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

A special issue of Acta Crystallographica Section B reports the great progress made recently in the determination, reporting, and archiving of magnetic structures, of which there are now more than 2000. The infrastructure needed to support the field is now in place. The special issue also highlights new science made possible by these developments.

The seventh blind test of crystal structure prediction (CSP) methods substantially increased the level of complexity of the target compounds relative to the previous tests organized by the Cambridge Crystallographic Data Centre. In this work, the performance of density-functional methods is assessed using numerical atomic orbitals and the exchange-hole dipole moment dispersion correction (XDM) for the energy-ranking phase of the seventh blind test. Overall, excellent performance was seen for the two rigid molecules (XXVII, XXVIII) and for the organic salt (XXXIII). However, for the agrochemical (XXXI) and pharmaceutical (XXXII) targets, the experimental polymorphs were ranked fairly high in energy amongst the provided candidate structures and inclusion of thermal free-energy corrections from the lattice vibrations was found to be essential for compound XXXI. Based on these results, it is proposed that the importance of vibrational free-energy corrections increases with the number of rotatable bonds.

Accurate modeling of conformational energies is key to the crystal structure prediction of conformational polymorphs. Focusing on molecules XXXI and XXXII from the seventh blind test of crystal structure prediction, this study employs various electronic structure methods up to the level of domain-local pair natural orbital coupled cluster singles and doubles with perturbative triples [DLPNO-CCSD(T1)] to benchmark the conformational energies and to assess their impact on the crystal energy landscapes. Molecule XXXI proves to be a relatively straightforward case, with the conformational energies from generalized gradient approximation (GGA) functional B86bPBE-XDM changing only modestly when using more advanced density functionals such as PBE0-D4, ωB97M-V, and revDSD-PBEP86-D4, dispersion-corrected second-order Møller-Plesset perturbation theory (SCS-MP2D), or DLPNO-CCSD(T1). In contrast, the conformational energies of molecule XXXII prove difficult to determine reliably, and variations in the computed conformational energies appreciably impact the crystal energy landscape. Even high-level methods such as revDSD-PBEP86-D4 and SCS-MP2D exhibit significant disagreements with the DLPNO-CCSD(T1) benchmarks for molecule XXXII, highlighting the difficulty of predicting conformational energies for complex, drug-like molecules. The best-converged predicted crystal energy landscape obtained here for molecule XXXII disagrees significantly with what has been inferred about the solid-form landscape experimentally. The identified limitations of the calculations are probably insufficient to account for the discrepancies between theory and experiment on molecule XXXII, and further investigation of the experimental solid-form landscape would be valuable. Finally, assessment of several semi-empirical methods finds r²SCAN-3c to be the most promising, with conformational energy accuracy intermediate between the GGA and hybrid functionals and a low computational cost.

Averaging of material coefficients of crystallites deposited at an angle to a rotating substrate is considered. A simple model is proposed, and applied to determine effective linear dielectric, piezoelectric, and elastic constants of all Laue groups. While these represent tensors of rank 2, 3, and 4, the method applies generally to tensors of any rank. Results are then particularized for 6mm point symmetry crystals, and applied numerically to zinc oxide, ZnO. It is shown that, by means of the rotating substrate method, depositions may be achieved having the equivalent of hexagonal anisotropy, enabling the creation of `engineered' structures.

The discovery of novel quaternary intermetallic compounds R₂MoSi₂C (R = Y, Gd) in the R-Mo-Si-C system is reported. Their crystal structure was investigated using single-crystal X-ray diffraction. It is shown that the Y₂MoSi₂C and Gd₂MoSi₂C compounds crystallize in the tetragonal space group P4/mbm (Pearson symbol tP12) with unit-cell parameters a = 6.9525 (18) Å, c = 4.1962 (16) Å, and a = 6.9945 (16) Å, c = 4.2020 (14) Å, respectively.

Introducing the new Section Editor for Acta Cryst. B.

A polycrystalline sample LuCrO₃ has been characterized by neutron powder diffraction (NPD) and magnetization measurements. Its crystal structure has been Rietveld refined from NPD data in space group Pnma; this perovskite contains strongly tilted CrO₆ octahedra with extremely bent Cr-O-Cr superexchange angles of ∼142°. The NPD data show that below Néel temperature (T_N ≃ 131 K), the magnetic structure can be defined as an A-type antiferromagnetic arrangement of Cr³⁺ magnetic moments, aligned along the b axis, with a canting along the c axis. A noticeable magnetostrictive effect is observed in the unit-cell parameters and volume upon cooling down across T_N. The AC magnetic susceptibility indicates the onset of magnetic ordering below 112.6 K; the magnetization isotherms below T_N show a nonlinear behaviour that is associated with the described canting of the Cr³⁺ magnetic moments. From the Curie-Weiss law, the effective moment of the Cr³⁺ sublattice is found to be μ_eff = 3.55 μ_B (calculated 3.7 μ_B) while the Θ_CW parameter yields a value of -155 K, indicating antiferromagnetic interactions. There is a conspicuous increase of T_N upon the application of external pressure, which must be due to shortening of the Cr-O bond length under compression that increases the orbital overlap integral.

In recent years, there is a significant interest from the crystallographic and materials science communities to have access to raw diffraction data. The effort in archiving raw data for access by the user community is spearheaded by the International Union of Crystallography (IUCr) Committee on Data. In materials science, where powder diffraction is extensively used, the challenge in archiving raw data is different to that from single crystal data, owing to the very nature of the contributions involved. Powder diffraction (X-ray or neutron) data consist of contributions from the material under study as well as instrument specific parameters. Having raw powder diffraction data can be essential in cases of analysing materials with poor crystallinity, disorder, micro structure (size/strain) etc. Here, the initiative and progress made by the International Centre for Diffraction Data (ICDD^R) in archiving powder X-ray diffraction raw data in the Powder Diffraction File^TM (PDF^R) database is outlined. The upcoming 2025 release of the PDF-5+ database will have more than 20 800 raw powder diffraction patterns that are available for reference.

A hydrated salt of decavanadate containing diprotonated metforminium(2+) (H₂Met²⁺), hydronium (H₃O⁺) and either neutral biguanide (Bg) or monoprotonated guanylurea (HGU⁺) exhibits a previously seen complex charge-stabilized hydrogen-bonded network [Chatkon et al. (2022). Acta Cryst. B78, 798-808]. Charge balance is achieved in two ways through substitutional disorder: a 0.6 occupied HGU⁺ cation is paired with a V₁₀O₂₈^6- anion, and a 0.4 occupied neutral Bg molecule is paired with a HV₁₀O₂₈^5- anion, with the remaining charge in both cases balanced by two H₂Met²⁺ dications and one H₃O⁺ monocation. Bg/HGU⁺ moieties exhibit bifurcated N-H...O hydrogen bonding to the H₃O⁺ cation and are substitutionally/positionally disordered along with the H₃O⁺ cation about an inversion center. The HGU⁺ V₁₀O₂₈^6- synthon seen in the previous study occurs again. Bg exhibits bifurcated hydrogen bonding from two amino groups to two rows of cluster O atoms running diagonally across the equatorial plane of the HV₁₀O₂₈^5- anion with a return hydrogen bond from the cluster H atom to the imino N atom of the Bg. Thus, a Bg...cluster synthon similar to the HGU⁺...cluster synthon previously reported is found. The disordered moieties occupy spaces with excess volume in the 3-D network structure. Interestingly, when the crystallographic unit cell of the current compound, whose X-ray data was collected at 100 K, is compared with that of a previous compound exhibiting the same supramolecular framework, unit-cell parameter c does not shorten as a and b expectantly do because of the lower data collection temperature. The lack of contraction on unit-cell parameter c is possibly due to the supramolecular structure.

Magnetic materials featuring triangular arrangements of spins are frequently investigated as platforms hosting magnetic frustration. Hexagonal perovskites with ordered vacancies serve as excellent candidates for two-dimensional triangular magnetism due to the considerable separation of the magnetic planes. In this work, the effects of chemical pressure on the ferromagnetic ground state of Ba₂La₂NiW₂O₁₂ by substitution of Ba²⁺ with Sr²⁺ to produce Sr₂La₂NiW₂O₁₂ are investigated. The two materials are characterized using synchrotron-based XRD, XANES and EXAFS in addition to magnetometry in order to correlate their crystal structures and magnetic properties. Both materials form in space group R3, yet as a result of the enhanced bending of key bond angles due to the effects of chemical pressure, the T_C value of the magnetic Ni²⁺ sublattice is reduced from ∼6 K in Ba₂La₂NiW₂O₁₂ to 4 K in Sr₂La₂NiW₂O₁₂.