Open-access and free articles in Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials
Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials publishes scientific articles related to the structural science of compounds and materials in the widest sense. Knowledge of the arrangements of atoms, including their temporal variations and dependencies on temperature and pressure, is often the key to understanding physical and chemical phenomena and is crucial for the design of new materials and supramolecular devices. Acta Crystallographica B is the forum for the publication of such contributions. Scientific developments based on experimental studies as well as those based on theoretical approaches, including crystal-structure prediction, structure-property relations and the use of databases of crystal structures, are published.
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Open-access and free articles in Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials
Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials
Subtle details revealed via powder diffraction and validated by charge flipping
Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials
The complex stacking disorder of Fe- and Ru-based 1,1′-(3,6-pyrazabolyl)metallocenes
1,1′-(3,6-Pyrazabolyl)ferrocene [Fc(BHpz)2] and the corresponding ruthenocene [Rc(BHpz)2] crystallize as order–disorder (OD) structures with layers of Pma(m) symmetry. Since the m[100] reflection and the a[010] glide reflection planes of adjacent layers do not overlap, given one layer, the adjacent layer can be placed in four distinct, but geometrically equivalent, positions. There are accordingly four polytypes of a maximum degree of order (MDO). One analyzed Fc(BHpz)2 crystal was composed of a mixture of the MDO1 polytype in four distinct orientations and the MDO3 polytype. A second crystal was essentially a single crystal of one out of two orientation of MDO3. In two analyzed crystals of Rc(BHpz)2, the MDO1 polytype (P1) was the major polytype and was observed in four or two orientation states, respectively. Pronounced diffuse scattering is attributed to a complex disorder, which cannot be explained by simple growth models. One Rc(BHpz)2 crystal features additional peaks in the diffraction pattern attributed to fragments of MDO3.
OD THEORY; DIFFUSE SCATTERING; SYNCHROTRON RADIATION; METALLOCENES
Prediction and analysis of symmetry-raising transitions in anilinium tetra­fluoro­borate
We report the structural and dielectric characterization of anilinium tetra­fluoro­borate (AnBF4), a molecular crystal identified as a candidate for symmetry-raising phase transitions using the FERROSCOPE routine. Variable-temperature powder X-ray diffraction and differential scanning calorimetry revealed a sequence of three structural phase transitions between room temperature and 490 K, yielding four polymorphs (A–D), three of which are previously unreported. All structures were solved either by single-crystal X-ray diffraction or ab initio from powder diffraction data. Single crystal experiments indicate diffuse scatter in polymorph C which can be related to the structural changes between the different polymorphs. The structural transitions have been rationalized using group–subgroup relationships. Dielectric measurements show a temperature-dependent response consistent with the structural changes, with the A to B transitions associated with a peak in the dielectric constant.
CRYSTAL STRUCTURE; PHASE TRANSITION; FERROELECTRICITY; DIELECTRIC CONSTANT; DISORDER
A case for resonant X-ray Bragg diffraction by a collinear antiferromagnet Li2Ni3P4O14
Magnetic axial and polar (Dirac) nickel multipoles contribute to resonant X-ray Bragg amplitudes in a symmetry-informed analysis of monoclinic Li2Ni3P4O14 presented for future diffraction experiments. Magnetic long-range order below a temperature of ≃ 14.5 K can be viewed as a two-dimensional trimerized antiferromagnet with Ni ions in two Wyckoff positions in the centrosymmetric (1) magnetic space group P21/c. It permits the coupling to circular polarization in the primary X-ray beam, unlike the corresponding diffraction by an antiferromagnet characterized by anti-inversion (1′) and a linear magnetoelectric effect, e.g. historically significant chromium sesquioxide (Cr2O3) and Cu2(MoO4)(SeO3) [Lovesey & van der Laan (2024). Phys. Rev. B 110, 174442]. The space group is inferred from neutron Bragg diffraction patterns, without an allowance for permitted Dirac dipoles (anapoles) and quadrupoles [Chikara et al. (2025). Phys. Rev. B 112, 014438].
ANTIFERROMAGNET; RESONANT X-RAY SCATTERING; BRAGG DIFFRACTION; ANAPOLES; CHIRAL SIGNATURE
Probing the local structure of Bi2O3 chemical derivatives: the neglected cation sublattice
High-resolution synchrotron X-ray diffraction data confirm the stabilization of the average structure at ambient temperatures as a defect fluorite structure in ternary and quaternary oxides of Bi2O3, where some Bi cations are replaced with Dy3+ and/or Er3+, Nb5+ and W6+. Rietveld refinement of the diffraction data indicates that when the larger cation Er3+ (0.89 Å) is replaced with smaller cations W6+ (0.42 Å) and/or Nb5+ (0.48 Å), the unit-cell parameter a, based on the relative ionic radii of the substituents, counterintuitively increases. Total scattering data show that the identity of the cation substituent is important in determining the positions of some Bi cations in the defect fluorite structure. Er3+ and Dy3+ cations induce monoclinic-like distortions that extend only locally in the Bi3+ sublattice. X-ray absorption spectroscopy provides evidence that some Dy cations, in the δ phases, prefer to keep a similar local environment to that in the parent Dy2O3. W cations are found to be predominantly tetrahedrally coordinated in the δ phases explored in this work.
LOCAL STRUCTURE; TOTAL SCATTERING DATA; X-RAY ABSORPTION SPECTROSCOPY; [DELTA] PHASE
Unveiling the structure and interactions of silicene grown on MoS2: insights from hybrid molecular dynamics simulations
The interaction of silicene with MoS2 surfaces has an important role in determining many properties of these systems. However, the intricate nature of silicene-based systems presents significant challenges for both experimental and computational investigations, often resulting in conflicting reports in the literature. Using molecular dynamics simulations with hybrid potentials, this work investigates the epitaxial growth of silicene on an MoS2 substrate using a vapor-deposition-like method. The findings reveal that silicene can achieve stability on an MoS2 substrate in an AB low-buckled configuration when both van der Waals and covalent interactions are considered. In this configuration, the silicene layer exhibits numerous defects, which can be significantly reduced by increasing the substrate temperature. In contrast, when only van der Waals interactions are accounted for, the system becomes unstable, leading to the collapse of the silicene structure into a three-dimensional disordered arrangement of Si atoms. A similar instability is observed in a heterostructure where a silicene sheet is intercalated between two MoS2 layers. These results, combined with the lattice mismatch observed between silicene and MoS2, suggest that van der Waals interactions alone are insufficient to ensure the mechanical stability of silicene.
EPITAXIAL GROWTH; MOLECULAR DYNAMICS; SILICENE; MOLYBDENUM DISULFIDE; VAN DER WAALS INTERACTIONS
OD (order–disorder) interpretation and diffuse scattering analysis of an organic polytype with allotwin character: a detailed how-to
2,3-Dihydroxy-1,3,4-trimethyl-6-oxo-1,4-cyclohexadiene-1-carboxylic acid crystallizes in an order–disorder (OD) structure with a high stacking fault probability. The diffraction pattern features diffuse scattering and broad peaks, which can be attributed to fragments of two polytypes of a maximum degree of order (MDO). Additional weak peaks indicate existence of a non-MDO polytype. A detailed step-by-step instruction is given on how to develop an OD description and interpret the diffuse scattering using a stochastic growth model.
ORDER-DISORDER THEORY; DIFFUSE SCATTERING; POLYTYPES; TWINNING; ALLOTWINNING
Crystal structure analysis of van der Waals layered phospho­rus chalcogenide CuVP2S6
The crystal structure of CuVP2S6 is investigated by transmission electron microscopy (TEM) and single-crystal X-ray diffraction. Cu, V and dimerized P2 are each octahedrally coordinated by S atoms. Each metal ion species forms a triangular lattice, which is interdigitated to form a two-dimensional sheet. Each sheet forms a layered structure with van der Waals gaps between them. The crystal has a rotational twin structure, which is manifested in the form of stacking disorder. Refined structure analysis reveals characteristic rotational distortion of the octahedra. The existence of Cu sites protruding into the van der Waals gap is directly shown by high-resolution scanning TEM.
VAN DER WAALS LAYERED COMPOUND; TRANSMISSION ELECTRON MICROSCOPY; SINGLE-CRYSTAL X-RAY DIFFRACTION
DMSO solvates of tert-butyl­calix[6]arene and related multisolvent structures
tert-Butyl­calix[6]arene (TBC6) exhibits a rich crystal energy landscape, with structural diversity driven by macrocycle conformation, guest inclusion and self-assembly. Using multicomponent crystallization mixtures containing DMSO and additional solvents (e.g. DMF, chloro­benzene, 1,2-di­chloro­benzene, anisole, THF), we obtained seven new TBC6 solvates. One structure, crystallized in the presence of pyridine, adopts a winged-cone conformation with a bilayer arrangement in the crystal. The remaining six are isostructural and feature a columnar packing of TBC6 in a 1,2,3-alternate conformation stabilized by four strong interactions with DMSO: two hydrogen bonds and two C—H⋯π interactions. Notably, DMSO in the TBC6 cavity is released at elevated temperatures (∼200°C), consistent with its stronger binding energy as calculated using DFT methods. TBC6–TBC6 interactions, both within and between columns, dominate the crystal packing, with intracolumnar contacts contributing twice as much stabilization as intercolumnar ones, where additional solvent guests are located. Hirshfeld surface analysis supports these findings, highlighting extensive contact areas between TBC6 molecules. This study contributes to understanding how solvent properties, interaction energies, and packing preferences drive the solid-state organization of calixarenes—molecules with broad applications in separation, recognition, and supramolecular chemistry.
CALIXARENES; SOLVATES; CRYSTALLIZATION; MULTICOMPONENT CRYSTAL; INTERMOLECULAR INTERACTIONS; HOST-GUEST SYSTEMS
The glycolipid flocculosin-A from the fungus Anthracocystis flocculosa, or how to deal with cotton-wool-like crystals
Flocculosin A is a strong antifungal metabolite secreted by the Basidiomycota fungus Anthracocystis flocculosa (formerly Pseudozyma flocculosa). It crystallizes in the form of long, soft cotton-wool-like needles, usually observed together with the mycelium in liquid culture conditions. Many crystallization tests were carried out. Among them, only a very slow cooling technique was able to yield suitable crystals, which were needle-like with a maximum thickness of 5 µm and several millimetres long. These crystals permitted us to record single-crystal X-ray diffraction data and solve the crystal structure as well as to determine the absolute configurations of the three chiral centres present in the two lipid chains.
FLOCCULOSIN-A; ANTHRACOCYSTIS FLOCCULOSA; FUNGUS; CRYSTAL GROWTH
EuPdSn2: magnetic structures in view of resonant X-ray Bragg diffraction
The magnetic properties of materials hosting Eu2+ (J = 7/2, 4f7) ions have attracted much attention in the science of strongly correlated electrons. In part because crystal electric field effects are impoverished for an S-state ion, as with Gd3+ intermetallics, and Eu2+ substitution in biological and optically active materials is resourceful. The magnetic structure of EuPdSn2 is not wholly resolved. Ferromagnetic and antiferromagnetic structures coexist in powder neutron diffraction patterns, and compete in the ground state. Moreover, the specific heat as a function of temperature is enigmatic and indicative of J = 5/2. We present symmetry-informed analytic magnetic structure factors for single crystal resonant X-ray Bragg diffraction using Eu atomic resonances that reveal significant potential for the technique. Europium ions use Wyckoff positions that are not centres of inversion symmetry in magnetic space groups inferred from neutron diffraction. In consequence, axial and polar Eu multipoles are compulsory components of both magnetic neutron and resonant X-ray Bragg diffraction patterns. The proposed antiferromagnetic phase of EuPdSn2 supports anapoles (magnetic polar dipoles) already observed in magnetic neutron diffraction patterns presented by Gd-doped SmAl2, and several resonant X-ray diffraction patterns.
MAGNETIC; RESONANT X-RAY DIFFRACTION; STRUCTURE FACTORS; MAGNETIC MULTIPOLES
Role of restraints on hydrogen atoms in Hirshfeld atom refinement: the case of tri-aspartic acid trihydrate
Two polymorphs of l-Asp-l-Asp-l-Asp (DDD) trihydrate as model compounds for biologically important proton-shuttle reactions were investigated with the quantum-crystallographic refinement technique Hirshfeld atom refinement (HAR). With HAR, hydrogen-atom positions are refined freely against the X-ray diffraction data and yield X—H bond distances close to those from neutron diffraction. However, the X-ray data of DDD trihydrate do not contain sufficient information to refine anisotropic displacement parameters (ADPs) for the hydrogen atoms, although the data quality is comparable to that of typical oligopeptide or protein datasets, including those with disordered fragments. Therefore, the following restraints were tested for the hydrogen-atom ADPs using NoSpherA2/olex2.refine: a restraint that approximates isotropic behaviour (ISOR), a restraint that enforces similar movement in any direction (SIMU), a rigid-bond restraint (DELU) and an advanced rigid-bond restraint (RIGU). Although it was found that there is no significant influence of the restraint weights and corresponding ADP values on the X—H distances, some recommendations on hydrogen-atom ADP restraint weights to be used in HAR are given. For ISOR, the suggested values are 10 times smaller (stricter) than the default values for non-hydrogen atoms in independent atom model (IAM) refinements, whereas those for RIGU are suggested to be less strict.
HYDROGEN ATOMS; RESTRAINTS; HIRSHFELD ATOM REFINEMENT; CRYSTAL WATER
New insights into structural diversity of open-framework silicates inspired by the exploration of the Cs2O–CaO–SiO2 system
CRYSTAL STRUCTURE; CS4CA[SI8O19]; INTERRUPTED FRAMEWORK SILICATE; MIXED TETRAHEDRAL-OCTAHEDRAL FRAMEWORK; TOPOLOGY; TILINGS
Unravelling the complexity of co-existing phases in piezoelectrics
PIEZOELECTRIC CERAMICS; HIGH CURIE TEMPERATURE; MORPHOTROPIC PHASE BOUNDARY; SYNCHROTRON X-RAY POWDER DIFFRACTION; NEUTRON DIFFRACTION
Multidisciplinary study of thorium mobility: formation of turkestanite and steacyite analogues, and structural insights using an XRD-directed microcrystal preparation technique
Minerals of the ekanite group typically contain Th, U or REE (rare earth elements) as primary structural constituents. The CNa analogues of turkestanite, AThB(Ca,Na)C(K1–x□x)T(Si8O20) (□ is vacancy), and steacyite, AThB(Ca,Na)C(K1–x□x)T(Si8O20), isostructural minerals of the ekanite group, were experimentally synthesized during hydro­thermal alteration of chevkinite-(Ce). The experiment was conducted at 550°C, 200 MPa, at an oxygen fugacity approximately equivalent to the Ni–NiO (NNO) buffer with NaF and Ca(PO4)2 added to the hydrous fluid. Both phases formed as a replacement of chevkinite-(Ce) and earlier alteration products. Their identity was confirmed by electron probe microanalysis and electron backscatter diffraction as CNa analogues of turkestanite and steacyite. Further SCXRD, and Raman spectroscopy analyses confirmed that it is a new CNa analogue with respect to the ekanite mineral group. Raman spectroscopy revealed the presence of H2O within the crystal structure. A dedicated FIB workflow was designed to extract single crystals ready for SCXRD analysis. Compositionally, the phase is Na rich and depleted in K, REE and Fe. The mean formula based on 20 O atoms can be written as A(Th0.94U0.03)0.97B(Na0.96Ca0.90Mn0.11Ce0.02Nd0.01Fe0.01)2.0C(Na0.83K0.07)0.9TSi8.05O20·0.1C(H2O). It crystallizes in space group P4/mcc with a = 7.4757 (2) Å, c = 14.9658 (7) Å, V = 836.38 (6) Å3, and Z = 2. Compositional variation is represented mainly by the relationship Ca2+ + □ → 2Na+, where □ is a vacancy which can also be filled by H2O during crystallization. The synthesis from this study represents the first record of CNa analogues of turkestanite and steacyite. A dedicated microcrystal selection technique is presented allowing for easy single-crystal X-ray diffraction.
MICROCRYSTAL PREPARATION; SINGLE-CRYSTAL X-RAY DIFFRACTION; TURKESTANITE; STEACYITE; HYDROTHERMAL ALTERATION; RARE EARTH ELEMENT
A linear/quadratic order parameter coupling description of the Verwey transition in magnetite, Fe3O4
The change in symmetry Fd3m → Cc at the Verwey transition in magnetite puts it in a class of phase transitions with linear/quadratic coupling between two separate order parameters. Direct coupling between an order parameter Qe, to represent an electronic instability, and an order parameter Qco, to represent cation charge ordering, has the form λQeQco2, with Tc(Qe) < Tc(Qco), but there must also be indirect coupling through the common strain, e6, due to strain coupling terms λe6Qe and λe6Qco2. Qe has the symmetry of irrep Γ5+ while the pattern of cation charge ordering of Fe2+ and Fe3+ on octahedral sites depends on some combination of irreps Δ5, X1, X3, W1 and W2. The software package ISOVIZ has been used to show how reported patterns of order for the simplified structure in space group P2/c can be understood in terms of a linearly dependent mix of patterns with symmetry Δ5 and X1, so that Qco can be treated in the first instance as though it has the symmetry of Δ5. Spontaneous strains calculated from published lattice parameters and symmetry-adapted atomic displacements from previous structural refinements in space group Cc have been used to confirm that the two order parameters have different temperature dependences, consistent with this phenomenological treatment. The effect of chemical doping can be understood in terms of the development of local strain heterogeneity which acts to suppress the macroscopic strains and which appears to have a greater influence on charge ordering than on the electronic structure.
MAGNETITE; VERWEY TRANSITION; CHARGE ORDER; ORDER PARAMETER COUPLING; SPONTANEOUS STRAIN
Structures of phospho­nitrides in light of the extended Zintl–Klemm concept
The extended Zintl–Klemm concept (EZKC) is applied to explain the crystalline structures of phospho­nitrides (also known as nitridophosphates in the chemical literature). The examples of (AE)2AlP8N15(NH) (AE = Ca, Sr, Ba), GeIVPN3 and MP2N4 (M = Be, Ca, Sr, Ba, GeII) are mainly discussed, although the examples of LiGaGe and LiGaGeO4 have been also commented on due to their relation with BeP2N4. It is shown that the EZKC provides a better understanding of the structures of these compounds than in previous descriptions. In most of these nitrides, P atoms behave as pseudo-Si atoms and N atoms behave as pseudo-O atoms, so providing a good explanation for the four-connectivity of P atoms forming PN4 units, which behave as pseudo-SiO4 units like the SiO4 units in many polymorphs of SiO2. In addition, the EZKC shows that the notation of these compounds as phospho­nitrides is more appropriate than as nitridophosphates because N atoms act as the anions in these compounds.
PHOSPHONITRIDES; CRYSTAL STRUCTURES; CRYSTAL CHEMISTRY; EXTENDED ZINTL-KLEMM CONCEPT
Magnetic structure determination and refinement using FullProf
The study of magnetic structures from a crystallographic perspective remains a niche field, largely confined to physical crystallography and microscopic magnetism. Unlike general or superspace crystallography, magnetic structure analysis lacks standardization, primarily due to the small research community and the reliance on specialized experimental techniques like neutron diffraction. However, emerging topics in condensed matter physics, such as topological materials, multiferroics, and skyrmions, have heightened the importance of understanding magnetic ordering. As a result, the analysis of magnetic neutron scattering data is becoming increasingly relevant, requiring unified methodologies. Since 2011, the IUCr Commission on Magnetic Structures has been working to improve the situation. This paper reviews the most commonly used methods for describing magnetic structures and highlights the capabilities of the FullProf Suite for analysing magnetic neutron diffraction data.
MAGNETIC (SUPER)SPACE GROUPS; INCOMMENSURATE MAGNETIC STRUCTURES; SIMULATED ANNEALING; MAGNETIC CRYSTALLOGRAPHY; SYMMETRY MODES
Multipolar model and Hirshfeld atom refinement of tetra­aqua­bis­(hydrogenmaleato)iron(II)
A high-resolution charge density study using the Hansen–Coppens multipolar model was performed on tetra­aqua­bis­(hydrogenmaleato)iron(II). The experimental electron density was subjected to Bader's topological analysis. Hirshfeld atom refinement and topological analysis of the molecular wavefunction were also conducted. A comparison of the properties obtained under different resolution and acquisition conditions are presented. The performance of these models is evaluated in terms of their ability to achieve bond lengths close to those from neutron diffraction, provide accurate anisotropic displacement parameters and model electron densities precisely, and to determine atomic charges under different experimental and modeling conditions. The structure presents a short intramolecular hydrogen bond, which is found to have a distinct character compared to other interactions, as the hydrogen interacts covalently with two oxygen atoms. Different models were evaluated, each outperforming the others in specific aspects. Overall, the analysis of these models provide deeper insights into electron density distribution and the nature of the interactions present in the structure.
ASPHERICAL REFINEMENTS; TOPOLOGICAL ANALYSIS; HYDROGEN BONDS; MULTIPOLE MODEL; HIRSHFELD ATOM REFINEMENT
Cs4Ca[Si8O19]: a new mixed tetrahedral–octahedral oxosilicate, its topological features and comparison with other interrupted framework silicates
Single crystals of a previously unknown caesium calcium silicate with the composition Ca4Ca[Si8O19] have been obtained during a systematic study of the phase relations and compound formation in the system Cs2O–CaO–SiO2. Structure determination was based on a single-crystal diffraction data set recorded at 288 (2) K. The compound crystallizes in the monoclinic space group P21/n and has the following basic crystallographic parameters: a = 7.1670 (6) Å, b = 12.0884 (10) Å, c = 12.4019 (10) Å, β = 90.044 (8)°, V = 1074.47 (15) Å3, Z = 2. The crystal structure was solved by direct methods. The sample showed twinning by pseudo-merohedry, which was accounted for in the subsequent least-squares refinements resulting in a residual of R1 = 0.036 for 1962 independent observed reflections and 149 parameters. The crystal structure of Cs4Ca[Si8O19] belongs to the group of interrupted framework silicates, in which the [SiO4] tetrahedra are linked in a three-dimensional network consisting of Q4 and Q3 groups in a 1:3 ratio. The linear backbones of the framework can be described as loop-branched dreier single chains. These ribbons are parallel to [100], and the translation period of about 7.2 Å along this axis reflects the periodicity of the chains. By sharing common corners, the condensation of these chains along the [001] direction leads to the formation of layers that are parallel to (010) and contain three- and nine-membered rings of tetrahedra. Alternatively, the crystal structure can be described as a mixed tetrahedral–octahedral framework between [SiO4] tetrahedra and [CaO6] octahedra containing cavities accommodating the caesium ions coordinated by seven and eight oxygen ligands, respectively. A detailed topological analysis of the mixed framework based on natural tiles is presented. Indeed, the net can be constructed from a total of only two different cages (tiles) having the following face symbols: [43] and [34.46.62.78]. A comparison with related silicates containing [Si8O19] anions and already classified as well as hitherto unclassified interrupted frameworks is presented. Finally, the thermal expansion tensor has been determined in the temperature interval between 193 K and 288 (2) K.
Pushing crystallography's frontiers through quantum mechanics
QUANTUM CRYSTALLOGRAPHY; DENSITY MATRICES; COMBINED ELASTIC-INELASTIC SCATTERING
The varying temperature- and pressure-induced phase transition pathways in hybrid improper ferroelectric Sr3Sn2O7
A variable-temperature and pressure single-crystal diffraction study of hybrid improper ferroelectric Sr3Sn2O7 is reported. In combination with symmetry analysis, we reveal that the application of pressure and temperature induce distinct phase transition pathways, driven by a differing response of the octahedral rotations to these stimuli. Contrary to what has been previously predicted, we observe the ferroelectric to paraelectric phase transition between 10.17(18) and 12.13(14) GPa, meaning the hybrid improper ferroelectric phase remains stable to significantly higher pressures than expected.
HIGH-PRESSURE CRYSTALLOGRAPHY; HYBRID IMPROPER FERROELECTRICITY; RUDDLESDEN-POPPER
Magnetic structure determination of multiple phases in the multiferroic candidate GdCrO3
Due to their potential applications in low-power consumption and/or multistate memory devices, multiferroic materials have attracted a lot of attention in the condensed matter community. As part of the effort to identify new multiferroic compounds, perovskite-based GdCrO3 was studied in both bulk and thin film samples. A strong enhancement of the capacitance in a field suggested ferroelectric behaviour but significant leakage and no well developed P–E hysteresis loops were observed. Measurements clearly indicate the existence of a polar phase but only below 2 K (likely connected to Gd ordering). Here the determination of the magnetic structure through neutron diffraction collected on an isotopic 160GdCrO3 sample at the WISH diffractometer at ISIS is reported. The presence of three successive magnetic phases as a function of temperature (commensurate, spin re-orientation and incommensurate phases once the Gd order), previously only seen by magnetization, is confirmed. Using the most recent guidelines for reporting the determined structures, we highlight the benefits of using such nomenclature for discussing physical properties and consider possible mechanisms and couplings that led this seemingly rather isotropic system to display the complex structures observed.
MAGNETIC STRUCTURE; PEROVSKITE; MULTIFERROICITY; NEUTRON DIFFRACTION; SPIN RE-ORIENTATION; INCOMMENSURATE MAGNETIC STRUCTURE
Electric charge and salting in/out effects on glucagon's dipole moments and polarizabilities using the GruPol database
This work demonstrates the use of the GruPol database to predict the functional group dipole moments and polarizabilities of glucagon in the presence of NaCl, simulating an electric charge distribution on the protein's backbone. A new feature of the database allows for the inclusion of ions on the protein backbone, effectively simulating a protein salt and predicting the impact on electrical properties. Glucagon was selected as a proof-of-concept molecule due to its relatively small chain, which enabled benchmarking against quantum mechanical calculations. Firstly, we simulated 70 different ionic configurations, varying the number of Na+ and Cl− ions from zero to four NaCl moieties. Additionally, we investigated the effects of solvation under two distinct conditions: one involving just the peptide and water, and the other also including NaCl at a concentration of approximately 4.2 mol L−1. Regarding the ab initio results, GruPol showed good accuracy, with an angular direction error of around 10° and a 15% difference in the magnitude of the dipole moments. However, the error in polarizability values was higher, most likely due to the lack of an augmented basis set in the ab initio quantum calculations (M06-HF/cc-pVDZ). The database entries were generated using the same functional along with the aug-cc-pVDZ basis set. In solution, a high ionic concentration lowered the overall dipole moment, while the main components of polarizability increased.
QUANTUM CRYSTALLOGRAPHY; PROTEINS; DISTRIBUTED POLARIZABILITY APPROACH; DIPOLE MOMENTS; SOLVATION EFFECTS
Intrusion of quantum crystallography into classical lands
This article, written on the occasion of the International Year of Quantum Science and Technology, explores the development of alternative approaches to pure-state N-electron wavefunctions in quantum physics. It focuses on Quantum Crystallography tools and explains how the combination of X-ray coherent-elastic and incoherent-inelastic scattering data has enabled the description of mean-electron's quantum behaviour in phase space. The article gives a numerical example using a urea crystal to demonstrate the attainability of recovering a one-electron reduced density matrix and its associated reduced Wigner function. It emphasizes the importance of momentum-space measurements in obtaining a more accurate phase-space picture of electron quantum physics in crystals.
QUANTUM PHYSICS; QUANTUM CRYSTALLOGRAPHY; WIGNER FUNCTION; REDUCED DENSITY MATRIX; COMPTON SCATTERING; HIGH-RESOLUTION X-RAY DIFFRACTION; PHASE SPACE; MOMENTUM SPACE
Computation screening for incorrectly determined cocrystal structures
Pharmaceutical solid forms, like salts and cocrystals, play a crucial role in drug formulation. Despite differing mainly by a single hydrogen atom, the regulatory requirements set by the US Food and Drug Administration for these forms vary significantly. We previously developed a DFT-based computational method to distinguish salts from cocrystals. This method, validated on 95 structures, performed well for systems where hydrogen bonds were longer than 2.613 (16) Å. Here, benefits of the rSCAN functional over the PBE functional are discussed. We expand the dataset to 404 cocrystal models. Analysis confirms that 301 of these forms are indeed cocrystals. Additionally, 87 salt–cocrystal continuum forms are identified and 16 cocrystals are classified as possible salts. These 16 problematic structures are further investigated and for seven of them, single crystals were grown and their structure determined using single-crystal X-ray diffraction. Among the phases exhibiting salt-like behaviour, five of them are identified as salts. In some cases, rSCAN alone gives unreliable results for strong hydrogen bonds, but these discrepancies are often corrected using better-renormalized or hybrid functionals (i.e. r2SCAN, PBE0 and PBE50). For future calculations, we recommend using the r2SCAN functional for salt–cocrystal differentiation, as it provides reliable results for O—H⋯N bonds longer than 2.554 (5) Å. The r2SCAN functional offers a good balance between accuracy and computational efficiency for systems with longer O—H⋯N bonds.
DFT-D; SALT; COCRYSTAL; COCRYSTAL STRUCTURES; VERIFICATION
Structural and magnetic properties of β-Li2IrO3 after grazing-angle focused ion beam thinning
Manipulating the size and orientation of quantum materials is often used to tune emergent phenomena, but precise control of these parameters is also necessary from an experimental point of view. Various synthesis techniques already exist, such as epitaxial thin film growth and chemical etching, that are capable of producing specific sample dimensions with high precision. However, certain materials exist as single crystals that are often difficult to manipulate, thereby limiting their studies to a certain subset of experimental techniques. One particular class of these materials includes lithium and sodium iridates, which are promising candidates for hosting a Kitaev quantum spin liquid state. Here a controlled method of using a focused ion beam at grazing incidence to reduce the size of a β-Li2IrO3 single crystal to a thickness of 1–2 µm is presented. Subsequent X-ray diffraction measurements show the lattice remains intact, albeit with a larger mosaic spread. The integrity of the magnetic order is also preserved as the temperature dependent magnetic diffraction peak follows the same trend as its bulk counterpart with a transition temperature at TN = 37.5 K. Our study demonstrates a technique that opens up the possibility of nonequilibrium experiments where submicron thin samples are often essential.
FOCUSED-ION BEAM; MAGNETIC DIFFRACTION; SAMPLE FABRICATION; X-RAY DIFFRACTION
Relaxing the restriction that valence can only adopt integer values removes the distinction between ionic and covalent bonds, resulting in a simple and powerful valence theory using the concept of the bonding strength of an element, defined as the ratio of its atomic valence to its average coordination number. Bonding strength, closely related to electronegativity, determines which atoms form stable bonds and the chemical properties of the resulting compounds of both organic and inorganic compounds.
CHEMICAL THEORY OF VALENCE; STRUCTURE; BONDING