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

K₈Ca₃(CO₃)₇ was synthesized at 1200°C and 6 GPa. Its layered triclinic crystal structure presents a new type characteristic of high-pressure K-Ca carbonates. It is also the second example of K/Ca disorder coupled with orientational disorder of carbonate anions. Descriptions based on the most symmetric (tri/hexagonal) and the densest (krd-type) cation-carbon nets reveal its peculiarities and common features with the crystal structures of chemically related carbonates. The four-layered ABA'B packing in K₈Ca₃(CO₃)₇ is based on hexagonal cation nets, wherein carbon atoms occupy two types of positions. Most of the carbon atoms are localized close to net B, within the centers of triangular loops. This forms a bütschliite layer, which is found in other alkali-bearing carbonates and is composed of alkali cations. Its presence in the K₈Ca₃(CO₃)₇ crystal structure implies that the K/Ca disorder is incomplete. In nets A and A', carbon atoms replace the cations at some nodes, resulting in the aggregation and increased concentration of carbon atoms and their stronger mixing with cations in the densest (krd-type) nets. Thus, a new type of densest mixed net is formed in the K₈Ca₃(CO₃)₇ crystal structure. A quantitative measure of the packing density of high-pressure K-Ca carbonates is the reticular density of the symmetric atomic nets, as well as the distance between the densest nets.

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.

The crystal structure of synthetic ludwigite-type oxoborate Cu₂FeO₂(BO₃) was first determined using in situ high-temperature single-crystal X-ray diffraction data collected at 300, 800 and 950 K. The structure is described as a heteropolyhedral framework composed of edge- and vertex-sharing [MO₆]^n- (M = Cu²⁺, Fe³⁺) octahedra that form extended zigzag chains in the bc plane with isolated trigonal planar [BO₃]^3- located in triangular cavities of the cationic framework. Oxo-centered [OM₄]ⁿ⁺ tetrahedra and [OM₅]ⁿ⁺ tetragonal pyramids are observed and described for the first time. The structure is disordered: the Cu1 and Cu3 sites are fully occupied by the Cu²⁺ ions, while the M2 and M4 sites are occupied by the Cu²⁺ and Fe³⁺ ions in ratios of 0.59:0.41 and 0.20:0.80, respectively. The O4 site is split into two sites, O4A and O4B, which leads to the formation of distorted [M2O₆]^n-, Cu3O₆]^n- and [M4O₆]^n- polyhedra. Thermal behavior of Cu₂FeO₂(BO₃) was investigated using in situ high-temperature single-crystal and powder X-ray diffraction, simultaneous thermal analysis and high-temperature heat capacity measurements. Unusual behavior of the unit-cell parameters and specific heat is observed at ∼690 K. The borate is stable up to ∼1040 K, when an incomplete solid-phase decomposition begins with the formation of CuO and (Cu,Fe)₃O₄ phases. The anisotropy of thermal expansion is weak and it is explained by (i) the preferable orientation of the most chemically rigid [BO₃]^3- units, (ii) the theory of shear deformations of the monoclinic ac plane and (iii) an arrangement of the oxo-centered [O₂M₇]ⁿ⁺ and [O₂M₈]ⁿ⁺ double chains. Volume expansion increases with an increase in temperature from 24.6 × 10^-6 K^-1 (at 300 K) to 35.4 × 10^-6 K^-1 (at 1000 K). The degree of anisotropy of the expansion of Cu₂FeO₂(BO₃) is similar to that of orthorhombic ludwigite-type oxoborates.

A large family of photosensitive [Co(NH₃)₅NO₂]XY compounds is known to manifest mechanical effects accompanying intramolecular photochemical linkage nitro-nitrito isomerization. A new member of this family, a malonate, [Co(NH₃)₅NO₂]C₃H₂O₄, was synthesized for the first time and shown to be photoinert. The compound appears to be the first photoinert member of the family. The crystal structure was analysed both at ambient conditions and under variable temperature and pressure, and compared to previously known members of the family in order to rationalize such behaviour. A reversible isosymmetric single-crystal to single-crystal phase transition was observed at pressures between 2.4 and 3.0 GPa, and the crystal structure of a new high-pressure phase was solved and refined. Possible driving forces for the phase transition are discussed.

Carbenoxolone (CBXH₂) is a biologically active compound first synthesized in the late 1950s. Four new solvates of CBXH₂ with propan-1-ol, tert-butanol, dimethylformamide and dimethyl sulfoxide were obtained and characterized using single-crystal and powder X-ray diffraction along with thermal analyses. Ribbons and layers, recurring building motifs in these structures, are primarily based on dispersion interactions. The degree of similarity (1D, 2D or 3D) between these new structures and four CBXH₂ structures from the Cambridge Structural Database was evaluated using geometrical parameters and XPac calculations.

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.

The ferroelectric diammonium hypodiphosphate, (NH₄)₂(H₂P₂O₆) (3), is the only known so far ammonium salt of hypodiphosphoric acid, H₄P₂O₆. A series of seven new ammonium salts is reported: triclinic (P1) (1) and monoclinic (C2/c) (2) polymorphs of monoammonium (NH₄)(H₃P₂O₆), isostructural (NH₄)₅(H₂P₂O₆)(HP₂O₆)·H₂O (4) and (NH₄)₃(HP₂O₆) (5), and tetraammonium (NH₄)₄(P₂O₆) (6), (NH₄)₄(P₂O₆)·H₂O (7) and (NH₄)₄(P₂O₆)·2H₂O (8). Monoammonium hypodiphosphates 1 and 2 crystallize in perovskite-type structures. Form 1 undergoes an order-disorder phase transition related to the dynamics of ammonium cations, the unit-cell doubling and no change in space group type. Dehydration-decomposition of 4 leads to the equimolar mixture of 3 and 5, whereas dehydration of dihydrate 8 proceeds in two steps, resulting in the formation of monohydrate 7 and then anhydrate 6. Compound 7 undergoes a reversible phase transition (P2₁/c ↔ C2/c) concerned with the reorganization of the hydrogen-bond network. Thermal analysis (DSC and TG-DTA) and variable-temperature microsample powder X-ray diffraction were used to analyse the phase transitions, dehydration and decomposition processes.

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), Ge^IVPN₃ and MP₂N₄ (M = Be, Ca, Sr, Ba, Ge^II) 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.

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 Q_e, to represent an electronic instability, and an order parameter Q_co, to represent cation charge ordering, has the form λQ_eQ_co², with T_c(Q_e) < T_c(Q_co), but there must also be indirect coupling through the common strain, e₆, due to strain coupling terms λe₆Q_e and λe₆Q_co². Q_e 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 Q_co 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.

The structure of unique compound Cs₄Ca[Si₈O₁₉] reported recently by Kahlenberg [(2025), Acta Cryst. B81, 325-336] contributes to the broader understanding of structure-property relationships in open-framework silicates (as well as silicates with mixed octahedral-tetrahedral frameworks), with potential implications for ion-exchange, catalysis, and materials with tailored thermal expansion.