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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Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials, Volume 82, Part 3, 2026
Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials
Introducing the special issue on current research in crystal growth and related characterization
The special issue on crystal growth and related characterization is introduced. The articles were originally published in recent regular issues of Acta Crystallographica Section B.
Structural and magnetic investigations of the Weyl semimetal family CeAlSi1–xGex
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 antiferromagnetic 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.
A systematic investigation is reported of the structural and magnetic evolution in the CeAlSi1–xGex series, combining X-ray and neutron diffraction with magnetization measurements on polycrystalline samples and single crystals. Chemical substitution leads to a continuous lattice expansion and a suppression of the ferromagnetic ordering temperature without a structural phase transition, revealing a crossover towards antiferromagnetic behaviour and establishing CeAlSi1–xGex as a model system for compositionally tuned magnetism in noncentrosymmetric materials.
Investigating the impact of molybdenum content on the room-temperature optical properties of Li2WO4 crystals grown by low-thermal-gradient Czochralski technique
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.
It is shown that even 1.25% of molybdenum in tungstate structure leads to significant decrease in X-ray luminescence intensity. On the other hand, 1.25% molybdenum doping shifts photoluminescence emission to longer wavelengths, the shift increasing with Mo concentration increasing to 5%, without decreasing photoluminescence intensity.
Maus's salts: an old family of iron sulfates with possible magnetic frustration
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.
Structural investigations into Maus's salt derivatives reveal great sensitivity toward alkali metal and cocrystallized water content. Magnetic properties of the Maus's salt iron trimer are explored.
Silicon allotropes by large-volume high-pressure techniques: crystal growth mechanisms, phase diagrams and hexagonal nanostructured Si-6H by in situ X-ray diffraction and computational methods
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.
In situ X-ray diffraction is used for exploring Si crystallography and for studying phase-transformation mechanisms in elemental Si and Na–Si systems. We present original results of hexagonal silicon, Si-6H and a critical analysis of the past-decade discoveries of silicon Si-4H and Si24 using large-volume high-pressure techniques.
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.
From experimental and theoretical techniques employed in this thorough reexamination of the structure of Al5C3N, the crystal structure is better described in centrosymmetric space group P63/mmc (No. 194) rather than in the previously reported noncentrosymmetric space group P63mc (No. 186).
Re-refinement of the structure of the planar hexagonal phase of ZnO nanocrystals
The planar hexagonal phase of ZnO, known as h-ZnO, g-ZnO, α-ZnO, the Bk structure, the 5-5 phase, the α-BN phase, etc., has P63/mmc symmetry and is implicated in ferroelectric switching mechanisms for wurtzite-ZnO. It is well known in thin films on substrates and can be stabilized by external pressure, but its possible existence is critical in high-purity nanocrystals under ambient conditions. Indeed, a crystal structure has been reported, but this work remains controversial as first-principles calculations predict very different structural properties. Herein, the original experimental data is re-refined, through phase-shift determination and Morlet wavelet transformation, that molecular dynamics simulations associate with a P63/mmc structure with unit-cell parameters at room temperature of a = 3.45±0.02 Å and c = 4.46±0.02 Å. These values are 0.35 Å and 0.80 Å, respectively, larger than those previously reported and in good agreement with computational predictions. This confirms that ZnO nanocrystals can form a metastable planar hexagonal phase. It provides key information pertaining to polarization switching in ZnO, its derivatives, and general wurtzite-structured materials.
The planar hexagonal phase of ZnO is significant for understanding ferroelectricity in this class of materials, and it is well established for ZnO in layered systems. Previously it has been reported as freestanding nanocrystals, but the fitted unit-cell parameters differed from predictions by up to 16%; we resolve this anomaly to provide fundamental understanding of ferroelectric switching.
Form (III) of artemisinin: discovery and crystallographic characterization of a new high-pressure polymorph
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 underscore the importance of external factors, such as PTM, in influencing solid-state transitions relevant to pharmaceutical processing and formulation.
The polymorphic behaviour of the three forms of artemisinin was investigated using single-crystal X-ray diffraction in a diamond anvil cell, with notable observation of a pressure-induced phase transition from form (II) to form (III).
α-l-Tyrosine under high pressure: validation of phase transitions and a curious similarity with l-alanine
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.
The effect of hydrostatic compression on the crystal structure of α-l-tyrosine was studied and compared with that reported previously for other amino acids, in particular, for structurally related l-alanine.
Structural modification of functionalized carbazoles: from Z′ = 1 to Z′ = 6
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.
The crystal structure of a 2,3,9,9a-tetrahydro-1H-carbazole in the space group Ia is reported, showing Z′ = 6 and approximate Fdd2 symmetry, and compared to the low Z′ crystal structures of two closely related congeners.
Incommensurate structure of AlPO4-5 and its stability
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.
The incommensurately modulated structure of AlPO4-5 was elucidated by single-crystal X-ray diffraction, revealing correlated rotational displacements of AlO4 and PO4 tetrahedra.
l-threo-d-galacto-Octitol: a curious non-classical order–disorder polytype with an 88 Å axis
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 sugar alcohol l-threo-d-galacto-octitol crystallizes in a structure with P43212 pseudo-symmetry and a highly elongated unit cell (c ≈ 88 Å). In distinct layers of the structure, symmetry is broken by an asymmetric hydrogen-bonding network leading to polytypism, which is discussed in the framework of order–disorder theory.
AB2X4 spinel structures: similarity and differences between the centrosymmetric, Fd3m, and non-centrosymmetric, F4132, space groups
Many compounds adopting the spinel AB2X4 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 Fd3m. 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 F43m, allowing thus small atomic displacements from their original position in Fd3m. In this work, we demonstrate that the loss of the inversion symmetry can occur without any atomic displacements, since the centrosymmetric Fd3m and non-centrosymmetric F4132 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.
Both centrosymmetric, Fd3m, and non-centrosymmetric, F4132, space groups cannot be distinguished in solving, refining and describing AB2X4 spinel structures in the harmonic approximation. They can be distinguished only by using the third-order tensor approximation of anisotropic displacement parameters.
centro- and non-centrosymmetric structures