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

Single crystals of bis(1,2-diaminepropane) di-μ-chloro-bis[diaquadichloromanganate(II)] dichloride have been prepared by evaporation from ethanoic solution. The triclinic X-ray crystal structure is built as layers of centrosymmetric dimers of [Mn(Cl)₄(H₂O)₂]^2- octahedra and 1,2-diaminopropane. The inorganic part consists of Mn octahedra sharing one edge and distributed in the basal ac plane along the a direction. These doubly negative charged layers are separated along the b axis by a positively charged diamine propane layer. One Cl^- anion contributes to the electroneutrality of the crystal interacting with both inorganic - through a hydrogen bond network to the two water molecules coordinated to Mn - and organic layers via the NH₃⁺ ammonium group. Differential scanning calorimetry shows two endothermic main peaks at T = 366 K and T = 375 K related to the release of the water molecules. The resulting dehydrated material is C-centered monoclinic as shown by powder X-ray diffraction.

VO and VO₂ vanadyl groups with short (typically 1.57-1.68 Å), essentially covalent, V-O bonds are common for V⁴⁺-bearing oxysalts with [5]- and [6]-coordinated vanadium. There is a clear negative correlation between vanadyl bond lengths and wavenumbers of the bands of V-O stretching vibrations in infrared spectra (in the range 1000-880 cm^-1). Optical, structural and chemical data for vanadyl minerals are used to calculate Gladstone-Dale compatibility coefficients. Gladstone-Dale compatibility indices of minerals containing vanadyl bonds are compared with total electronic polarizabilities of V⁴⁺. Unlike compounds of [5]-coordinated Ti⁴⁺, for most minerals with V⁴⁺=O (vanadyl) bonds there is good agreement between measured refractive indices and those calculated based on the polarizability concept.

Like many complex intermetallic phases, the crystal structures of REZn_5+x compounds (RE = lanthanide or Group 3 element) based on the EuMg₅ type have gradually unfolded. The original reports described a complex hexagonal structure with an unusual combination of tetrahedrally close-packed regions and open spaces, as well as observations of superstructure reflections. More recently, we reinvestigated the structure of YZn₅, reclassifying it as the EuMg_5+x-type compound YZn_5+x (x ≃ 0.2), in which disordered channels run along c through the spaces formerly considered open. In addition, DFT-chemical pressure (DFT-CP) analysis of ordered models of YZn_5+x highlighted paths for communication between neighboring channels setting the stage for superstructure formation. Herein, the experimental elucidation of this effect is presented with the synthesis and structure determination of a modulated form of YZn_5+x. By slow-cooling samples of YZn_5+x from the annealing temperature, crystals were obtained that exhibit satellite reflections with the modulation wavevector q = {1over 3}a* + {1over 3}b* + 0.3041c*. Structure solution and refinement using a (3+1)D model in superspace group P31c({1over 3},!{1over 3}σ₃)00s reveals incommensurate order in the structure's channels. Here, two Zn sites associated with the channels are present, each with discontinuous atomic domains that are slanted in the x₃x₄ plane. Their slanting corresponds to adjustments along the c axis for the presence or absence of close neighbors along that axis, while the occupation patterns of neighboring channels are shifted by {1over 3} of the modulation period. These features follow earlier predictions from CP analysis, highlighting how this approach can be used predictively in search of new phenomena.

Introducing the new Section Editor of Acta Crystallographica Section B.

A series of Bi_1-xLa_xFeO₃ samples with 0.00 ≤ x ≤ 0.30 was synthesized by the sol-gel method. The effects of lanthanum concentration on the phase formation, microstructure and cycloidal spin ordering were studied using X-ray diffraction, scanning electron microscopy and Mössbauer spectroscopy. The crystal structure of the La-doped bismuth ferrite transformed from rhombohedral R3c (x ≤ 0.05) to a mixture of R3c and cubic Pm3m (0.07 ≤ x ≤ 0.15) and finally to a mixture of R3c, Pm3m and orthorhombic Pbam (0.20 ≤ x ≤ 0.30). The Pbam phase, with characteristic porous microstructure shown by microscopy images, was observed in Bi_1-xLa_xFeO₃ compounds for the first time. Based on the Mössbauer spectroscopy, it was found that the cycloidal spin ordering started to disappear at x = 0.07. With increasing La concentration the share of the cycloid decreased from 100% at 0.00 ≤ x ≤ 0.05 to 0% at x = 0.30. At the beginning, for x ≤ 0.02, the anharmonicity parameter, m, of the cycloidal spin ordering was about 0.5, which is typical of a pure BiFeO₃ compound. In the range 0.05 ≤ x ≤ 0.25, the m parameter was of the order of 0.1, which indicated the practically harmonic character of the cycloid. The structural transition at x = 0.07 was accompanied by a substantial increase in magnetization.

Investigations simultaneously involving multiple techniques of quantum crystallography could be very useful to prove the consistency of obtained results or to highlight different facets of the same scientific phenomenon or problem. Pinto et al. [Acta Cryst. (2023), B79, 282-296] exploit three different quantum crystallographic techniques (Hansen & Coppens multipole model refinement, QTAIM analysis of the electron density, and Hirshfeld atom refinement) to characterize the nature of chemical bonds and of intra/intermolecular interactions in an organometallic compound.

A method for the visualization of noncovalent interactions using examples of the conformational polymorphs of four organic compounds: 2-(phenylamino)nicotinic, 2-(3-chloro-2-methylphenylamino)nicotinic, N-(3-chloro-2-methylphenyl)anthranilic and 2-(methylphenylamino)nicotinic acids is examined. The changes in noncovalent contacts are plotted against the angle between the planes of aromatic rings allowing a visual representation of conformational adjustment of molecules as well as packing features of crystal structures. According to the k-Φ criterion, the studied structures represent conformational polymorphs. Different types of hydrogen bonding are discussed within the framework of the method of visualization and molecular Voronoi-Dirichlet polyhedra. Good correlations are found between calculated and experimental data for several cases, such as the agreement between π stacking and polymorphic transition temperatures as well as between the area of a contact and the energy of conjugation. Also, an attempt has been made to assess the relative contributions of conformational and packing polymorphism in the formation of polymorphs.

A novel approach is proposed for the description of possible reconstructive solid-state transformations, which is based on the analysis of topological properties of atomic periodic nets and relations between their subnets and supernets. The concept of a region of solid-state reaction that is the free space confined by a tile of the net tiling is introduced. These regions (tiles) form the reaction zone around a given atom A thus unambiguously determining the neighboring atoms that can interact with A during the transformation. The reaction zone is independent of the geometry of the crystal structure and is determined only by topological properties of the tiles. The proposed approach enables one to drastically decrease the number of trial structures when modeling phase transitions in solid state or generating new crystal substances. All crystal structures which are topologically similar to a given structure can be found by the analysis of its topological vicinity in the configuration space. Our approach predicts amorphization of the phase after the transition as well as possible single-crystal-to-single-crystal transformations. This approach is applied to generate 72 new carbon allotropes from the initial experimentally determined crystalline carbon structures and to reveal four allotropes, whose hardness is close to diamond. Using the tiling model it is shown that three of them are structurally similar to other superhard carbon allotropes, M-carbon and W-carbon.

The study of azoproite by Biryukov et al. [Acta Cryst. (2022), B78, 809-816) continues the systematic and complex investigation of the crystal structures, temperature behavior and magnetic properties of natural oxoborates.

The unique phase in a crystal of triphenylsilanol (Ph₃SiOH) is reported. It is observed that after the crystal is flash-cooled from room temperature to 100 K, a new stable ordered phase II occurs with an increase in the unit-cell parameters compared to the earlier reported phase I. The new phase II occurs upon fast cooling while on slow cooling the disordered phase I is present. Gradual heating from 100 K (phase II) causes the crystal to return to the original phase I at about 150 K. The crystal undergoes the observed transformation in a reversible manner in many consecutive flash cooling/heating cycles without cracking.