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

The crystal structures and hyperfine magnetic parameters of EuFe₃(BO₃)₄ and mixed Eu_0.82La_0.18Fe₃(BO₃)₄ were studied over a wide temperature range in order to analyze correlations of the structural and magnetic features and the phase transitions in multiferroic compounds of the rare-earth iron borate family. The chemical compositions of the crystals are reported from X-ray fluorescence analysis. The crystal structures of EuFe₃(BO₃)₄ and Eu_0.82La_0.18Fe₃(BO₃)₄ were determined using single-crystal X-ray diffraction in the temperature range 25-500 K. A structural phase transition is observed in EuFe₃(BO₃)₄ below 89 K which is related to distortions in the interatomic distances and angles. The most significant of which are for R-O, R-B, R-Fe, Fe-O and Fe-Fe distances, and the angles between the BO₃ triangles and the ab plane. There is no structural phase transition in lanthanum-doped EuFe₃(BO₃)₄ based on specific heat measurements (2.2-101.3 K) and structure analysis (25-500 K), and the temperature dependences of the interatomic distances and angles are smooth. The lengths of the superexchange paths needed for the appearance of a structural phase transition in RFe₃(BO₃)₄ have been proposed. Negative thermal expansion is observed for both compounds below 90 K, resulting from a growth of the interatomic Fe-Fe distances in the iron chains during cooling. The largest atomic displacement parameters are observed for O atoms (O2), indicating that they have the highest mobility. The magnetic properties of EuFe₃(BO₃)₄ and Eu_0.82La_0.18Fe₃(BO₃)₄ were analyzed using Mössbauer spectroscopy in the temperature range 4.5-298 K. Néel temperatures (T_N) of 34.57 (1) and 32.22 (1) K are obtained based on Mössbauer spectroscopy for the pure and doped crystals, respectively. The maximum specific heat capacity temperature dependence related to the magnetic phase transition for the doped crystal is observed at 31.2 K. A violation of the strict arrangement of antiferromagnetic ordering in the ab plane in the La-doped crystals at low temperatures is suggested. The magnetic contributions of the two structural positions of the iron ions to the Mössbauer spectra could not be distinguished in either pure and doped compounds, regardless of whether they are in the paramagnetic and antiferromagnetic regions.

We report a complete set of elastic, piezooptic and photoelastic tensor constants of scheelite crystals CaMoO₄, BaMoO₄, BaWO₄ and PbWO₄ determined by density functional theory (DFT) calculations using the quantum chemical software package CRYSTAL17. The modulation parameter, i.e. the change in the crystal optical path normalized by thickness and mechanical stress, was calculated based on piezooptic and elastic compliance tensor constants. For the geometries of the most effective piezo-optic interactions, this parameter reaches rather large values (16-17) × 10^-12 m² N^-1. Anisotropy of the photoelastic and acoustooptic effects is explored by means of indicative surfaces, considering the directions of light propagation and polarization, the direction of uniaxial compression or lattice distortion caused by the propagation of the acoustic wave. DFT calculations indicate BaWO₄ and PbWO₄ crystals as the most effective acousto-optic materials, predicting the figure of merit constant M₂ ∼ 20 × 10^-15 s³ kg^-1. The methodology proposed combines the DFT calculations and photoelasticity caused by uniaxial compression of the crystal lattice, with particular emphasis on its anisotropy. It can be considered as part of optical engineering aimed at preliminary assessment of the photoelastic properties of crystal materials, thus assisting in their selection for synthesis and relevant applications.

A series of Li⁺/Fe³⁺-doped enstatite crystals of composition Mg_(2-2x)Li_xFe_xSi₂O₆ were synthesized and structurally characterized. Under the selected experimental conditions, we grew three crystals of Pbca orthopyroxene (OPX: x = 0.270-0.313) and two crystals of Pbcn protopyroxene (PPX: x = 0.156-0.164) using the flux-growth technique. The observed variation in the polyhedral volume and distortion of the M2 octahedron as a function of Li/Fe³⁺ doping suggests the presence of an upper limit, at least for the OPX samples. The same linear relation was observed between the polyhedral volume and ⟨M1-O⟩ bond length across all analysed samples, including the endmembers protoenstatite (PEN), orthoenstatite (OEN) and LiFe³⁺Si₂O₆. It seems that the M2 octahedron plays a crucial role in stabilizing the pyroxene topology in either the PEN or the OEN form, because the PPX and OPX samples show two distinct linear relations between the M2O₆ polyhedral volume and ⟨M2-O⟩, with the PPX trend converging toward the parameters of the LiFe³⁺Si₂O₆ endmember, whereas the OPX trend, including OEN, diverges largely from these parameters.

A novel approach to computationally enhance the sampling of molecular crystal structures is proposed and tested. This method is based on the use of extended variables coupled to a Monte Carlo based crystal polymorph generator. Inspired by the established technique of quasi-random sampling of polymorphs using the rigid molecule constraint, this approach represents molecular clusters as extended variables within a thermal reservoir. Polymorph unit-cell variables are generated using pseudo-random sampling. Within this framework, a harmonic coupling between the extended variables and polymorph configurations is established. The extended variables remain fixed during the inner loop dedicated to polymorph sampling, enforcing a stepwise propagation of the extended variables to maintain system exploration. The final processing step results in a polymorph energy landscape, where the raw structures sampled to create the extended variable trajectory are re-optimized without the thermal coupling term. The foundational principles of this approach are described and its effectiveness using both a Metropolis Monte Carlo type algorithm and modifications that incorporate replica exchange is demonstrated. A comparison is provided with pseudo-random sampling of polymorphs for the molecule coumarin. The choice to test a design of this algorithm as relevant for enhanced sampling of crystal structures was due to the obvious relation between molecular structure variables and corresponding crystal polymorphs as representative of the inherent vapor to crystal transitions that exist in nature. Additionally, it is shown that the trajectories of extended variables can be harnessed to extract fluctuation properties that can lead to valuable insights. A novel thermodynamic variable is introduced: the free energy difference between ensembles of Z' = 1 and Z' = 2 crystal polymorphs.

A seventh blind test of crystal structure prediction has been organized by the Cambridge Crystallographic Data Centre. The results are presented in two parts, with this second part focusing on methods for ranking crystal structures in order of stability. The exercise involved standardized sets of structures seeded from a range of structure generation methods. Participants from 22 groups applied several periodic DFT-D methods, machine learned potentials, force fields derived from empirical data or quantum chemical calculations, and various combinations of the above. In addition, one non-energy-based scoring function was used. Results showed that periodic DFT-D methods overall agreed with experimental data within expected error margins, while one machine learned model, applying system-specific AIMnet potentials, agreed with experiment in many cases demonstrating promise as an efficient alternative to DFT-based methods. For target XXXII, a consensus was reached across periodic DFT methods, with consistently high predicted energies of experimental forms relative to the global minimum (above 4 kJ mol^-1 at both low and ambient temperatures) suggesting a more stable polymorph is likely not yet observed. The calculation of free energies at ambient temperatures offered improvement of predictions only in some cases (for targets XXVII and XXXI). Several avenues for future research have been suggested, highlighting the need for greater efficiency considering the vast amounts of resources utilized in many cases.

The presence of magnetic atoms at the A and B sites and the coupling between these two spin subsystems in perovskites gives rise to a variety of exciting effects. In particular this coupling attracts interest from the field of novel multiferroic and magnetoelectric oxides. Moreover, magnetic double perovskites presenting cationic order at the B sites incorporate an additional modulation that can favor symmetry breaking, multiferroic, magnetoelectric and polar phases. Here, we describe the magnetic structures obtained from neutron diffraction and analyze the symmetry properties of well ordered A₂MnB'O₆ double perovskites with A = Lu, Yb, Tm, Er, Ho, Y, Tb, La_0.5Tb_0.5, La and B' = Co or Ni. A rich variety of magnetic orders is formed that have been identified and described, and their symmetry properties are discussed in relation to the multiferroic and magnetoelectric properties of the different compounds.

A seventh blind test of crystal structure prediction was organized by the Cambridge Crystallographic Data Centre featuring seven target systems of varying complexity: a silicon and iodine-containing molecule, a copper coordination complex, a near-rigid molecule, a cocrystal, a polymorphic small agrochemical, a highly flexible polymorphic drug candidate, and a polymorphic morpholine salt. In this first of two parts focusing on structure generation methods, many crystal structure prediction (CSP) methods performed well for the small but flexible agrochemical compound, successfully reproducing the experimentally observed crystal structures, while few groups were successful for the systems of higher complexity. A powder X-ray diffraction (PXRD) assisted exercise demonstrated the use of CSP in successfully determining a crystal structure from a low-quality PXRD pattern. The use of CSP in the prediction of likely cocrystal stoichiometry was also explored, demonstrating multiple possible approaches. Crystallographic disorder emerged as an important theme throughout the test as both a challenge for analysis and a major achievement where two groups blindly predicted the existence of disorder for the first time. Additionally, large-scale comparisons of the sets of predicted crystal structures also showed that some methods yield sets that largely contain the same crystal structures.

During the synthetic exploration targeting the polycrystalline compound LK-99, an unexpected phase, Pb₅(PO₄)₃OH_δ, was identified as a byproduct. We elucidated the composition of this compound through single-crystal X-ray diffraction analysis. Subsequent synthesis of the target compounds was achieved via high-temperature solid-state pellet reactions. The newly identified Pb₅(PO₄)₃OH_δ has an orthorhombic crystal structure with space group Pnma, representing a unique structure differing from the hexagonal apatite phases of Pb₁₀(PO₄)₆O and Pb₅(PO₄)₃OH. Comprehensive temperature- and magnetic-field-dependent magnetization studies unveiled a temperature-independent magnetic characteristic of Pb₅(PO₄)₃OH_δ. Solid-state nuclear magnetic resonance spectroscopy was employed to decipher the origins of the phase stability and confirm the presence of hydrogen atoms in Pb₅(PO₄)₃OH_δ. These investigations revealed the presence of protonated oxygen sites, in addition to the interstitial water molecules within the structure, which may play critical roles in stabilizing the orthorhombic phase.

Two reports on the seventh blind test on crystal structure prediction extensively discuss the cutting-edge avant-garde methods of structure generation and energy ranking.

Y_xOs₄B₄ (x = 1.161) crystallizes as a tetragonal incommensurate composite of columns of Y atoms extending along [001] in an Os₄B₄ framework. The structure was refined using the superspace approach. The basic structure of the Y subsystem can be idealized as having I4/mmm symmetry, with a crystallographically unique Y atom located on the 4/mmm position. The actual superspace symmetry is P4₂/nmc(00σ₃)s0s0. The Y atoms feature only subtle positional modulation in the [001] direction. The Os₄B₄ subsystem [P4₂/ncm(00σ₃)00ss superspace symmetry] is built of columns of edge-sharing Os₄ tetrahedra extending along [001] and B₂ dumbbells. The Os₄ tetrahedra feature pronounced positional modulation with a distinct variation of the Os-Os bond lengths. Modulation of the B₂ dumbbells is best described as a rotation about the [001] axis.