(3) and µ(4) bridging fluoride. Structures exhibiting inter-layer spaces and channels frequently have these lined by terminal fluoride anions of the PO(3)F, PO(2)(OH,F)(2) and Co(O,F)(n) polyhedra.