Many compounds adopting the spinel AB(2)X(4) structure are technologically important owing to their tunable physical and chemical properties enabling diverse applications in energy storage, catalysis, magnetism, and functional ceramics. Most of them are traditionally assigned to the centrosymmetric space group Fd (3) over barm. However, the physical properties of some spinels are incompatible with centrosymmetry. This discrepancy is often accounted for by reducing the symmetry to the non-centrosymmetric space group F (4) over bar 3m, allowing thus small atomic displacements from their original position in Fd (3) over barm. In this work, we demonstrate that the loss of the inversion symmetry can occur without any atomic displacements, since the centrosymmetric Fd (3) over barm and non-centrosymmetric F4(1)32 space groups are equivalent for structure determination and refinement based on X-ray diffraction data. If consistent with experiment, only the use of an anharmonic model of atomic displacements can distinguish these space groups. This study aims to clarify certain misconceptions regarding the structural symmetry and physical properties of spinel-type compounds.
Metastable allotropes of silicon recovered from high-pressure conditions exhibit a wide range of crystal structures, physical properties and transformation pathways that remain only partially understood despite decades of study. This article combines original crystallographic observations with a critical review of phase transformations, nucleation mechanisms and crystal growth processes in elemental Si and Na-Si systems synthesized under high-pressure, high-temperature conditions. Using in situ diffraction data, structural characterization and computational approaches, we analyze how symmetry breaking, lattice instabilities and kinetic constraints govern the formation of dense polymorphs (Si-II, Si-III, Si-XI) and open-framework structures, including clathrate and channel phases. Particular attention is given to the role of large-volume synthesis and chemically assisted growth routes in controlling phase selection, defect formation and recoverability. The evolution of hexagonal polytypes, including nanostructured 6H silicon, is discussed in terms of stacking modifications driven by stress release and thermal treatment. By integrating crystallographic relations, thermodynamic considerations and growth kinetics, this work identifies phase-transformation mechanisms as the key factor linking structure, synthesis conditions and functional properties of silicon allotropes. The results provide a unified framework for understanding crystal growth at high pressure and offer guidance for the controlled synthesis of advanced silicon materials.
GalC8 [L-threo-D-galacto-octitol, systematic name (2S,3R,4R,5R,6R,7S)-octane-1,2,3,4,5,6,7,8-octol] crystallizes with a pseudo-tetragonal lattice featuring a long (∼88 Å) c axis. The bulk of the structure possesses P43212 (Z' = 1) pseudo-symmetry, whereby molecules are arranged linearly with eight molecules in a translation period, thus explaining the long c axis. According to this pseudo-symmetry, adjacent molecules are alternately related by 21 screw rotations (in 〈100〉 directions) and twofold rotations (in 〈110〉 directions). Due to an asymmetrical hydrogen-bonding network, symmetry is broken in the vicinity of the twofold rotation axes, leading to an overall P21 (Z' = 4) space-group symmetry. The lost symmetry is retained as a twin operation. The structure can be classified as a `non-classical' OD polytype, in the sense that there are nonequivalent layer contacts, yet all members of the polytype family are locally equivalent. The main polytype is not of a maximum degree of order (MDO) and can be described as an alternation of fragments of the two MDO polytypes with P43 and P212121 symmetry, respectively.
The incommensurate structure of AlPO4-5 and its stability have been investigated in detail. Under ambient conditions, single-crystal X-ray diffraction measurements reveal weak satellite reflections, which become significantly enhanced upon exposure to vacuum. Temperature-dependent X-ray diffraction and Raman scattering measurements of the O-H stretching region show that the appearance of satellite reflections is due to the desorption of water molecules under vacuum, and the satellite reflections disappear above 335 (5) K, whereas their intensity increases again upon cooling to room temperature. To elucidate the origin of the modulation, single-crystal X-ray diffraction experiments were performed on a dehydrated sample. Diffraction data including satellite reflections up to second order were successfully collected, with a modulation vector of q = 0.36710 (5)c*. Analysis of systematic absences up to second-order satellites indicate that the (3+1)D superspace group is P6(00γ)h. On the basis of these data, the first-order positional modulation amplitudes were refined. The results indicate that the modulation originates from correlated rotational displacements of AlO4 and PO4 tetrahedra. This behaviour suggests that phonon-like motions, as proposed in previous studies, persist over a long modulation period and give rise to the observed satellite reflections. These findings highlight a fundamental connection between the intrinsically spatial description of atomic positional modulations in the incommensurate structure and the underlying temporal dynamics.
Crystal structures with asymmetric units comprising large numbers of molecules (Z ' > 4) remain rare. Their study provides a unique opportunity to further the understanding of the physical basis for crystal formation, supporting the advancement of crystal structure prediction, crystal engineering and the study of polymorphism. Here the crystal structures of a series of three related 2,3,9,9a-tetrahydro-1H-carbazoles are discussed, one of which exhibits an asymmetric unit comprising six crystallographically independent molecules (Z ' = 6). Careful analysis of this structure reveals approximate Fdd2 symmetry and strong evidence for this being linked to a symmetry-reducing, low-temperature phase transition. Comparison of the structures reveals that the variation in Z ' values is linked to conformational changes related to the lack of classical hydrogen bonding in the high-Z ' structure.
Single-crystal X-ray diffraction is one of the most reliable techniques for elucidating the molecular structures, while it requires high-quality single crystals. The crystalline mate (CM) strategy has recently emerged as an effective method to induce rapid and selective co-crystallization. Here, we demonstrate that this strategy can be reliably extended to ionic systems using the neutral trinuclear silver pyrazolate complex Ag(3)Pz(3) as a crystallization mate. By regulating solvent polarity, two co-crystals of silver pyrazolate berberine hydrochloride Ag(3)Pz(3)center dot BCl, SF (sandwich form) and NSF (non-sandwiched form), were obtained. In SF, neutral Ag(3)Pz(3) simultaneously engages the Cl anion via pi-acid/base contacts and the berberine cation through weak Ag center dot center dot center dot O coordination. Remarkably, SF can also be crystallized directly from crude Coptis chinensis extract without prior purification. In NSF, due to the influence from methanol or ethanol, Ag(3)Pz(3) rearranges into a rare anionic Ag(3)Pz(4) cluster that serves as the counterion to berberine. These results establish solvent polarity as a decisive factor governing CM-mediated co-crystal assembly and broaden the applicability of the crystalline-mate concept to ionic systems.
A series of 12 compounds: Na(H3P2O6) (1 and 2), Na(H3P2O6)center dot H2O (3), Na-2(H2P2O6) (4 and 5), Na-2(H2P2O6)center dot 2H(2)O (6), Na-2(H2P2O6)center dot 6H(2)O (7 and 8), Na-5(H2P2O6)(HP2O6)center dot 21H(2)O (9), Na-3(HP2O6) (10 and 11) and Na-3(HP2O6)center dot 9H(2)O (12) has been characterized by variable-temperature optical microscopy, thermogravimetry, single-crystal and powder X-ray diffraction. Crystalline salts 1-4, 6-10 and 12 have been obtained in typical solution crystallization processes. Dehydrations of Na(H3P2O6)center dot H2O (3), Na-2(H2P2O6)center dot 6H(2)O (both 7 and 8) and Na-3(HP2O6)center dot 9H(2)O (12) were monitored by TGA, variable-temperature microscopy and powder or microsample powder X-ray diffraction, and turned out to be one-step, destructive transformations, resulting in almost pure triclinic form of Na(H3P2O6) (1), alpha-Na-2(H2P2O6) (4) and alpha-Na-3(HP2O6) (10), respectively. Sodium ionic conductivity was calculated using the bond valence sum method for all obtained anhydrous substances and measured for the best candidate, compound 4 in the temperature range 300-420 K, reaching 10(-4) S m(-1).
The nature of the phenomenon of negative thermal expansion was studied at the atomic level using the in-situ single-crystal and powder high-temperature X-ray diffraction (HTXRD) of Ho,Tm-doped Yb2Mo3O12 crystals grown by the fluxmelt technique. Phase transitions, structure deformation and luminescence were investigated in the temperature range 303-1273 K in air. Under ambient conditions, Ho,Tm-codoped Yb2Mo3O12 has monoclinic crystal structure of Al2W3O12 (P2(1)/c) structure type: a = 16.554 (2), b = 9.859 (1), c = 16.667 (2) angstrom, beta = 107.88 (1)degrees, V = 2588.6 (5) angstrom(3). A reversible transformation monoclinic <-> orthorhombic occurs at about 320 K according to the HTXRD data. The orthorhombic structure [Pbcn, a = 13.7388 (2), b = 9.8582 (2), c = 9.9472 (2), V = 1347.25 (4) angstrom(3) at 373 K] shows remarkable negative volumetric thermal expansion up to about 1173 K (average alpha(v) = 15 x 10(-6) K-1); above this temperature molybdate starts to evaporate. Monoclinic modification in contrast to the orthorhombic one expands only positively. Careful analysis of the orthorhombic crystal structure from non-ambient single-crystal diffraction data showed that the bond lengths in MoO4 and YbO6 polyhedra do not change with temperature; the decrease of structure volume is due to the angular deformation of the framework built from MoO4 and YbO6 polyhedra. This study demonstrates the potential of Yb3+, Ho3+, Tm3+- codoped crystalline systems for developing a highly sensitive ratiometric optical thermometer by exploiting efficient energy transfer, a negative thermal expansion-induced emission stabilization at.655 nm within 570 to 800 K, and phonon-activated population of the Tm3+ F-3(3) level for similar to 700 nm emission under 980 nm excitation.
Supramolecular polymers have attracted considerable attention owing to their potential applications in biomedicine, molecular machines, sensing, and self-healing materials. Precise evaluation of hydrogen-bond energies is crucial for the rational design of these materials. Here, the experimental electron densities (EEDs) of three representative quadruple hydrogen-bonded dimers, ureido-pyrimidinone (DDAA, DADA. D: donor; A: acceptor) and ureidotriazine (DADA), are investigated using high-resolution single-crystal X-ray diffraction. Comparative energy decomposition analyses (EDA) of the experimental and theoretical electron densities are performed. The total interaction energies from the EDA of EEDs are in good agreement with the experimentally determined binding constants, correcting the discrepancies observed in theoretical EDA and in the typical secondary electrostatic interaction model. Our results confirm the pivotal role of long-range electrostatic interactions in governing multiple hydrogen-bonded systems, and the EDA-EED approach provides a benchmark for theoretical methods that accurately describe long-range electrostatic interactions, which are fundamentally important for the development of supramolecular systems.
The baric behavior of α-L-tyrosine, an aromatic amino acid, was investigated using single-crystal X-ray diffraction. Around 2 GPa, the nonlinear behavior in the compression of unit-cell parameters a and c was observed, as N-H...O hydrogen bonds involving the NH3+ group switched over. This rearrangement is remarkably similar to the changes observed in crystalline L-alanine at comparable pressures. A comparison of intermolecular interactions in L-alanine and α-L-tyrosine is performed in order to suggest an interpretation of structural strain on hydrostatic compression.
Artemisinin (ART) is mainly used for the treatment of malaria and exhibits polymorphism with two known crystalline forms. In this study, the high-pressure behaviour of these two polymorphs was investigated to evaluate their compressibility and identify if any pressure-induced phase transitions occur with a view to assessing the impact of manufacturing pressure on the active pharmaceutical ingredient. Form (I), the orthorhombic polymorph, is found to be the most compressible of the three. Form (II), a triclinic phase, undergoes a phase transition to a new polymorph that is observed at different pressures depending on the pressure-transmitting medium (PTM) used. The transition to form (III) occurs at 0.75 GPa when compressed in petroleum ether, however, this transition is delayed to 2.02 GPa in silicone oil. This highlights the influence of the PTM on the stability of the crystal structure. The newly characterized form (III) shares structural similarities with form (II) but differs in symmetry where a pseudo-21 screw axis in form (II) becomes a formal 21 screw axis in form (III), resulting in a change from triclinic to monoclinic and a reduction of the asymmetric unit from Z' = 4 to Z' = 2. These findings contribute to a deeper understanding of pressure-induced polymorphism in ART and underscores the importance of external factors such as PTM in influencing solid-state transitions relevant to pharmaceutical processing and formulation.
The family of Maus's salts has been known for several centuries, but many aspects of their structures remain elusive, particularly because most examples are highly solvated and readily lose water. To examine the role of alkali cations and water on the structure, and to resolve some ambiguities of previously reported supercells, several new Maus's salt derivatives were synthesized and characterized by low-temperature X-ray diffraction. The parent Maus's salt, K5[Fe3O(SO4)6(H2O)3]·5H2O, grows in hexagonal P63/m, with trigonal symmetry of the Fe3+ clusters, conditions that allow for potential magnetic frustration. A new caesium ferric sulfate complex, Cs5[Fe4O2(HSO4)(SO4)6(H2O)3]·1.75H2O, was also synthesized under the same conditions. It is not a Maus's salt derivative but instead contains Fe3+ tetramers of corner- and edge-sharing iron octahedra, decorated by seven sulfate tetrahedra, crystallizing in monoclinic space group C2/c. Preliminary orientation-dependent magnetic data relative to the trigonal c axis were collected on single crystals of the parent Maus's salt. The data suggests very weak short-range antiferromagnetic spin reorientations with a subtle broad rolling feature having a local maximum near 60-70 K. Magneto-structural anisotropy and spin saturation moments, one-tenth those expected for high-spin Fe3+, suggest a splayed magnetic structure with spin moments predominantly in the trimer plane directed inward about the local z axis toward the shared oxygen between trimers.
Li2WO4 single crystals and Mo-doped Li2W1-xMoxO4 (x = 0.0125, 0.05) were grown by the low-thermal-gradient Czochralski technique. Optimal crystallization conditions were determined for obtaining single crystals with a length of 50 mm and a diameter of 30 mm: crystallization rate of 0.5 mm h-1 and rotation rate of 5 rpm. The crystal structure was investigated using powder X-ray diffraction, which confirmed the single-phase nature and trigonal structure (space group R3) for all compositions. Differential scanning calorimetry did not reveal phase transitions in the temperature range 303-1153 K. The influence of partial substitution of W with Mo on the luminescence properties is demonstrated and a correlation between Mo concentration and emission characteristics is established.
We report a systematic study of the structural and magnetic evolution in CeAlSi1-xGex, a series of materials which provide a tunable platform for exploring magnetism in noncentrosymmetric Ce-based intermetallics. Polycrystalline samples and single crystals were synthesized using arc melting and flux growth techniques. Structural characterization by X-ray diffraction shows a continuous increase in the unit-cell parameters with increasing Ge content, with no evidence of a structural phase transition across the series. Magnetization measurements reveal a suppression of the ferromagnetic ordering temperature of CeAlSi with increasing Ge substitution, indicating a crossover toward anti-ferromagnetic behaviour in Ge-rich compositions. Neutron diffraction measurements performed on selected compositions show that weak magnetic intensity appears on some structural Bragg peaks below the magnetic ordering temperature. These results elucidate the relationship between chemical substitution, crystal structure, and magnetic ground states in CeAlSi1-xGex, and establish this system as a model platform for studying compositionally tuned magnetic order in noncentrosymmetric materials.
High-temperature ceramic compound Al5C3N belongs to the scarcely studied Al-C-N system. It was originally reported as an ordered compound in noncentrosymmetric space group P63mc and described as a nanolaminate with an -Al2C-AlN-Al2C2- stacking sequence. The recently reported structural disorder in the related compound Al4SiC4 led us to question this proposed structure for Al5C3N and investigate the possibility of a disordered structure in the centrosymmetric space group P63/mmc. In the present work, we employed different synthesis routes to maximize the yield and quality of the desired phase, and applied a variety of techniques to probe the Al5C3N crystal structure. Our single-crystal X-ray diffraction analysis clearly indicates that noncentrosymmetric space group P63mc must be rejected. From a joint refinement of single-crystal X-ray and neutron powder diffraction data, the occupancies of C and N were refined at two sites in P63/mmc resulting in the stacking sequence -Al2C-Al(C/N)-Al2(C/N)2-. Furthermore, DFT calculations show that a centrosymmetric disordered structure described in a supercell has the lowest energy, 0.2 eV per formula unit, relative to the previously reported P63mc structure. The calculated band structure shows both direct and indirect bandgaps, which leads to implications for the physical properties. Finally, STEM analysis provides additional evidence that the crystal structure of Al5C3N is better described in centrosymmetric space group P63/mmc.
Crystal chemical rules are widely applied to understand and rationalize inorganic crystal structures. Examples of such include Pauling rules and Beck's extended coordination number rule. For the latter no large-scale benchmark has been performed yet. A rigorous analysis of the validity of the extended coordination number rule was performed on a dataset consisting of 1038 ternary fluorides. 59.9% of the examined crystal structures agree with Beck's rule. An additional 22.3% of structures the deviations can be explained by a more or equally homogenous distribution of charges, thus yielding a total rationalization power of 82.2%. The structural complexity of the crystal structures was identified as a main reason for disagreement with Beck's rule.
Although there have been several studies on the powder form, there has yet to be a report on the bulk crystal growth and its property studies of LiNiO 2 . In the present study, we report the first successful growth of a LiNiO 2 single crystal by employing the optical floating-zone technique. Structural properties have been studied using single-crystal X-ray diffraction (XRD). The structural refinement of the single-crystal XRD data, along with the Laue diffraction patterns, confirms that this system crystallizes in a rhombohedral unit cell in space group R 3 m and the presence of a single grain along the length of the grown crystal. Furthermore, for the first time, we have observed and determined their superstructures as a function of temperature using single-crystal XRD. We have also conducted a study using the high flux of synchrotron X-rays to demonstrate the mechanism which drives the superstructure witnessed by the single-crystal XRD. Resonant elastic X-ray scattering was used to confirm the superstructure and the mixed valence state of different Ni sites. No additional ordering phenomena were observed, including magnetic or electronic ordering. Our study demonstrates the optimization of LiNiO 2 growth parameters and provides information about atomic and electronic ordering in the system, including the onset of a superstructure phase. This will provide a basis for further work in developing improved cathode materials and understanding quantum spin liquids.
Molecular dynamics simulation method is used to study the process of methane hydrate formation induced by sodium dodecyl sulfate (SDS) at a concentration of 28.6 mmol L-1. We demonstrate the transformation of water molecules from disordered arrangement in liquid state to ordered hydrate state using F3 and F4 order parameters, the number of water molecules in different phase states, and the system energy. Besides, not only are the 512 and 51262 type water cages formed, but also some uncommon cages, such as 4151062 cages, proving that the obtained hydrate is not a standard structure I hydrate. The water molecules layer near SDS cannot form four hydrogen bonds with the outer water molecules, and thus do not contribute to the formation of solid hydrate, but rather appears as liquid. So, some methane cannot be confined in the water cage and form bubbles near the SDS hydrophobic tail chain. Thus, SDS contributes to the formation of methane hydrates at 28.6 mmol L-1, but the final methane hydrate structure is messy.
The growth dynamics of faceted materials exhibit slow attachment kinetics, strong solid-liquid anisotropy, and specific kinetic coefficients. The formation of structures, facet dynamics and defects during growth strongly influence the properties of the resulting materials. Using the transparent model material salol and the method of controlled directional solidification, this work aims to provide a deeper understanding of the formation of structures in faceted materials, which remain insufficiently understood. The study reveals that after an initial transient period, the solid-liquid interface stabilizes, with its position linked to the interface undercooling. Facet size heterogeneity is characterized with a tendency to larger facets for lower thermal gradients. A methodology is developed to exploit experimentally measured angles into a three-dimensional angle in the crystals, allowing the identification of the crystallographic nature of the facets. Despite random and unknown initial orientations, the interface is predominantly bounded by {111} planes. Facet velocities are analyzed, and deviations from theoretical predictions are attributed to competitive growth processes. The analysis of facet velocities as a function of undercooling suggests a growth mechanism primarily controlled by two-dimensional nucleation. It is shown that the experimental data relating the growth rate of {111} facets to the undercooling correspond to bi-dimensional kinetic growth law in most cases. Only a few data point can be related to a kinetic growth law driven by screw dislocation emerging at the {111} facet surface.
Polymers have been widely used to physically stabilize amorphous drugs by forming amorphous solid dispersions (ASDs), resulting in commercial and clinical success as a pharmaceutical technique to improve the bioavailability of a poorly water-soluble drug. However, the role of polymers in maintaining the physical stability of ASDs has not been fully understood. Herein, we investigated how poly(methyl methacrylates) (PMMAs) with different tacticities impact the liquid dynamics and crystallization kinetics of amorphous griseofulvin (GSF). PMMAs with similar chain lengths and identical monomer structures were selected, aiming to exclude effects arising from differences in monomer structure and end groups. The syndiotactic form of PMMA (s-PMMA) exhibited a stronger inhibitory effect on the crystal growth of amorphous GSF in comparison with isotactic (i-PMMA) and atactic (a-PMMA) forms. Effects of the isotactic atactic forms of PMMA on the crystal growth of GSF can be mainly attributed to their molecular mobility, as shown by the overlapping of the logarithm growth rate curves versus viscosity and α-relaxation time. However, the crystal growth rate curves of GSF in the system containing 10 wt% s-PMMA did not overlap with those of the pure GSF system. These results suggest that liquid dynamics is not a main contributor to the inhibitory effect of s-PMMA during drug crystallization.
