Orthorhombic charge density wave on the tetragonal lattice of EuAl.

EuAl possesses the BaAl crystal structure type with tetragonal symmetry 4/. It undergoes a charge density wave (CDW) transition at  = 145 K and features four consecutive antiferromagnetic phase transitions below 16 K. Here we use single-crystal X-ray diffraction to determine the incommensurately modulated crystal structure of EuAl in its CDW state.

Incommensurate Phase in Λ-cobalt (III) Sepulchrate Trinitrate Governed by Highly Competitive N-H⋅⋅⋅O and C-H⋅⋅⋅O Hydrogen Bond Networks.

Phase transitions in molecular crystals are often determined by intermolecular interactions. The cage complex of [Co(C H N )]  ⋅ 3 NO is reported to undergo a disorder-order phase transition at T ≈133 K upon cooling. Temperature-dependent neutron and synchrotron diffraction experiments revealed satellite reflections in addition to main reflections in the diffraction patterns below T .

Single-crystal-to-single-crystal phase transitions of commensurately modulated sodium saccharinate 1.875-hydrate.

This work reports reversible, single-crystal-to-single-crystal phase transitions of commensurately modulated sodium saccharinate 1.875-hydrate [Na(sac)(15/8)HO]. The phases were studied in the temperature range 298 to 20 K.

Boranes: The Boron Subhydride BH with a Distorted β-Boron Crystal Structure.

A single crystal of the boron subhydride BH was serendipitously obtained while attempting to synthesize β-boron. An accurate crystal structure analysis revealed a distorted β-boron framework with the noncentrosymmetric space group 3. We have found one interstitial site occupied by boron.

Charge density of 4-methyl-3-[(tetrahydro-2H-pyran-2-yl)oxy]thiazole-2(3H)-thione. A comprehensive multipole refinement, maximum entropy method and density functional theory study.

The structure of 4-methyl-3-[(tetrahydro-2H-pyran-2-yl)oxy]thiazole-2(3H)-thione (MTTOTHP) was investigated using X-ray diffraction and computational chemistry methods for determining properties of the nitrogen-oxygen bond, which is the least stable entity upon photochemical excitation. Experimentally measured structure factors have been used to determine and characterize charge density via the multipole model (MM) and the maximum entropy method (MEM). Theoretical investigation of the electron density and the electronic structure has been performed in the finite basis set density functional theory (DFT) framework.

On the puzzling case of sodium saccharinate 1.875-hydrate: structure description in (3+1)-dimensional superspace.

The structure of sodium saccharinate 1.875-hydrate is presented in three- and (3+1)-dimensional space. The present model is more accurate than previously published superstructures, due to an excellent data set collected up to a high resolution of 0.

Pressure dependence of spin canting in ammonium metal formate antiferromagnets.

High-pressure single-crystal X-ray diffraction at ambient temperature and high-pressure SQUID measurements down to 2 K were performed up to ∼2.5 GPa on ammonium metal formates, [NH][M(HCOO)] where M = Mn, Fe, and Ni, in order to correlate structural variations to magnetic behaviour. Similar structural distortions and phase transitions were observed for all compounds, although the transition pressures varied with the size of the metal cation.

The role of PbI in CHNHPbI perovskite stability, solar cell parameters and device degradation.

We report a systematic investigation on the role of excess PbI content in CHNHPbI perovskite film properties, solar cell parameters and device storage stability. We used the CHNHI vapor assisted method for the preparation of PbI-free CHNHPbI films under a N atmosphere. These pristine CHNHPbI films were annealed at 165 °C for different time intervals in a N atmosphere to generate additional PbI in these films.

The electrostatic potential of dynamic charge densities.

A procedure to derive the electrostatic potential (ESP) for dynamic charge densities obtained from structure models or maximum-entropy densities is introduced. The ESP essentially is obtained by inverse Fourier transform of the dynamic structure factors of the total charge density corresponding to the independent atom model, the multipole model or maximum-entropy densities, employing dedicated software that will be part of the software package. Our approach is also discussed with respect to the Ewald summation method.

Incommensurately modulated structure of morpholinium tetrafluoroborate and configurational versus chemical entropies at the incommensurate and lock-in phase transitions.

Morpholinium tetrafluoroborate, [CHNO][BF], belongs to a class of ferroelectric compounds ABX. However, [CHNO][BF] does not develop ferroelectric properties because the incommensurate phase below T = 153 K is centrosymmetric with superspace group Pnam(σ00)00s and σ = 0.42193 (12) at T = 130 K; the threefold superstructure below T = 117-118 K possesses the acentric but non-ferroelectric space group P222.

Unusual ground states in [Formula: see text] (R = rare earth; T = Rh, Ir; and X = Si, Ge, Sn): a review.

Rare earth compounds of the type [Formula: see text] (R  =  rare earth; T  =  Rh, Ir, and X  =  Si, Ge, Sn) display a variety of phase transitions towards exotic states, including charge density waves (CDW), local moment magnetism, antiferromagnetism in the heavy fermion state, superconductivity and giant positive magnetoresistance. They support strongly correlated electron systems. In particular, R Ir [Formula: see text] (R  =  Dy-Lu) exhibit strong coupling CDWs with high transition temperatures, and superconductivity or magnetic ordering at lower temperatures.

High-Pressure Phase Transformations in TiPO : A Route to Pentacoordinated Phosphorus.

Titanium(III) phosphate, TiPO  , is a typical example of an oxyphosphorus compound containing covalent P-O bonds. Single-crystal X-ray diffraction studies of TiPO reveal complex and unexpected structural and chemical behavior as a function of pressure at room temperature. A series of phase transitions lead to the high-pressure phase V, which is stable above 46 GPa and features an unusual oxygen coordination of the phosphorus atoms.

Time-dependent growth of crystalline Au(0)-nanoparticles in cyanobacteria as self-reproducing bioreactors: 2. Anabaena cylindrica.

Microbial biosynthesis of metal nanoparticles as needed in catalysis has shown its theoretical ability as an extremely environmentally friendly production method in the last few years, even though the separation of the nanoparticles is challenging. Biosynthesis, summing up biosorption and bioreduction of diluted metal ions to zero valent metals, is especially ecofriendly, when the bioreactor itself is harmless and needs no further harmful reagents. The cyanobacterium Anabaena cylindrica (SAG 1403.

The Z' = 12 superstructure of Λ-cobalt(III) sepulchrate trinitrate governed by C-H...O hydrogen bonds.

Λ-Cobalt(III) sepulchrate trinitrate crystallizes in P6322 with Z = 2 (Z' = 1/6) at room temperature. Slabs perpendicular to the hexagonal axis comprise molecules Co(sepulchrate) alternating with nitrate groups A and B. Coordinated by six sepulchrate molecules, highly disordered nitrate groups C are accommodated between the slabs.

Charge-ordering transition in iron oxide Fe4O5 involving competing dimer and trimer formation.

Phase transitions that occur in materials, driven, for instance, by changes in temperature or pressure, can dramatically change the materials' properties. Discovering new types of transitions and understanding their mechanisms is important not only from a fundamental perspective, but also for practical applications. Here we investigate a recently discovered Fe4O5 that adopts an orthorhombic CaFe3O5-type crystal structure that features linear chains of Fe ions.

Disorder and defects are not intrinsic to boron carbide.

A unique combination of useful properties in boron-carbide, such as extreme hardness, excellent fracture toughness, a low density, a high melting point, thermoelectricity, semi-conducting behavior, catalytic activity and a remarkably good chemical stability, makes it an ideal material for a wide range of technological applications. Explaining these properties in terms of chemical bonding has remained a major challenge in boron chemistry. Here we report the synthesis of fully ordered, stoichiometric boron-carbide B13C2 by high-pressure-high-temperature techniques.

Pressure-induced normal-incommensurate and incommensurate-commensurate phase transitions in CrOCl.

The high-pressure behavior of layered CrOCl is shown to be governed by non-bonded interactions between chlorine atoms in relation to a rigid framework composed of Cr and O atoms. The competition between optimizing intra- and interlayer Cl-Cl distances and the general trend towards denser packing defines a novel mechanism for high-pressure phase transitions of inorganic materials. CrOCl possesses an incommensurate phase for 16-51 GPa.

Resonance-stabilized partial proton transfer in hydrogen bonds of incommensurate phenazine-chloranilic acid.

The co-crystal of phenazine (Phz) and chloranilic acid (H2ca) becomes ferroelectric upon cooling through the loss of inversion symmetry. Further cooling results in the development of an incommensurate ferroelectric phase, followed by a lock-in transition towards a twofold superstructure. Here we present the incommensurately modulated crystal structure of Phz-H2ca at T = 139 K with a symmetry given by the superspace group P2(1)(½ σ(2) ½)0 and σ(2) = 0.

N-H···O and C-H···F hydrogen bonds in the incommensurately modulated crystal structure of adamantan-1-ammonium 4-fluorobenzoate.

At low temperatures the organic salt adamantan-1-ammonium 4-fluorobenzoate, C10H18N(+)·C7H4FO2(-), possesses an incommensurately modulated crystal structure. The effect of the modulation on the atomic arrangement and intermolecular interactions is studied by analysing single-crystal X-ray diffraction data within the (3 + 1)-dimensional superspace approach and superspace group P2(1)/n(α0γ)00. The modulation strongly affects the position of the atoms as well as their atomic displacement parameters.

The active site of hen egg-white lysozyme: flexibility and chemical bonding.

Chemical bonding at the active site of hen egg-white lysozyme (HEWL) is analyzed on the basis of Bader's quantum theory of atoms in molecules [QTAIM; Bader (1994), Atoms in Molecules: A Quantum Theory. Oxford University Press] applied to electron-density maps derived from a multipole model. The observation is made that the atomic displacement parameters (ADPs) of HEWL at a temperature of 100 K are larger than ADPs in crystals of small biological molecules at 298 K.

Charge density distribution of 3-(1-aminoethylidene)-2-methoxy-2-oxo-2,3-dihydro-2λ(5)-benzo[e][1,2]oxaphosphinin-4-one.

A combined experimental and theoretical study of one oxaphosphinane derivative was made on the basis of a topological analysis of its electron density distributions. The electron density was determined from a high-resolution X-ray diffraction data set measured with synchrotron radiation at 100 K, whereas theoretical calculations were performed using density functional theory (DFT) methods at the B3LYP\6-311++G(3df,3pd) level of approximation. The charge-density distribution and analysis of topological properties revealed that the P-O bond is of the transit closed-shell type.

Topological Properties of Chemical Bonds from Static and Dynamic Electron Densities.

Dynamic and static electron densities (EDs) based on the independent spherical atom model (IAM) and multipole (MP) models of crambin were successfully computed, holding no series-termination effects. The densities are compared to EDs of small biological molecules at diverse temperatures. It is outlined that proteins exhibit an intrinsic flexibility, present as frozen disorder at 100 K, in contrast to small molecules.

Electron densities by the maximum entropy method (MEM) for various types of prior densities: a case study on three amino acids and a tripeptide.

Dynamic model densities according to Mondal et al. [(2012), Acta Cryst. A68, 568-581] are presented for independent atom models (IAM), IAMs after high-order refinements (IAM-HO), invariom (INV) models and multipole (MP) models of α-glycine, DL-serine, L-alanine and Ala-Tyr-Ala at T ≃ 20 K.

Anisotropic thermal expansion of La(n)(Ti,Fe)(n)O(3n + 2) (n = 5 and 6).

Crystal structures are reported for two perovskite-related compounds with nominal compositions La5(Ti(0.8)Fe(0.2))5O17 and La6(Ti(0.