Understanding the Degradation of Methylenediammonium and Its Role in Phase-Stabilizing Formamidinium Lead Triiodide.

Formamidinium lead triiodide (FAPbI3) is the leading candidate for single-junction metal-halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl2) has been used as an additive in FAPbI3. MDA2+ has been reported as incorporated into the perovskite lattice alongside Cl-. However, the precise function and role of MDA2+ remain uncertain. Here, we grow FAPbI3 single crystals from a solution containing MDACl2 (FAPbI3-M). We demonstrate that FAPbI3-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA2+ is not the direct cause of the enhanced material stability. Instead, MDA2+ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI3 crystals grown from a solution containing HMTA (FAPbI3-H) replicate the enhanced α-phase stability of FAPbI3-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA+ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H+). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H+ is selectively incorporated into the bulk of both FAPbI3-M and FAPbI3-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.

A quantum crystallographic approach to short hydrogen bonds.

In this work we use high-resolution synchrotron X-ray diffraction for electron density mapping, in conjunction with ab initio modelling, to study short O-H⋯O and O+-H⋯O- hydrogen bonds whose behaviour is known to be tuneable by temperature. The short hydrogen bonds have donor-acceptor distances in the region of 2.45 Šand are formed in substituted urea and organic acid molecular complexes of N,N'-dimethylurea oxalic acid 2 : 1 (1), N,N-dimethylurea 2,4-dinitrobenzoate 1 : 1 (2) and N,N-dimethylurea 3,5-dinitrobenzoic acid 2 : 2 (3). From the combined analyses, these complexes are found to fall within the salt-cocrystal continuum and exhibit short hydrogen bonds that can be characterised as both strong and electrostatic (1, 3) or very strong with a significant covalent contribution (2). An additional charge assisted component is found to be important in distinguishing the relatively uncommon O-H⋯O pseudo-covalent interaction from a typical strong hydrogen bond. The electron density is found to be sensitive to the extent of static proton transfer, presenting it as a useful parameter in the study of the salt-cocrystal continuum. From complementary calculated hydrogen atom potentials, we attribute changes in proton position to the molecular environment. Calculated potentials also show zero barrier to proton migration, forming an 'energy slide' between the donor and acceptor atoms. The better fundamental understanding of the short hydrogen bond in the 'zone of fluctuation' presented in a salt-cocrystal continuum, enabled by studies like this, provide greater insight into their related properties and can have implications in the regulation of pharmaceutical materials.

Publisher Correction: Establishing magneto-structural relationships in the solid solutions of the skyrmion hosting family of materials: GaV4S8-ySey.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Structure and vibrational spectroscopy of lithium and potassium methanesulfonates.

In this work, we have determined the structures of lithium methanesulfonate, Li(CH3SO3), and potassium methanesulfonate, K(CH3SO3), and analysed their vibrational spectra. The lithium salt crystallizes in the monoclinic space group C2/m with two formula units in the primitive cell. The potassium salt is more complex, crystallizing in I4/m with 12 formula units in the primitive cell. The lithium ion is fourfold coordinated in a distorted tetrahedron, while the potassium salt exhibits three types of coordination: six-, seven- and ninefold. Vibrational spectroscopy of the compounds (including the 6Li and 7Li isotopomers) confirms that the correlation previously found, that in the infrared spectra there is a clear distinction between coordinated and not coordinated forms of the methanesulfonate ion, is also valid here. The lithium salt shows a clear splitting of the asymmetric S-O stretch mode, indicating a bonding interaction, while there is no splitting in the spectrum of the potassium salt, consistent with a purely ionic material.

Understanding the Correlation between Oxide Ion Mobility and Site Distributions in Ba3NbWO8.5.

A correlation between oxygen site distributions and ionic conductivity has been established in the recently discovered family of oxide-ion conductors Ba3M2O8.5±Î´ (M = Nb, V, Mo, W). We rationalize this observation on the basis of structural insights gained from the first single-crystal neutron diffraction data collected for a member of this family, Ba3NbWO8.5, and theoretical considerations of bonding and O site energies.

Establishing magneto-structural relationships in the solid solutions of the skyrmion hosting family of materials: GaV4S8-ySey.

The GaV4S8-ySey (y = 0 to 8) family of materials have been synthesized in both polycrystalline and single crystal form, and their structural and magnetic properties thoroughly investigated. Each of these materials crystallizes in the F[Formula: see text][Formula: see text]3m space group at ambient temperature. However, in contrast to the end members GaV4S8 and GaV4Se8, that undergo a structural transition to the R3m space group at 42 and 41 K respectively, the solid solutions (y = 1 to 7) retain cubic symmetry down to 1.5 K. In zero applied field the end members of the family order ferromagnetically at 13 K (GaV4S8) and 18 K (GaV4Se8), while the intermediate compounds exhibit a spin-glass-like ground state. We demonstrate that the magnetic structure of GaV4S8 shows localization of spins on the V cations, indicating that a charge ordering mechanism drives the structural phase transition. We conclude that the observation of both structural and ferromagnetic transitions in the end members of the series in zero field is a prerequisite for the stabilization of a skyrmion phase, and discuss how the absence of these transitions in the y = 1 to 7 materials can be explained by their structural properties.

Mechanism of enhancement of ferroelectricity of croconic acid with temperature.

A detailed study of the thermal behaviour of atomic motions in the organic ferroelectric croconic acid is presented in the temperature range 5-300 K. Using high-resolution inelastic neutron scattering and first-principles electronic-structure calculations within the framework of density functional theory and a quasiharmonic phonon description of the material, we find that the frequencies of the well defined doublet in inelastic neutron scattering spectra associated with out-of-plane motions of hydrogen-bonded protons decrease monotonically with temperature indicating weakening of these bonding motifs and enhancement of proton motions. Theoretical mean-square displacements for these proton motions are within 5% of experimental values. A detailed analysis of this observable shows that it is unlikely that there is a facile proton transfer along the direction of ferroelectric polarization in the absence of an applied electric field. Calculations predict constrained thermal motion of proton along crystallographic lattice direction c retaining the hydrogen bond motif of the crystal at high temperature. Using the Berry-phase method, we have also calculated the spontaneous polarization of temperature dependent cell structures, and find that our computational model provides a satisfactory description of the anomalous and so far unexplained rise in bulk electric polarization with temperature. Correlating the thermal motion induced lattice strain with temperature dependent spontaneous polarizations, we conclude that increasing thermal strain with temperatures combined with constrained thermal motion along the hydrogen bond motif are responsible of this increase in ferroelectricity at high temperature.

Multi-temperature study of potassium uridine-5'-monophosphate: electron density distribution and anharmonic motion modelling.

Uridine, a nucleoside formed of a uracil fragment attached to a ribose ring via a ß-N1-glycosidic bond, is one of the four basic components of ribonucleic acid. Here a new anhydrous structure and experimental charge density distribution analysis of a uridine-5'-monophosphate potassium salt, K(UMPH), is reported. The studied case constitutes the very first structure of a 5'-nucleotide potassium salt according to the Cambridge Structural Database. The excellent crystal quality allowed the collection of charge density data at various temperatures, i.e. 10, 100, 200 and 300 K on one single crystal. Crystal structure and charge density data were analysed thoroughly in the context of related literature-reported examples. Detailed analysis of the charge density distribution revealed elevated anharmonic motion of part of the uracil ring moiety relatively weakly interacting with the neighbouring species. The effect was manifested by alternate positive and negative residual density patterns observed for these atoms, which `disappear' at low temperature. It also occurred that the potassium cation, quite uniformly coordinated by seven O atoms from all molecular fragments of the UMPH- anion, including the O atom from the ribofuranose ring, can be treated as spherical in the charge density model which was supported by theoretical calculations. Apart from the predominant electrostatic interactions, four relatively strong hydrogen bond types further support the stability of the crystal structure. This results in a compact and quite uniform structure (in all directions) of the studied crystal, as opposed to similar cases with layered architecture reported in the literature.

Nuclear dynamics and phase polymorphism in solid formic acid.

We apply a unique sequence of structural and dynamical neutron-scattering techniques, augmented with density-functional electronic-structure calculations, to establish the degree of polymorphism in an archetypal hydrogen-bonded system - crystalline formic acid. Using this combination of experimental and theoretical techniques, the hypothesis by Zelsmann on the coexistence of the ß1 and ß2 phases above 220 K is tested. Contrary to the postulated scenario of proton-transfer-driven phase coexistence, the emerging picture is one of a quantitatively different structural change over this temperature range, whereby the loosening of crystal packing promotes temperature-induced shearing of the hydrogen-bonded chains. The presented work, therefore, solves a fifty-year-old puzzle and provides a suitable framework for the use neutron-Compton-scattering techniques in the exploration of phase polymorphism in condensed matter.

Glycine zinc sulfate penta-hydrate: redetermination at 10†K from time-of-flight neutron Laue diffraction.

Single crystals of glycine zinc sulfate penta-hydrate [systematic name: hexa-aqua-zinc tetra-aquadiglycinezinc bis-(sulfate)], [Zn(H2O)6][Zn(C2H5NO2)2(H2O)4](SO4)2, have been grown by isothermal evaporation from aqueous solution at room temperature and characterized by single-crystal neutron diffraction. The unit cell contains two unique ZnO6 octa-hedra on sites of symmetry -1 and two SO4 tetra-hedra with site symmetry 1; the octa-hedra comprise one [tetra-aqua-diglycine zinc]2+ ion (centred on one Zn atom) and one [hexa-aqua-zinc]2+ ion (centred on the other Zn atom); the glycine zwitterion, NH3+CH2COO-, adopts a monodentate coordination to the first Zn atom. All other atoms sit on general positions of site symmetry 1. Glycine forms centrosymmetric closed cyclic dimers due to N-H⋯O hydrogen bonds between the amine and carboxyl-ate groups of adjacent zwitterions and exhibits torsion angles varying from ideal planarity by no more than 1.2°, the smallest values for any known glycine zwitterion not otherwise constrained by a mirror plane. This work confirms the H-atom locations estimated in three earlier single-crystal X-ray diffraction studies with the addition of independently refined fractional coordinates and Uij parameters, which provide accurate inter-nuclear X-H (X = N, O) bond lengths and consequently a more accurate and precise depiction of the hydrogen-bond framework.

Selective Synthesis of Cyclooctanoids by Radical Cyclization of Seven-Membered Lactones: Neutron Diffraction Study of the Stereoselective Deuteration of a Chiral Organosamarium Intermediate.

Seven-membered lactones undergo selective SmI2 -H2 O-promoted radical cyclization to form substituted cyclooctanols. The products arise from an exo-mode of cyclization rather than the usual endo-attack employed in the few radical syntheses of cyclooctanes. The process is terminated by the quenching of a chiral benzylic samarium. A labeling experiment and neutron diffraction study have been used for the first time to probe the configuration and highly diastereoselective deuteration of a chiral organosamarium intermediate.

Electron density, disorder and polymorphism: high-resolution diffraction studies of the highly polymorphic neuralgic drug carbamazepine.

Analysis of neutron and high-resolution X-ray diffraction data on form (III) of carbamazepine at 100 K using the atoms in molecules (AIM) topological approach afforded excellent agreement between the experimental results and theoretical densities from the optimized gas-phase structure and from multipole modelling of static theoretical structure factors. The charge density analysis provides experimental confirmation of the partially localized π-bonding suggested by the conventional structural formula, but the evidence for any significant C-N π bonding is not strong. Hirshfeld atom refinement (HAR) gives H atom positional and anisotropic displacement parameters that agree very well with the neutron parameters. X-ray and neutron diffraction data on the dihydrate of carbemazepine strongly indicate a disordered orthorhombic crystal structure in the space group Cmca, rather than a monoclinic crystal structure in space group P2(1)/c. This disorder in the dihydrate structure has implications for both experimental and theoretical studies of polymorphism.

Computation of diffuse scattering arising from one-phonon excitations in a neutron time-of-flight single-crystal Laue diffraction experiment.

Direct phonon excitation in a neutron time-of-flight single-crystal Laue diffraction experiment has been observed in a single crystal of NaCl. At room temperature both phonon emission and excitation leave characteristic features in the diffuse scattering and these are well reproduced using abinitio phonons from density functional theory (DFT). A measurement at 20 K illustrates the effect of thermal population of the phonons, leaving the features corresponding to phonon excitation and strongly suppressing the phonon annihilation. A recipe is given to compute these effects combining DFT results with the geometry of the neutron experiment.

Structural studies of P-type ATPase-ligand complexes using an X-ray free-electron laser.

Membrane proteins are key players in biological systems, mediating signalling events and the specific transport of e.g. ions and metabolites. Consequently, membrane proteins are targeted by a large number of currently approved drugs. Understanding their functions and molecular mechanisms is greatly dependent on structural information, not least on complexes with functionally or medically important ligands. Structure determination, however, is hampered by the difficulty of obtaining well diffracting, macroscopic crystals. Here, the feasibility of X-ray free-electron-laser-based serial femtosecond crystallography (SFX) for the structure determination of membrane protein-ligand complexes using microcrystals of various native-source and recombinant P-type ATPase complexes is demonstrated. The data reveal the binding sites of a variety of ligands, including lipids and inhibitors such as the hallmark P-type ATPase inhibitor orthovanadate. By analyzing the resolution dependence of ligand densities and overall model qualities, SFX data quality metrics as well as suitable refinement procedures are discussed. Even at relatively low resolution and multiplicity, the identification of ligands can be demonstrated. This makes SFX a useful tool for ligand screening and thus for unravelling the molecular mechanisms of biologically active proteins.

Structure of magnesium selenate enneahydrate, MgSeO4·9H2O, from 5 to 250†K using neutron time-of-flight Laue diffraction.

The complete structure of MgSeO4·9H2O has been refined from neutron single-crystal diffraction data obtained at 5, 100, 175 and 250 K. It is monoclinic, space group P21/c, Z = 4, with unit-cell parameters a = 7.222 (2), b = 10.484 (3), c = 17.327 (4) Å, ß = 109.57 (2)°, and V = 1236.1 (6) Å(3) [ρ(calc) = 1770 (1) kg m(-3)] at 5 K. The structure consists of isolated [Mg(H2O)6](2+) octahedra, [SeO4](2-) tetrahedra and three interstitial lattice water molecules, all on sites of symmetry 1. The positions of the H atoms agree well with those inferred on the basis of geometrical considerations in the prior X-ray powder diffraction structure determination: no evidence of orientational disorder of the water molecules is apparent in the temperature range studied. Six of the nine water molecules are hydrogen bonded to one another to form a unique centrosymmetric dodecamer, (H2O)12. Raman spectra have been acquired in the range 170-4000 cm(-1) at 259 and 78 K; ab initio calculations, using density functional theory, have been carried out in order to aid in the analysis of the Raman spectrum as well as providing additional insights into the geometry and thermodynamics of the hydrogen bonds. Complementary information concerning the thermal expansion, crystal morphology and the solubility are also presented.

Structure, hydrogen bonding and thermal expansion of ammonium carbonate monohydrate.

We have determined the crystal structure of ammonium carbonate monohydrate, (NH4)2CO3·H2O, using Laue single-crystal diffraction methods with pulsed neutron radiation. The crystal is orthorhombic, space group Pnma (Z = 4), with unit-cell dimensions a = 12.047 (3), b = 4.453 (1), c = 11.023 (3) Šand V = 591.3 (3) Å(3) [ρcalc = 1281.8 (7) kg m(-3)] at 10 K. The single-crystal data collected at 10 and 100 K are complemented by X-ray powder diffraction data measured from 245 to 273 K, Raman spectra measured from 80 to 263 K and an athermal zero-pressure calculation of the electronic structure and phonon spectrum carried out using density functional theory (DFT). We find no evidence of a phase transition between 10 and 273 K; above 273 K, however, the title compound transforms first to ammonium sesquicarbonate monohydrate and subsequently to ammonium bicarbonate. The crystallographic and spectroscopic data and the calculations reveal a quite strongly hydrogen-bonded structure (EHB ≃ 30-40 kJ mol(-1)), on the basis of H...O bond lengths and the topology of the electron density at the bond critical points, in which there is no free rotation of the ammonium cation at any temperature. The barrier to free rotation of the ammonium ions is estimated from the observed librational frequency to be ∼ 36 kJ mol(-1). The c-axis exhibits negative thermal expansion, but the thermal expansion behaviour of the a and b axes is ormal.

Crystal structure of magnesium selenate hepta-hydrate, MgSeO4·7H2O, from neutron time-of-flight data.

MgSeO4·7H2O is isostructural with the analogous sulfate, MgSO4·7H2O, consisting of isolated [Mg(H2O)6](2+) octa-hedra and [SeO4](2-) tetra-hedra, linked by O-H⋯O hydrogen bonds, with a single inter-stitial lattice water mol-ecule. As in the sulfate, the [Mg(H2O)6](2+) coordination octa-hedron is elongated along one axis due to the tetra-hedral coordination of the two apical water mol-ecules; these have Mg-O distances of ∼2.10 Å, whereas the remaining four trigonally coordinated water mol-ecules have Mg-O distances of ∼2.05 Å. The mean Se-O bond length is 1.641 Šand is in excellent agreement with other selenates. The unit-cell volume of MgSeO4·7H2O at 10 K is 4.1% larger than that of the sulfate at 2 K, although this is not uniform; the greater part of the expansion is along the a axis of the crystal.

The crystal structure and magnetic properties of a Zintl phase EuIrIn4: the first member of the Eu-Ir-In family.

The indium-rich intermetallic compound EuIrIn4 was synthesized using indium as the active metal flux. The crystal structure of EuIrIn4 was investigated by X-ray powder and single crystal diffraction. EuIrIn4 crystallizes in the YNiAl4 type, Cmcm space group, with lattice parameters, a = 4.5206(9) Å, b = 16.937(3) Å, c = 7.2661(15) Å. Europium atoms in EuIrIn4 are surrounded by three dimensional [IrIn4] polyanionic networks. EuIrIn4 shows two successive antiferromagnetic transitions at 5.4 and 10.8 K. Modified Curie-Weiss fitting on the magnetic susceptibility data within the temperature region 15-300 K gives the effective magnetic moment of 8.45µB/Eu. EuIrIn4 shows pronounced magnetic anisotropy perpendicular to the direction of the applied magnetic field.

Modulating reactivity in iridium bis(N-heterocyclic carbene) complexes: influence of ring size on E-H bond activation chemistry.

The changes in the steric and electronic properties of N-heterocyclic carbenes (NHCs) as a function of ring size have a profound effect on the reactivity of their late transition metal complexes. Comparison of closely related complexes featuring either a saturated 5- or 6-membered NHC, reveals that the larger ring is associated with an increased propensity towards intramolecular C-H activation, but with markedly lower reactivity towards external substrates. Thus, systems of the type [IrL2(H)2](+) give rise to contrasting chemical behaviour, primarily reflecting the differing possibilities for secondary stabilization of the metal centre by the N-bound aryl substituents: highly labile [Ir(5-Mes)2(H)2](+) can only be studied by trapping experiments, while [Ir(6-Mes)2(H)2](+) is air and moisture stable, and unreactive towards many external reagents. With appropriate substrates, this heightened reactivity can be exploited, and in situ generated [Ir(5-Mes)2(H)2](+) is capable of intermolecular B-H and N-H activation chemistry. In the case of H3B·NMe2H, this affords a rare opportunity to study amine/aminoborane coordination via single crystal neutron diffraction methods.

An improved method for calibrating time-of-flight Laue single-crystal neutron diffractometers.

A robust and comprehensive method for determining the orientation matrix of a single-crystal sample using the neutron Laue time-of-flight (TOF) technique is described. The new method enables the measurement of the unit-cell parameters with an uncertainty in the range 0.015-0.06%, depending upon the crystal symmetry and the number of reflections measured. The improved technique also facilitates the location and integration of weak reflections, which are often more difficult to discern amongst the increased background at higher energies. The technique uses a mathematical model of the relative positions of all the detector pixels of the instrument, together with a methodology that establishes a reproducible reference frame and a method for determining the parameters of the instrument detector model. Since all neutron TOF instruments require precise detector calibration for their effective use, it is possible that the method described here may be of use on other instruments where the detector calibration cannot be determined by other means.