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.

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.

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.

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.

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.

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.

New structure type in the mixed-valent compound YbCu4Ga8.

The new compound YbCu(4)Ga(8) was obtained as large single crystals in high yield from reactions run in liquid gallium. Preliminary investigations suggest that YbCu(4)Ga(8) crystallizes in the CeMn(4)Al(8) structure type, tetragonal space group I4/mmm, and lattice constants are a = b = 8.6529(4) Å and c = 5.3976(11) Å. However, a detailed single-crystal XRD revealed a tripling of the c axis and crystallizing in a new structure type with lattice constants of a = b = 8.6529(4) Å and c = 15.465(1) Å. The structural model was further confirmed by neutron diffraction measurements on high-quality single crystal. The crystal structure of YbCu(4)Ga(8) is composed of pseudo-Frank-Kasper cages occupying one ytterbium atom in each ring which are shared through the corner along the ab plane, resulting in a three-dimensional network. The magnetic susceptibility of YbCu(4)Ga(8) investigated in the temperature range 2-300 K showed Curie-Weiss law behavior above 100 K, and the experimentally measured magnetic moment indicates mixed-valent ytterbium. Electrical resistivity measurements show the metallic nature of the compound. At low temperatures, variation of ρ as a function of T indicates a possible Fermi-liquid state at low temperatures.

MgSO4·11H2O and MgCrO4·11H2O based on time-of-flight neutron single-crystal Laue data.

Hexaaquamagnesium(II) sulfate pentahydrate, [Mg(H2O)6]SO4·5H2O, and hexaaquamagnesium(II) chromate(II) pentahydrate, [Mg(H2O)6][CrO4]·5H2O, are isomorphous, being composed of hexaaquamagnesium(II) octahedra, [Mg(H2O)6](2+), and sulfate (chromate) tetrahedral oxyanions, SO4(2-) (CrO4(2-)), linked by hydrogen bonds. There are two symmetry-inequivalent centrosymmetric octahedra: M1 at (0, 0, 0) donates hydrogen bonds directly to the tetrahedral oxyanion, T1, at (0.405, 0.320, 0.201), whereas the M2 octahedron at (0, 0, ½) is linked to the oxyanion via five interstitial water molecules. Substitution of Cr(VI) for S(VI) leads to a substantial expansion of T1, since the Cr-O bond is approximately 12% longer than the S-O bond. This expansion is propagated through the hydrogen-bonded framework to produce a 3.3% increase in unit-cell volume; the greatest part of this chemically induced strain is manifested along the b* direction. The hydrogen bonds in the chromate compound mitigate ~20% of the expected strain due to the larger oxyanion, becoming shorter (i.e. stronger) and more linear than in the sulfate analogue. The bifurcated hydrogen bond donated by one of the interstitial water molecules is significantly more symmetrical in the chromate analogue.

Boron-boron σ-bond formation by two-electron reduction of a H-bridged dimer of monoborane.

Diborane(6) as a H-bridged dimer of monoborane can be converted cleanly by two-electron reduction into diborane(6) dianion, which is isoelectronic with ethane, through B-B σ-bond formation when each boron atom has a bulky ligand on it. The existence of the B-B σ bond is supported by the X-ray molecular structure [B-B bond length of 1.924(3) Å], NMR studies, magnetic susceptibility measurements, and DFT calculations. Stepwise hydride abstraction reactions of the diborane(6) dianion produce the corresponding H-bridged diborane(5) anion and doubly H-bridged diborane(4) without B-B bond scission.

The structure of water in p-sulfonatocalix[4]arene.

Tetrasodium p-sulfonatocalix[4]arene exists as a hydrate with approximately 14 water molecules and has three polymorphic modifications, all of which contain a water molecule in the molecular cavity that is engaged in OH···π interactions. Single-crystal neutron structures are reported for two of these three forms and reveal a "compressed" water molecule with short OH bonds. Partial atomic charges and hardness analysis (PACHA) calculations based on the neutron coordinates give an OH···π interaction energy of 6.9-7.5 kJ mol(-1). The PACHA analysis also reveals the dominance of the charge-assisted hydrogen bonds from the Na(+)-coordinated water molecules. The instability of the crystal towards dehydration can be traced to an uncoordinated lattice water site. The remarkable calixarene-Na(+)-hydrate motif is conserved almost unchanged across all three polymorphs. A single-crystal neutron structure is also reported for pentasodium p-sulfonatocalix[4]arene·12H(2)O, which exhibits an intracavity water molecule that is engaged in both OH···π and OH···O hydrogen bonding. The shorter covalent bond to the hydrogen atom that forms the interaction with the aromatic ring is again apparent.

Development and loss of ferromagnetism controlled by the interplay of Ge concentration and Mn vacancies in structurally modulated Y4Mn(1-x)Ga(12-y)Ge(y).

The cubic intermetallic phase Y(4)Mn(1-x)Ga(12-y)Ge(y) (x = 0-0.26, y = 0-4.0) has been isolated from a molten gallium flux reaction. It presents a rare example of a system where ferromagnetism can be induced by controlling the vacancies of the magnetic centers. The Y(4)PdGa(12) type crystal structure is made up of a corner-sharing octahedral network of Ga and Ge atoms with Mn atoms at the centers of half the octahedra and Y atoms in the voids. At the highest Ge concentration, y = 4.0, the Mn site is nearly fully occupied, x = 0.05, and the samples are paramagnetic. At a lower Ge concentration, y = 1.0, Mn deficiency develops with x = 0.10. Surprisingly, strong ferromagnetism is observed with T(c) = 223 K. When Ge is excluded, y = 0, Mn is substantially deficient at x = 0.26 and ferromagnetism is maintained with a T(c) of approximately 160 K. In addition, a 6-fold modulated superstructure appears owing to an ordered slab-like segregation of Mn atoms and vacancies. Corresponding bond distortions propagate throughout the octahedral Ga network. Structure-property relationships are examined with X-ray and neutron diffraction, magnetic susceptibility, and electrical resistivity measurements.

Insights into metal borohydride and aluminohydride bonding: X-ray and neutron diffraction structures and a DFT and charge density study of [Na(15-crown-5)][EH(4)] (E = B, Al).

The neutron and X-ray structures of [Na(15-crown-5)][BH(4)] and [Na(15-crown-5)][AlH(4)], respectively, are reported, along with a topological analysis of their DFT-computed charge densities that explores the bonding between the anionic complex hydride [EH(4)](-) (E = B, Al) and the counterion [Na(15-crown-5)](+). In each case, the interaction is weak and mainly electrostatic in nature; however, notable differences are observed in the manner in which [BH(4)](-) and [AlH(4)](-) bind to the metal, which explains their different coordination modes. A range of unconventional E-H···H-C contacts is revealed to play an important role in the overall bonding and crystal packing of both complexes. These interactions can be classified as weak dihydrogen bonds based on the atoms in molecules approach.

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.

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.

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.

Experimental and theoretical charge density study of polymorphic isonicotinamide-oxalic acid molecular complexes with strong O...H...N hydrogen bonds.

Two polymorphs of the 2:1 molecular complex of isonicotinamide and oxalic acid have been characterized by combined X-ray charge density and single-crystal neutron diffraction studies at 100 K. Both polymorphs show strong O-H...N intermolecular hydrogen bonding between the acid and the pyridine base. As is typical of short, strong hydrogen bonds (SSHBs), the covalent O-H bonds are considerably elongated to 1.161(3) and 1.235(5) A, and the H...N interactions are correspondingly short at 1.398(3) and 1.313(6) A in Forms I and II, respectively. The neutron diffraction data indicate no pronounced H dynamics in the SSHBs, and in the case of Form II the SSHB can be described as quasicentered. In addition to the experimental charge densities, theoretical charge densities have been determined from ab initio calculations within the full periodic environment of the crystalline state. The SSHBs are found to be covalent in nature according to the topological analysis of the experimental and theoretical charge densities and application of the source function. Aside from the SSHBs, moderate N-H...O and weak C-H...O interactions are also present in the molecular complexes, for which hydrogen bond energies are estimated from energy densities and independent ab initio calculations. Finally, an attempt is made to evaluate the intermolecular interactions governing the manifestation of polymorphism in this compound.

Neutron diffraction studies on a 4-coordinate hydrogen atom in an yttrium cluster.

A four-coordinate hydrogen atom has been unambiguously located, by single-crystal neutron diffraction for the first time, in the center of the tetrahedral metal complex Y4H8(Cp')4(THF) [Cp'=C5Me4(SiMe3)]. The core of the molecule consists of a tetranuclear cluster with one interstitial, one face-bridging, and six edge-bridging hydride ligands. The compound was prepared via the reaction of YCp'(CH2SiMe3)2(THF) with gaseous H2. Neutron data were collected on a 4 mm3 crystal at the Quasi-Laue diffractometer VIVALDI at ILL (Grenoble)1a and on an 8 mm3 crystal at the SXD diffractometer at ISIS (Didcot). The final agreement factor is R = 8.9% for 4171 reflections. The existence of 4-coordinate hydrogen now completes the series of high-connectivity hydride ligands located in the interstitial cavities of molecular cluster complexes. We had previously reported the existence of 6-coordinate hydrogen in the octahedral cavity of [HCo6(CO)15]- in 1979, and 5-coordinate hydrogen in the square pyramidal cavities of [H2Rh13(CO)24]3- in 1997, also via single-crystal neutron analyses.

Temperature-dependent single-crystal neutron diffraction study of the strong OHN hydrogen bond in pyridinium 2,4-dinitrobenzoate.

The structure of pyridinium 2,4-dinitrobenzoate was studied by neutron diffraction at 300, 270, 240, 210, 180, 150, 120, 90, 60, and 30 K. With temperature change, the O...H bond length changes from 1.403(10) A at 300 K to 1.424(4) A at 30 K. The proton shifts in the hydrogen bridge toward the acceptor nitrogen atom. Temperature-dependent changes in the strong OHN hydrogen bond were analyzed by using both the neutron structure and the atom-in-molecule approach. The results are compared with those for other strong OHN hydrogen bonds.

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.