Low-temperature Crystal Structure Research Articles

1,3,5-Tris(4-bromophenyl)benzene prenucleation clusters from metadynamics

Recent experimental evidence has shown that the nucleation of 1,3,5-tris(4-bromophenyl)benzene (3BrY) follows a two-step mechanism. In this work, the formation of clusters of 3BrY from homogeneous water and methanol solutions is simulated using metadynamics. The local structure of 3BrY molecules in the clusters is then compared with the low-temperature crystal structure of 3BrY, as well as with an alternative 3BrY high-pressure crystal packing obtained through Parrinello-Rahaman molecular dynamics simulations. It is found that the interactions between the aromatic cores of 3BrY represent the main supramolecular motif observed in both the local structure of the prenucleation clusters and in the crystalline state. Within the clusters, these interactions lead to the presence of dimers and trimers locally arranged in crystal-like configurations.

Series of thermally stable Li1+2x Mg4−x V3O12 ceramics: low temperature sintering characteristic, crystal structure and microwave dielectric properties

Series of novel low-firing microwave dielectric ceramics with the composition of Li1+2x Mg4−x V3O12 (−0.2 ≤ x ≤ 0.1) were prepared by a solid-state reaction process. The crystal structure, sintering behavior and microwave dielectric properties of ceramics were investigated using X-ray diffraction, scanning electron microscopy and dielectric measurement. As x increased from −0.2 to 0.1, the optimized sintering temperature was reduced from 880 to 700 °C, the quality factors (Q × f) were effectively enhanced from 21,000 to 33,200 GHz. A good combined microwave dielectric properties with er = 12.1, Q × f = 21,800 GHz, τ f = 1.8 ppm/°C were obtained for x = −0.1.

Low temperature crystal structure and magnetic properties of RAl2

Low temperature crystal structure and magnetic properties of RAl2 (R = Pr and Nd) have been studied using temperature dependent powder x-ray diffraction, magnetization, and heat capacity measurements. Unlike PrAl2, NdAl2 retains cubic MgCu2-type structure from room temperature down to 5 K, which is also confirmed from first principles electronic structure calculations. The magnetization measurements show both PrAl2 and NdAl2 order ferromagnetically at TC = 32 K and 77 K, respectively. However, the magnetization measurements show the former is a hard ferromagnet compared to the latter which is a soft ferromagnetic material. The magnetic entropy change obtained from heat capacity measurements at ΔH = 30 kOe for PrAl2 and NdAl2 are 3.15 J mol−1 K−1 and 1.18 J mol−1 K−1, respectively.

Hydrogen-Bonding Motifs in MS2N2H Metallacycles: A Crystallographic and Computational Study of [CpCoS2N2H][BF4

The first preparation and full characterization of highly insoluble [{η5–Cp}CoS2N2H][BF4] is reported. Proof of structure and composition is established from a single-crystal X-ray diffraction experiment at 173 K, which determined a crystal structure containing two independent cations and two independent anions in the asymmetric unit. The site of protonation is the terminal N of the S–N–S–N2– ligand which is coordinated to cobalt in the metallacycle. The crystal lattice contains numerous interatomic interactions, of which the most important are formed from multiple hydrogen bonds that link four [{η5–Cp}CoS2N2H]+ ions and two [BF4]− ions into a tetrameric cluster. The second kind of [BF4]− is not involved in intermolecular bonding. Short S···S′ interactions link such tetramers into planes; the resulting double-layer planes are described by the (101) Miller planes. DFT calculations using the B3PW91/6-311+G(3df)(2p) and B3LYP/6-311+G(3df)(2p) methods are in excellent agreement with the structure of the title cation from crystallography. Comparison to newly calculated and literature-reported DFT calculations on the neutral starting material [{η5–Cp}CoS2N2] and to a recent low-temperature crystal structure of the latter shows that there are only minor changes (≤3%) in the bond lengths within the CoS2N2 ring upon protonation, whereas the bond angles change by up to 9%. The B3PW91 functional is shown to be superior to B3LYP for computing the molecular structures of these formally CoIII metallacycles.

Synthesis, Crystal Structures, Magnetic, and Thermal Properties of Divalent Metal Formate–Formamide Layered Compounds.

A series of layered divalent metal formate compounds, [M(HCOO)2(HCONH2)2] (M = Mn (1Mn), Ni (2Ni), Cu(3Cu), Zn(4Zn), Mg(5Mg)), have been prepared by solvothermal synthesis and their room temperature (RT) and low-temperature (LT) crystal structures, and thermal and magnetic properties determined. All the compounds contain octahedral metal ions connected by four anti-anti formato ligands to form (4,4) nets with the composition of M(HCOO)2. The oxygen atoms from two coordinating formamide ligands above and below the layer complete the MO6 distorted octahedral coordination. Order-disorder phase transformations involving the formamide ligands were observed in the 1Mn, 2Ni, and 4Zn compounds. Like transitions in related formate structures with perovskite like topology, the transitions correspond to the ordering of the amine groups of the terminating formamide ligands which are disordered at ambient temperature. The magnetic properties of the three magnetic members of the series 1Mn, 2Ni, and 3Cu were investigated using microcrystalline samples, over the temperature range of 2 K-300 K under different applied fields. All compounds belong to antiferromagnetic square lattices with S = 5/2, 1, and 1/2. Exchange constants for a nearest neighbor model are presented here. Specific heat measurements indicate magnetic long-range order at lower temperatures, S = 5/2 (antiferromagnetic) and S = 1 (ferrimagnetic).

Nature of the magnetic ground state in the mixed valence compound CeRuSn: a single-crystal study

We report on detailed low-temperature measurements of the magnetization, the specific heat and the electrical resistivity on high-quality CeRuSn single crystals. The compound orders antiferromagnetically at TN = 2.8 K with the Ce3+ ions locked within the a–c plane of the monoclinic structure. Magnetization shows that below TN CeRuSn undergoes a metamagnetic transition when applying a magnetic field of 1.5 and 0.8 T along the a- and c-axis, respectively. This transition manifests in a tremendous negative jump of ∼25% in the magnetoresistance. The value of the saturated magnetization along the easy magnetization direction (c-axis) and the magnetic entropy above TN derived from specific heat data correspond to the scenario of only one third of the Ce ions in the compound being trivalent and carrying a stable Ce3+ magnetic moment, whereas the other two thirds of the Ce ions are in a nonmagnetic tetravalent and/or mixed valence state. This is consistent with the low-temperature CeRuSn crystal structure i.e., a superstructure consisting of three unit cells of the CeCoAl type piled up along the c-axis, and in which the Ce3+ ions are characterized by large distances from the Ru ligands while the Ce–Ru distances of the other Ce ions are much shorter causing a strong 4f-ligand hybridization and hence leading to tetravalent and/or mixed valence Ce ions.

Low temperature crystal structure and large lattice discontinuity at Tc in superconducting FeTeOx films

We report a high resolution x-ray diffraction study of the crystal structure of superconducting FeTeOx films. The crystal symmetry of FeTeOx matches the parent FeTe, unlike most iron-based superconductors. However, at the superconducting transition there is a large change in the c-axis lattice parameter. Such a discontinuity in the thermal expansion is known in normal-to-superconducting phase transitions, but here the effect is far larger than for other iron-based superconductors. Following the typical analysis of such a discontinuity in thermal expansion using the Ehrenfest-relation leads to a prediction of a large enhancement of Tc in strained FeTeOx films.

Synchrotron X-ray powder diffraction study on synthetic Sr-Fresnoite

The Sr analogue of the mineral fresnoite (Sr2TiSi2O8) is of interest as a potential storage medium for radioactive Sr from nuclear waste. No high or low temperature crystal structure information is known on this phase. Therefore high-resolution synchrotron X-ray powder diffraction measurements have been done on a synthetic sample of Sr-fresnoite in the temperature range 87-1223K. This was done as a test experiment using the HRPD beamline P02.1 at PETRA-III, DESY. Synchrotron X-ray wavelengths of 0.2067(3)Å (293K and 573-1223K) and 0.2079(3)Å (87-499K) were used. Powder diffraction data were collected with a counting time of 30s using a PerkinElmer XRD 1621 flat panel image plate detector. CeO2 was included as an internal standard to calibrate the sample to detector distance. The P4bm tetragonal crystal structure of fresnoite (Ba2TiSi2O8) was used as a starting model for Sr-fresnoite. Small amounts of SrTiO3 and SrSiO3 were also found as impurities in this sample; therefore four-phase Rietveld refinements were done. The P4bm fresnoite structure is retained over the temperature range 87-1223K.

Pressure effect on the magnetic order of LuFe2O4

The effects of applied external pressure on the low temperature crystal and magnetic structures of LuFe2O4 have been investigated by means of synchrotron X-ray and neutron diffraction experiments in the range 0–3 GPa. Although the crystal structure is nearly rhombohedral at RT and ambient pressure, a marked monoclinic distortion is seen at low temperature, which increases with increasing pressure. In the magnetically ordered regime, the ordered magnetic moment value at 50 K decreases by up to 30% at 3 GPa, although no evidence for magnetic diffuse scattering can be inferred from the data. A tentative explanation is proposed, based on the delocalization of the magnetic moment, and possibly related with a melting of the charge ordered state in this pressure range.

Low Temperature Crystal Structure Behaviour of Complex Yttrium Aluminium Oxides YAlO_3 and Y_3Al_5O_{12}

Crystal structures of two yttrium aluminium oxides, namely YAlO3 and Y3Al5O12, were investigated in the temperature range 3.4 300 K by high-resolution neutron powder di raction. Neither traces of phase transformations nor discontinuous changes of physical properties were observed. Thermal expansion of yttrium aluminium oxides was evaluated in terms of 1st order Gruneisen approximation, where the Debye temperatures and the Gruneisen parameters have been estimated for both compositions. Anomalies in the thermal expansion of yttrium aluminium perovskite have been observed and modelled using the Einstein oscillator with negative Gruneisen parameter. Extended bond length analysis revealed signi cant thermally-driven modi cations of the aluminium oxygen framework.

The Role of the Distal Histidine in H2O2 Activation and Heme Protection in both Peroxidase and Globin Functions

The distal histidine mutations of dehaloperoxidase-hemoglobin A (DHP A) to aspartate (H55D) and asparagine (H55N) have been prepared to study the role played by the distal histidine in both activation and protection against oxidation by radicals in heme proteins. The H55D and H55N mutants of DHP A have ~6-fold and ~11-fold lower peroxidase activities than wild type enzyme toward the oxidation of 2,4,6-trichlorophenol (TCP) to yield 2,6-dichloroquinone (DCQ) in the presence of H(2)O(2). The origin of the lower rate constants may be the solvent-exposed conformations of distal D55 and N55, which would have the dual effect of destabilizing the binding of H(2)O(2) to the heme iron, and of removing the acid-base catalyst necessary for the heterolytic O-O bond cleavage of heme-bound H(2)O(2) (i.e., compound 0). The partial peroxidase activity of H55D can be explained if one considers that there are two conformations of the distal aspartate (open and closed) by analogy with the distal histidine. We hypothesize that the distal aspartate has an active conformation in the distal pocket (closed). Although the open form is observed in the low-temperature X-ray crystal structure of ferric H55D, the closed form is observed in the FTIR spectrum of the carbonmonoxy form of the H55D mutant. Consistent with this model, the H55D mutant also shows inhibition of TCP oxidation by 4-bromophenol (4-BP). Consistent with the protection hypothesis, compound ES, the tyrosyl radical-containing ferryl intermediate observed in WT DHP A, was not observed in H55D.

Spin-singlet dimerization in La2RuO5investigated using magnetic susceptibility and specific heat measurements

The origin of spin-dimerization and concomitant spin-gap opening in the triclinic phase of poly- and single-crystalline La${}_{2}$RuO${}_{5}$ at unusually high temperatures was investigated using magnetic susceptibility and specific-heat measurements. From the low-temperature crystal structure the formation of antiferromagnetically coupled Ru${}^{4+}$ ($S=1$) dimers within the quasi-two-dimensional magnetic system can be deduced, resulting in a nonmagnetic singlet state. It was found that the antiferromagnetic coupling within the dimers is much stronger than the interaction with neighboring dimers. La${}_{2}$RuO${}_{5}$ exhibits a step-like change in the magnetic susceptibility at 161 K, indicating a first-order transition of combined magnetic and structural character. The size of the spin-gap has been estimated from the thermally activated behavior in the low-temperature dimerized phase and was found to be significantly different in the polycrystalline sample when compared to the results obtained from the single crystals. The magnetic entropy obtained from specific-heat measurements amounts to roughly $0.5R\mathrm{ln}(3)$, reflecting solely the contribution of spin degrees of freedom to the entropy change during the phase transition.

Two types of adjacent dimer layers in the low-temperature phase of BaCuSi2O6

The low-temperature crystal structure of ${\mathrm{BaCuSi}}_{2}{\mathrm{O}}_{6}$ has been investigated with high-resolution synchrotron x-ray and neutron powder diffraction techniques and has been found to be on average (ignoring the incommensurate modulation) orthorhombic, with the most probable space group $Ibam$. The Cu-Cu dimers in this material are forming two types of layers with distinctly different interatomic distances. Subtle changes also modify the partially frustrated interlayer Cu-Cu exchange paths. The present results corroborate the interpretation of low-temperature nuclear magnetic resonance and inelastic neutron scattering data in terms of distinct dimer layers.

Study of low-temperature ordering and crystal structure in FePtBi/Au nanocomposite films

Based on interfacial microstructure control of FePt/Au multilayer structure and surfactant Bi atoms, an FePtBi/Au nanocomposite film was designed to achieve good properties [low-temperature ordering, good perpendicular magnetic anisotropy (PMA), and no interparticle exchange coupling (IEC)] in this paper. The ordering process was promoted due to the increase of concentration of defects by adding appropriate surfactant Bi atoms. Thus, the ordering temperature of this film can be decreased from 500 to 450 °C. Meantime, the FePtBi/Au nanocomposite film maintains good PMA and low IEC with the texture manipulation of Au layers and control of total film thickness.

Evolution of two-step structural phase transition in Fe1+dTe detected by low-temperature x-ray diffraction

The low-temperature crystal structure of Fe1.13Te, which exhibits an anomalous two-step magnetic transition, was clarified by the systematic x-ray diffraction measurements. It was found that two-step structural phase transition, tetragonal–orthorhombic–monoclinic, occurred correspondingly to the two-step magnetic transition. The detailed analysis of the profile at 5K indicated the coexistence of the minor orthorhombic area inside the major, monoclinic lattice, which could explain the lower-shift (suppression) of the antiferromagnetic transition temperature in Fe1.13Te and suggest a possibility of superconductivity at the domain boundary.

Structural distortion, charge modulation and local anisotropies in magnetite below the Verwey transition using resonant X-ray scattering

The pattern of charge modulations and local anisotropies below the Verwey transition has been determined and quantified in high-quality Fe(3)O(4) single crystals and thin films grown on MgO by using resonant X-ray scattering at the Fe K-edge. The energy, polarization and azimuthal angle dependencies of an extensive set of reflections with potential sensitivity to charge or local anisotropy orderings have been analyzed to explore their origins. A charge disproportion on octahedral B sites of 0.20 ± 0.05 e(-) with [0 0 1] and [1 1 0] cubic periodicities has been confirmed, while no significant charge disproportion has been obtained with [0 0 1/2] cubic periodicity. Additional charge modulations in the monoclinic a-b plane are also present. In addition, the occurrence of new forbidden (1, 1, 0) and (0, 0, 2n + 1/2) cubic reflections that arise from the anisotropy of the local structure around different tetrahedral and octahedral Fe atoms is shown. This complex pattern of weak charge modulations and local anisotropies is fully compatible with the low-temperature crystal structure refined in the non-polar C2/c space group and disproves any bimodal charge disproportion of the octahedral Fe atoms.

An expedient synthesis and conformational features of acylhydrazone macrocycles derived from tartaric acid: evidence of water and π aromatic hydrogen bond interactions

Tartaric acid dihydrazides with acetal protected hydroxy groups react with terephthalaldehyde to preferentially form [2+2] bisacylhydrazone macrocycles. The structures of these macrocycles display all anti N–N and CN bonds. Both trans- and cis-C(O)–NH bonds are present in the macrocycle thus allowing the formation of a rather compact macrocyclic structure. The structures of the acylhydrazone macrocycle are shown to be different in the crystal and in the isolated molecule due to the structure determining role of water included in the crystal lattice: in the former all of the OC–C–H bonds are anti, while in the latter both syn- and anti-bonds are present. Both the non-planarity of the bisacylhydrazone molecules and their chiral interchromophoric interactions contribute to the rotatory power of the molecule. The low-temperature X-ray crystal structure of this compound provides direct evidence for hydrogen bonding between water and the aromatic π-electrons in the solid state.

Covalent assistance to supramolecular synthesis: modifying the drug functionality of the antituberculosis API isoniazidin situ during co-crystallization with GRAS and API compounds

The anti-tuberculosis molecule isonicotinic acid hydrazide (isoniazid) is a promising molecule in the supramolecular synthesis of multi-component molecular complexes and also allows for its biological activity to be improved and modified by a simple covalent reaction. The low temperature crystal structures of both isoniazid (1) and the related N′-(propan-2-ylidene)isonicotinohydrazide molecule (2), the latter known to be a more effective agent against Mycobacterium tuberculosis, are reported. In addition, these two molecules were then co-crystallized with the same three Generally Regarded As Safe (GRAS) molecules, succinic acid, 4-hydroxybenzoic acid and 2-hydroxybenzoic acid, to produce the following pharmaceutical co-crystals: (isonicotinic acid hydrazide)2·(succinic acid) 3, (N′-(propan-2-ylidene)isonicotinohydrazide)2·(succinic acid) 4, (isonicotinic acid hydrazide)·(4-hydroxybenzoic acid) 6, (N′-(propan-2-ylidene)isonicotinohydrazide)·(4-hydroxybenzoic acid) 7, (isonicotinic acid hydrazide)·(2-hydroxybenzoic acid) 8, and (N′-(propan-2-ylidene)isonicotinohydrazide)·(2-hydroxybenzoic acid) 9. In addition, a co-crystal using 2-butanone as a modifier is also reported, (N′-(butan-2-ylidene)isonicotinohydrazide)2·(succinic acid) 5. Drug–drug co-crystals were also made with the anti-HIV compound 2-chloro-4-nitrobenzoic acid: (isonicotinic acid hydrazide)·(2-chloro-4-nitrobenzoic acid) 10, and (N′-(propan-2-ylidene)isonicotinohydrazide)·(2-chloro-4-nitrobenzoic acid) 11. All the co-crystals use the carboxylic acid⋯pyridine hydrogen bond to connect the GRAS or drug molecule to the pyridine ring. In general, the co-crystals with the modified isoniazid feature the C(4) homosynthon, and those with the original isoniazid a variety of homo- and heterosynthons. By comparing the melting points of the co-crystals that use isoniazid and those that use the modified isoniazid, a reduction in melting point is observed.

Using a Dual Plasma Process to Produce Cobalt-Polypyrrole Catalysts for the Oxygen Reduction Reaction in Fuel Cells: II. Analysing the Chemical Structure of the Films

The chemical structure of cobalt–polypyrrole – produced by a dual plasma process – is analyzed by means of X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption spectroscopy (NEXAFS), X-ray diffraction (XRD), energy-dispersive X-Ray spectroscopy (EDX) and extended X-ray absorption spectroscopy (EXAFS). It is shown that only nanoparticles of a size of 3 nm with the low temperature crystal structure of cobalt are present within the compound. Besides that, cobalt–nitrogen and carbon–oxygen structures are observed. Furthermore, more and more cobalt–nitrogen structures are produced when increasing the magnetron power. Linking the information on the chemical structure to the results about the catalytic activity of the films – which are presented in part I of this contribution – it is concluded that the cobalt–nitrogen structures are the probable catalytically active sites. The cobalt–nitrogen bond length is calculated as 2.09 Å and the carbon–nitrogen bond length as 1.38 Å.

Pressure dependence of the low-temperature crystal structure and phase transition behavior of CaFeAsF and SrFeAsF: A synchrotron x-ray diffraction study

We report systematic investigation of high pressure crystal structures and structural phase transition upto 46 GPa in CaFeAsF and 40 GPa in SrFeAsF at 40 K using powder synchrotron x-ray diffraction experiments and Rietveld analysis of the diffraction data. We find that CaFeAsF undergoes orthorhombic to monoclinic phase transition at Pc = 13.7 GPa while increasing pressure. SrFeAsF exhibits coexistence of orthorhombic and monoclinic phases over a large pressure range from 9 to 39 GPa. The coexistence of the two phases indicates that the transition is of first order in nature. Unlike in the 122 compounds (BaFe2As2 & CaFe2As2) we do not find any collapse tetragonal transition. The transition to a lower symmetry phase (orthorhombic to monoclinic) in 1111 compounds under pressure is in contrast with the transition to a high symmetry phase (orthorhombic to tetragonal) in 122 type compounds. On heating from 40 K at high pressure, CaFeAsF undergoes monoclinic to tetragonal phase transition around 25 GPa and 200 K. Further, it does not show any post-tetragonal phase transition and remains in the tetragonal phase upto 25 GPa at 300 K. The dPc/dT is found to be positive for the CaFeAsF & CaFe2As2, however the same was not found in case of BaFe2As2. We discuss observations of structural evolution in the context of superconductivity in these and other Fe-based compounds. It appears that the closeness of the Fe-As-Fe bond angle to its ideal tetrahedral value of 109.470 might be associated with occurrence of superconductivity at low temperature.