Acta crystallographica Section B, Structural science, crystal engineering and materials最新文献

The magnetic structure of chromium arsenide CrAs is studied with neutron powder diffraction at ambient pressure in the temperature range 1.5-300 K as well as with neutron single-crystal diffraction at 2 K and 0.12 GPa. The material undergoes an anti-isostructural phase transition at T_N = 267 K and atmospheric conditions, in which both orthorhombic phases have the same space-group symmetry (Pnma, Z = 4) but different distortions of the parent hexagonal structure of the NiAs type (P6₃/mmc, Z = 2). The magnetic structure below T_N is incommensurate with the propagation vector k = (0, 0, k_c). At ambient pressure, the component k_c decreases from k_c = 0.3807 (7) at 260 K to k_c = 0.3531 (6) at 50 K. Below this temperature, it is basically constant. With increasing pressure at 2 K, k_c is also constant within standard uncertainties [k_c = 0.353 (2)]. For the analysis of the magnetic structure, a group-theoretical approach based on the space group of the nuclear structure and its subgroups is used. To avoid falling into false minima in the refinements, a random search for magnetic moments in the models is implemented. In the literature, the magnetic structure has been determined on the basis of powder diffraction data as a double helix propagating along the c axis. Although this double-helical model leads to satisfactory agreement factors for our powder data, it does not reproduce the intensities of the magnetic satellite reflections measured on single-crystal data in a satisfactory way and can therefore be discarded. Instead, several other models are found that lead to better agreement. Each of them is spiral-like with directional components in all three directions and with no spin-density wave character that would cause a non-constant magnetic moment. In all these models, the ordering of the spins is neither a pure helix nor a pure cycloid. Instead, the unit vectors of the spin rotation planes make an angle α, 0° < α < 90°, with respect to the c* direction. The model in superspace group P2₁.1'(α0γ)0s yields the best agreement factors in the refinements of the neutron single-crystal and powder diffraction data. This model is unique as it is the only one in which all the magnetic moments rotate with the same chirality.

Tetrakis-4-(4-pyridyl)phenylmethane (TPPM) is a tetrahedral rigid molecule that crystallizes forming a dynamically responsive supramolecular organic framework (SOF). When exposed to different stimuli, this supramolecular network can reversibly switch from an empty to a filled solvated solid phase. This article describes a novel expanded form of a TPPM-based SOF that has been mechanochemically synthesized and whose crystal structure has been determined by 3D electron diffraction analysis using a novel electron diffractometer.

Density functional theory methods are applied to crystal structures and stabilities of phases from the aleksite homologous series, Pb_nBi₄Te₄S_n+2 (n = homologue number). The seven phases investigated correspond to n = 0 (tetradymite), 2 (aleksite-21R and -42R), 4 (saddlebackite-9H and -18H), 6 (unnamed Pb₆Bi₄Te₄S₈), 8 (unnamed Pb₈Bi₄Te₄S₁₀), 10 (hitachiite) and 12 (unnamed Pb₁₂Bi₄Te₄S₁₄). These seven phases correspond to nine single- or double-module structures, each comprising an odd number of atom layers, 5, 7, (5.9), 9, (7.11), 11, 13, 15 and 17, expressed by the formula: S(M_pX_p+1)·L(M_p+1X_p+2), where M = Pb, Bi and X = Te, S, p ≥ 2, and S and L = number of short and long modules, respectively. Relaxed structures show a and c values within 1.5% of experimental data; a and the interlayer distance d_sub decrease with increasing PbS content. Variable Pb-S bond lengths contrast with constant Pb-S bond lengths in galena. All phases are n-fold superstructures of a rhombohedral subcell with c/3 = d_sub*. Electron diffraction patterns show two brightest reflections at the centre of d_sub*, described by the modulation vector q_F = (i/N) · d_sub*, i = S + L. A second modulation vector, q = γ · c_sub*, shows a decrease in γ, from 1.8 to 1.588, across the n = 0 to n = 12 interval. The linear relationship between γ and d_sub allows the prediction of any theoretical phases beyond the studied compositional range. The upper PbS-rich limit of the series is postulated as n = 398 (Pb₃₉₈Bi₄Te₄S₄₀₀), a phase with d_sub (1.726 Å) identical to that of trigonal PbS within experimental error. The aleksite series is a prime example of mixed layer compounds built with accretional homology principles.

A drug-drug cocrystal created with two antithrombotic-active ingredients from herbs, honokiol (HON) and ligustrazine (TMP, 1:1), was synthesized and characterized. The structure of HON-TMP (1:1) was determined by single-crystal X-ray diffraction. Then co-amorphous HON-TMP was prepared by honey-assisted grinding, which was inspired by a grinding process for a Chinese patent medicine-Shijunzi honey pill. This co-amorphous drug-drug cocrystal (20% honey) exhibits improved solubility over HON and a significantly reduced sublimation tendency than TMP.

Five different electron density datasets obtained from conventional and synchrotron single crystal X-ray diffraction experiments are compared. The general aim of the study is to investigate the quality of data for electron density analysis from current state-of-the-art conventional sources, and to see how the data perform in comparison with high-quality synchrotron data. A molecular crystal of melamine was selected as the test compound due to its ability to form excellent single crystals, the light atom content, and an advantageous suitability factor of 3.6 for electron density modeling. These features make melamine an optimal system for conventional X-ray diffractometers since the inherent advantages of synchrotron sources such as short wavelength and high intensity are less critical in this case. Data were obtained at 100 K from new in-house diffractometers Rigaku Synergy-S (Mo and Ag source, HyPix100 detector) and Stoe Stadivari (Mo source, EIGER2 1M CdTe detector), and an older Oxford Diffraction Supernova (Mo source, Atlas CCD detector). The synchrotron data were obtained at 25 K from BL02B1 beamline at SPring-8 in Japan (λ = 0.2480 Å, Pilatus3 X 1M CdTe detector). The five datasets were compared on general quality parameters such as resolution, ⟨I/σ⟩, redundancy and R factors, as well as the more model specific fractal dimension plot and residual density maps. Comparison of the extracted electron densities reveals that all datasets can provide reliable multipole models, which overall convey similar chemical information. However, the new laboratory X-ray diffractometers with advanced pixel detector technology clearly measure data with significantly less noise and much higher reliability giving densities of higher quality, compared to the older instrument. The synchrotron data have higher resolution and lower measurement temperature, and they allow for finer details to be modeled (e.g. hydrogen κ parameters).

Spin-crossover (SCO) compounds are promising materials for a wide variety of industrial applications. However, the fundamental understanding of their nature of transition and its effect on the physical properties are still being fervently explored; the microscopic knowledge of their transition is essential for tailoring their properties. Here an attempt is made to correlate the changes in macroscopic physical properties with microscopic structural changes in the orthorhombic and monoclinic polymorphs of the SCO compound Fe(PM-Bia)₂(NCS)₂ (PM = N-2'-pyridylmethylene and Bia = 4-aminobiphenyl) by employing single-crystal X-ray diffraction, magnetization and DSC measurements. The dependence of macroscopic properties on cooperativity, highlighting the role of hydrogen bonding, π-π and van der Waals interactions is discussed. Values of entropy, enthalpy and cooperativity are calculated numerically based on the Slichter-Drickamer model. The particle size dependence of the magnetic properties is probed along with the thermal exchange and the kinetic behavior of the two polymorphs based on the dependence of magnetization on temperature scan rate and a theoretical model is proposed for the calculation of the non-equilibrium spin-phase fraction. Also a scan-rate-dependent two-step behavior observed for the orthorhombic polymorph, which is absent for the monoclinic polymorph, is reported. Moreover, it is found that the radiation dose from synchrotron radiation affects the spin-crossover process and shifts the transition region to lower temperatures, implying that the spin crossover can be tuned with radiation damage.

A short commentary is given on the paper by Vosegaard et al. [Acta Cryst. (2023), 79, 380-391], which compares charge density models derived from four datasets measured on conventional diffractometers with the results of a high-quality dataset from SPring-8.

Human tooth enamel (HTE) is the hardest tissue in the human body and its structural organization shows a hierarchical composite material. At the nanometric level, HTE is composed of approximately 97% hydroxyapatite [HAP, Ca₁₀(PO₄)₆(OH)₂] as inorganic phase, and of 3% as organic phase and water. However, it is still controversial whether the hexagonal HAP phase crystallizes in P6₃/m or another space group. The observance in HTE of Ca²⁺, Mg²⁺ and Na⁺ ions using X-ray characteristic energy-dispersive spectroscopy in the scanning electron microscope has been explained by substitutions in the HAP unit cell. Thus, Ca²⁺ can be replaced by Na⁺ and Mg²⁺ ions; the PO₄^3- group can be replaced by CO₃^2- ions; and the OH^- ions can also be replaced by CO₃^2-. A unit-cell model of the hexagonal structure of HTE is not fully defined yet. In this work, density functional theory calculations are performed to study the hexagonal HAP unit cell when substitution by OH^-, CO₃^2-, Mg²⁺ and Na⁺ ions are carried out. An approach is presented to study the crystallographic unit cell of HTE by examining the changes resulting from the inclusion of these different ions in the unit cell of HAP. Enthalpies of formation and crystallographic characteristics of the electron diffraction patterns are analysed in each case. The results show an enhancement in structural stability of HAP with OH defects, atomic substitution of Mg²⁺, carbonate and interstitial Na⁺. Simulated electron diffraction patterns of the generated structures show similar characteristics to those of human tooth enamel. Hence, the results explain the indiscernible structural changes shown in experimental X-ray diffractograms and electron diffraction patterns.

Aluminium alloy 7005 is widely used for structural purposes because of its attractive properties such as good weldability and age-hardening capability. However, since the workability of this alloy falls after a short period of natural aging, the application of cold rolling for the production of strain-hardened sheets of this alloy is a challenge. Two solutions proposed to overcome this challenge are as follows: (a) immediate rolling of the alloy after solution treatment and (b) rolling of the alloy after artificial aging. However, there is no comprehensive study comparing the effect of pre-rolling aging treatments on the evolutions of microstructure and texture of the alloy through heavy cold rolling. This subject is the aim of the present study. For this purpose, different pieces of the alloy are subjected to three different heat treatments before rolling, and afterward, they are rolled to obtain a thickness reduction of 80%. Scanning electron microscopy with electron backscattered diffraction observations are applied to study the evolutions of the microstructure and the texture of the alloy. Results show that the progression of pre-rolling aging decreases the incidence of micro-scaled shear bands by rolling. In addition, the rolling texture intensity decreases with the advancement of pre-rolling aging. Mechanisms responsible for this effect are discussed.

The structure of a recently found hyperhydrated form of sodium chloride (NaCl·13H₂O and NaCl·13D₂O) has been determined by in situ single-crystal neutron diffraction at 1.7 GPa and 298 K. It has large hydrogen-bond networks and some water molecules have distorted bonding features such as bifurcated hydrogen bonds and five-coordinated water molecules. The hydrogen-bond network has similarities to ice VI in terms of network topology and disordered hydrogen bonds. Assuming the equivalence of network components connected by pseudo-symmetries, the overall network structure of this hydrate can be expressed by breaking it down into smaller structural units which correspond to the ice VI network structure. This hydrogen-bond network contains orientational disorder of water molecules in contrast to the known salt hydrates. An example is presented here for further insights into a hydrogen-bond network containing ionic species.