The composition of phosgenite (ideal formula Pb-2(CO3)Cl-2, sp. gr. P4/mbm with a approximate to 8.15 and c approximate to 8.87 & Aring;) from Monteponi Mine, Iglesias, Sardinia, Italy, its crystal structure and its high-T behaviour up to the onset of decomposition were investigated by a series of chemical analytical and diffraction techniques, including single-crystal X-ray (data collected at 293 K) and neutron diffraction (at 293 and 20 K), in situ high-T powder X-ray diffraction (XRD) and thermogravimetric analysis. Concentrations of >65 elements were measured. The empirical mineralogical formula of phosgenite, obtained by the multi-analytical approach used in this study, is almost identical to the ideal one, with only a few elements measured above the detection limit: Sigma(Na2O+K2O+CaO+SiO2) = 0.11 wt.%. The concentration of other industrially relevant elements is insignificant. X-ray and neutron refinements, based on data collected at room T, confirm the previously reported general structural model of phosgenite, while providing a full description of the displacement parameters of all the atomic sites. The building unit of the crystal structure of phosgenite is represented by a Pb-polyhedron, in which Pb is coordinated by 5Cl + 4O (coordination number CN = 9), forming a monocapped square antiprism. The combination of face-sharing Pb-polyhedra generates dense layers parallel to (001), which are connected by (edge-sharing) CO3-groups to form the crystalline edifice. Low-T neutron diffraction data show evidence of a temperature-mediated phase transition towards a lower symmetry (space group P$\bar 4$), with a modest distortion of the building units of the structure. A tentative description of the low-T mechanisms, at the atomic scale, that can lead to the phase transition is provided.

Neutron diffraction studies of the low-temperature relaxor ferroelectric phases of [NH 4 ]M(HCO 2 ) 3 , where M = Mn 2+ and Zn 2+ , show that a third of the NH 4 + cations remain subtly structurally disordered to low temperature. All NH 4 + cations within the channels are well separated from each other, with significant hydrogen bonds only with the anionic M(HCO 2 ) 3 framework. Complementary studies of the dynamics using 2 H solid state NMR and quasielastic neutron scattering indicate significant rotational motion in both paraelectric and ferroelectric phases, which evolves gradually with increasing temperature with no abrupt change at the phase transition. Nudged elastic band calculations suggest that the activation barrier for flipping between “up” and “down” orientations of the NH 4 + cations is low in the ferroelectric phase, with the NH 4 + cations primarily interacting with the framework rather than neighbouring NH 4 + cations. It is likely this motion that is responsible for scrambling the NH 4 + cation orientation locally in the ferroelectric phase. We propose that this disorder, with the same basic motion active above and below the phase transition, induces the significant dielectric relaxation in these materials. This suggests that orientational disorder may be an effective substitution for compositional disorder commonly associated with relaxor ferroelectrics in molecular materials.

Altermagnetism has been proposed as a distinct class of antiferromagnets exhibiting momentum-dependent spin splitting band structures without requiring spin-orbit coupling. Recently, KV2Se2O has been identified as a metallic room-temperature altermagnet with d-wave spin-momentum locking. Here, we investigate the magnetic structure of KV2Se2O in both polycrystalline and single-crystal samples using neutron diffraction techniques. The system exhibits G-type antiferromagnetic structure with a N & eacute;el temperature TN approximate to 400 K. Notably, substantial broadening of magnetic peaks was observed along L at low temperatures in single crystals, consistent with the coexistence of G-type AFM order and a c-axis spin density wave. These results demonstrate that bulk KV2Se2O cannot host altermagnetism.

The crystal chemistry of inderite, a hydrous borate with known ideal formula MgB3O3(OH)5·5H2O from the Kramer deposit, was re-investigated by electron probe micro-analysis in wavelength dispersive mode, laser ablation-(multi collector-)inductively coupled plasma-mass spectrometry and single-crystal neutron diffraction. The chemical data prove that the real composition of the investigated inderite is substantially identical to the ideal one, with insignificant content of potential isomorphic substituents, so that, excluding B, inderite does not contain any other industrially-relevant element (e.g., Li concentration is lower than 2.5 wt ppm, Be or REE lower than 0.1 wt ppm). The average δ11BNIST951 value of ca. − 7 ‰ lies within the range of values in which the source of boron is ascribable to terrestrial reservoirs (e.g., hydrothermal brines), rather than to marine ones. Neutron structure refinements, at both 280 and 10 K, confirm that the building units of the structure of inderite consist of: two BO2(OH)2 tetrahedra (B-ion in sp3 electronic configuration) and one BO2(OH) triangle (B-ion in sp2 electronic configuration), linked by corner-sharing to form a (soroborate) B3O3(OH)5 ring, and a Mg-octahedron Mg(OH)2(OH2)4. The B3O3(OH)5 ring and the Mg-octahedron are connected, by corner-sharing, to form an isolated Mg(H2O)4B3O3(OH)5 (molecular) cluster. The tri-dimensional edifice of inderite is therefore built by heteropolyhedral Mg(H2O)4B3O3(OH)5 clusters mutually connected by H-bonds, mediated by the zeolitic (“interstitial”) H2O molecules lying between the clusters, so that the correct form of the chemical formula of inderite is Mg[B3O3(OH)5](H2O)4·H2O, rather than MgB3O3(OH)5·5H2O. All the thirteen independent oxygen sites of the structure are involved in H-bonding, as donors or as acceptors. This confirms the pervasive nature and the important role played by the H-bonding network on the structural stability of inderite. The differences between the crystal structure of the two dimorphs inderite and kurnakovite are discussed.

We report detailed structural studies of the low-temperature ferroelectric phases of [NH4]M(HCO2)3, where M = Mn2+ and Zn2+, finding that a third of the NH4+ cations remain subtly rotationally disordered to low temperature in both compounds. All NH4+ cations within the channels are well separated from each other, with significant hydrogen bonds only with the anionic M(HCO2)3 framework. Complementary studies of the dynamics using 2H solid state NMR and quasielastic neutron scattering indicate significant motion in both paraelectric and ferroelectric phases, which evolves gradually with increasing temperature with no abrupt change at the phase transition. Nudged elastic band calculations suggest that the activation barrier for flipping between “up” and “down” orientations of the NH4+ cations is low in the ferroelectric phase, with the NH4+ cations primarily interacting with the framework rather than the neighbouring molecular cations. It is likely this motion that is responsible for scrambling the NH4+ cation orientation locally in the ferroelectric phase. We propose that this disorder, with the same basic motion active above and below the phase transition, induces the significant dielectric relaxation associated with these materials’ relaxor-like dielectric properties.

A number of transition metal hydrides reveal intriguing temperature-dependent JHD in their deuterated derivatives and possibly the temperature dependent hydrogen–hydrogen distance (r(H–H)) as well. Previously, theoretical studies rationalized JHD and r(H–H) changes in such compounds through a “temperature-elastic” structure model with a significant population of vibrational states in an anharmonic potential. Based on the first variable temperature neutron diffraction study of a relevant complex, (p-H-POCOP)IrH2, observation of its elusive counterpart with longer r(H–H), crystallized as an adduct with C6F5I, and thorough spectroscopic and computational study, we argue that the model involving isomeric species in solution at least in some cases is more relevant. The existence of such isomers is enabled or enhanced by solvation and weak non-covalent interactions with solvent, such as halogen or dihydrogen bonds. “Non-classical” hydrides with r(H–H) ≈ 1.0–1.6 Å are especially sensitive to the above-mentioned factors.

The crystal structure and crystal chemistry of meyerhofferite, ideally CaB 3 O 3 (OH) 5 ·H 2 O, was investigated by a multi-methodological approach based on titrimetric determination of boron, gravimetric determination of calcium, determination of fluorine by ion selective electrode, determination of water content by heating, other minor elements by inductively coupled plasma atomic emission spectroscopy, along with single-crystal synchrotron X-ray and neutron diffraction. The concentration of more than 50 chemical elements was measured. The combination of these techniques proves that the composition of meyerhofferite approaches the ideal one (i.e., (Ca 1.012 Mg 0.003 ) (B 2.984 Si 0.001 )O 3 (OH) 5 ·1.018H 2 O), with only a modest fraction of Mg (with MgO ≈ 0.03 wt%) replacing Ca, and with Si the only potential substituent of tetrahedral B (with SiO 2 ≈ 0.02 wt%). The content of REE and other minor elements is, overall, not significant, including that of fluorine as a potential OH − substituent (i.e., < 0.01 wt%). These findings have some relevant geochemical and technical implications, here discussed. The X-ray and neutron structure model obtained in this study prove that the building units of the structure of meyerhofferite consist of: two BO 2 (OH) 2 tetrahedra and one BO 2 (OH) triangle, linked by corner-sharing to form [B 3 O 3 (OH) 5 ] 2− rings, and distorted Ca-polyhedra (with CN = 8, CaO 3 (OH) 4 (OH 2 )), linked by edge-sharing to form infinite chains along [001]. The B 3 O 3 (OH) 5 rings are connected to the Ca-polyhedra chains by corner- and edge-sharing, on two sides of the chains. These heteropolyhedral chains, made by Ca-polyhedra and B 3 O 3 (OH) 5 rings, are mutually connected through hydrogen bonding only, giving rise to the tri-dimensional edifice of meyerhofferite. The neutron structure refinement showed no evidence of static or dynamic disorder pertaining to the H sites; their libration regime was found to be significantly anisotropic. At least seven of the nine oxygen sites of the structure are involved in H-bonding, as donors or as acceptors . The role played by the H-bonding scheme on the physical properties of meyerhofferite is discussed.

The self-assembly and self-organization of water molecules are relevant in many fields of research. When water spontaneously reacts with 2,2,6,6-tetramethylpiperidine (TMP) to form colourless and crystalline discrete needles, only in the exact ratio of 2:1, it is important to understand the phenomenon. Single-crystal X-ray and neutron diffraction data have unveiled that TMP self-assembles around columns of water molecules, and as such, the resulting adduct may be described as a series of molecular water pipes.

A crystal chemical investigation of a natural specimen of whitlockite, ideally Ca9Mg(PO4)6[PO3(OH)], from Palermo Mine (USA), was achieved by means of a combination of electron microprobe analysis (EMPA) in WDS mode, single-crystal neutron diffraction probe (NDP) and single-crystal X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. The crystal-chemical characterization resulted in the empirical formula (Ca8.682Na0.274Sr0.045)Σ9.000(Ca0.034□0.996)Σ1.000(Mg0.533Fe2+0.342Mn2+0.062Al0.046)Σ0.983(P1.006O4)6[PO3(OH0.968F0.032)Σ1.000]. Crystal-structure refinement, in the space group R3c, converged to R1 = 7.12% using 3273 unique reflections from NDP data and to R1 = 2.43% using 2687 unique reflections from XRD data. Unit cell parameters from NDP are a = 10.357(3) Å, c = 37.095(15) Å and V = 3446(2) Å3, and from XRD, the parameters are a = 10.3685(4) Å, c = 37.1444(13) Å and V = 3458.2(3) Å3. NDP results allowed a deeper definition of the hydrogen-bond system and its relation with the structural unit [PO3(OH)]. The FTIR spectrum is very similar to that of synthetic tricalcium phosphate Ca3(PO4)2 and displays minor band shifts due to slightly different P-O bond lengths and to the presence of additional elements in the structure. A comparison between whitlockite, isotypic phases from the largest merrillite group, and its synthetic counterpart Ca3(PO4)2 is provided, based on the XRD/NDP and FTIR results.

High-pressure neutron diffraction data from powder and single-crystal samples of atomically disordered (Fm3¯m) and ordered (Pm3¯m) Pd3Fe were collected up to pressures of 15 GPa, and high-pressure SQUID magnetometry data were collected up to 6 GPa. The data show a subtle decrease in the magnetic moment with applied pressure, resulting in a transition to a paramagnetic state by approximately 8 GPa at 300 K. Diffraction results have been used to determine the equation of state, resulting in a bulk modulus of 176.78(9) GPa for the disordered powder and 187.96(7) GPa for the ordered single-crystal samples, approximately 20% more compressible than previously reported from X-ray measurements. High-temperature superconducting quantum interference device magnetometry was used to confirm the ambient pressure Curie temperature of the sample (545 K), which was further investigated using high-temperature single-crystal neutron diffraction at ambient pressure.

The wavelength dependence of the effective neutron cross section for hydrogen has been investigated by measuring the transmitted total scattering cross section in urea, β-alanine, tartaric acid and polyethylene over the energy range 3 meV to 10 eV. Under the assumption that carbon, nitrogen and oxygen atoms contribute a small and invariant amount to the measured total cross section, these data represent a direct measure of the wavelength dependence of the overall scattering contribution of the hydrogen atoms to the total cross section. These experimental data can be used to apply effective wavelength-dependent corrections to neutron scattering data of hydrogen-rich simple organic compounds.

In this contribution we attempt to answer a general question: can X-ray diffraction data combined with theoretical computations be a source of information about the thermodynamic properties of a given system? Newly collected sets of high-quality multi-temperature single-crystal X-ray diffraction data and complementary periodic DFT calculations of vibrational frequencies and normal mode vectors at the Γ point on the yellow and white polymorphs of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate are combined using two different approaches, aiming to obtain thermodynamic properties for the two compounds. The first approach uses low-frequency normal modes extracted from multi-temperature X-ray diffraction data (normal coordinate analysis), while the other uses DFT-calculated low-frequency normal mode in the refinement of the same data (normal mode refinement). Thermodynamic data from the literature [Yang et al. (1989), Acta Cryst. B45, 312–323] and new periodic ab initio DFT supercell calculations are used as a reference point. Both approaches tested in this work capture the most essential features of the systems: the polymorphs are enantiotropically related, with the yellow form being the thermodynamically stable system at low temperature, and the white form at higher temperatures. However, the inferred phase transition temperature varies between different approaches. Thanks to the application of unconventional methods of X-ray data refinement and analysis, it was additionally found that, in the case of the yellow polymorph, anharmonicity is an important issue. By discussing contributions from low- and high-frequency modes to the vibrational entropy and enthalpy, the importance of high-frequency modes is highlighted. The analysis shows that larger anisotropic displacement parameters are not always related to the polymorph with the higher vibrational entropy contribution.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.