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basq crystal  basq  crystal  model  modulation  mullite  ordered  phase  ray diffraction  single crystal  structure  superspace  temperature 
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Acta Crystallographica Section B



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



Published: 2017-05-12

 



Temperature-induced reversible structural phase transition and X-ray diffuse scattering in 2-amino-3-nitropyridinium hydrogen sulfate

2017-05-12

The novel polar material 2-amino-3-nitropyridinium hydrogen sulfate, C5H6N3O2(HSO4) (abbreviated as 2A3NP-HS), was obtained and structurally characterized by means of single-crystal X-ray diffraction. At room temperature, 2A3NP-HS crystallizes as a non-centrosymmetric disordered phase (I) in the orthorhombic Pna21 space group. On cooling below 298 K, 2A3NP-HS undergoes a reversible phase transition to phase (II) with the monoclinic non-centrosymmetric P21 space group. This transition might be classified as an `order–disorder' type. The structural details in both phases are analysed. Additionally, for phase (I), in the 304–365 K temperature range, diffuse scattering was found to be present in the form of elongated streaks parallel to the a* direction. This can be unravelled when implementing a short-range order affecting anionic cationic ribbons occurring in the structure, with correlations acting both in the a-direction and in the bc-plane. The results of Monte Carlo simulations, adapting a two-dimensional Ising-type model, reveal the formation of domains, which are b-elongated and thin along a. Locally, the stacking of the ribbons in the domains reflects the ordered arrangement observed in the low-temperature monoclinic phase (II).



Growth, crystal structure, Hirshfeld surface, optical, piezoelectric, dielectric and mechanical properties of bis(l-asparaginium hydrogensquarate) single crystal

2017-05-12

Molecular organic single crystals of bis(l-asparaginium hydrogensquarate) monohydrate [BASQ; (C8H10N2O7)2·H2O] have been grown by solution technique. Crystallographic information was investigated by single-crystal X-ray diffraction (SCXRD) analysis. Hirshfeld surface and fingerprint plot studies were performed to understand the intermolecular interactions of the BASQ crystal in graphical representation. Functional group identification was studied with FT–IR (Fourier transform–IR) spectroscopy. The positions of proton and carbon atoms in the BASQ compound were analyzed using NMR spectroscopy. High transparency and a wide band gap of 3.49 eV were observed in the linear optical study by UV–vis–NIR spectroscopy. Intense and broad photoluminescence emissions at room temperature were observed in blue and blue–green regions. The frontier molecular orbitals of the BASQ molecule were obtained by the DFT/B3LYP method employing 6-311G** as the basis set. The dielectric study was carried out with temperature at various frequency ranges. The piezoelectric charge coefficient (d33) value of BASQ crystal was found to be 2 pC/N, which leads to its application in energy harvesting, mechanical sensors and actuators applications. In the non-linear optical study, the BASQ crystal showed promising SHG conversion efficiency. Mechanical properties of the BASQ crystal were studied experimentally by Vicker's microhardness technique, which revealed that the grown crystal belonged to the softer category. BASQ crystal void estimation reveals the mechanical strength and porosity of the material.



Identification of a deleterious phase in photocatalyst based on Cd1 − xZnxS/Zn(OH)2 by simulated XRD patterns

2017-05-12

The X-ray diffraction (XRD) pattern of a deleterious phase in the photocatalyst based on Cd1 − xZnxS/Zn(OH)2 contains two relatively intense asymmetric peaks with d-spacings of 2.72 and 1.56 Å. Very small diffraction peaks with interplanar distances of (d) ≃ 8.01, 5.40, 4.09, 3.15, 2.49 and 1.35 Å are characteristic of this phase but not always observed. To identify this phase, the XRD patterns for sheet-like hydroxide β-Zn(OH)2 and sheet-like hydrozincite Zn5(CO3)2(OH)6 as well as for turbostratic hydrozincite were simulated. It is shown that the XRD pattern calculated on the basis of the last model gives the best correspondence with experimental data. Distances between layers in the turbostratically disordered hydrozincite fluctuate around d ≃ 8.01 Å. This average layer-to-layer distance is significantly higher than the interlayer distance 6.77 Å in the ordered Zn5(CO3)2(OH)6 probably due to a deficiency of CO32− anions, excess OH− and the presence of water molecules in the interlayers. It is shown by variable-temperature XRD and thermogravimetric analysis (TGA) that the nanocrystalline turbostratic nonstoichiometric hydrozincite-like phase is quite thermostable. It decomposes into ZnO in air above 473 K.



Could incommensurability in sulfosalts be more common than thought? The case of meneghinite, CuPb13Sb7S24

2017-05-12

The structure of meneghinite (CuPb13Sb7S24), from the Bottino mine in the Apuan Alps (Italy), has been solved and refined as an incommensurate structure in four-dimensional superspace. The structure is orthorhombic, superspace group Pnma(0β0)00s, cell parameters a = 24.0549 (3), b = 4.1291 (6), c = 11.3361 (16) Å, modulation vector q = 0.5433 (4)b*. The structure was refined from 6604 reflections to a final R = 0.0479. The model includes modulation of both atomic positions and displacement parameters, as well as occupational waves. The driving forces stabilizing the modulated structure of meneghinite are linked to the occupation modulation of Cu and some of the Pb atoms. As a consequence of the Cu/[] and Pb/Sb modulations, three- to sevenfold coordinations of the M cations (Pb/Sb) occur in different parts of the structure. The almost bimodal distribution of the occupation of Cu/[] and Pb/Sb at M5 conforms with the coupled substitution Sb3+ + [] → Pb2+ + Cu+, thus corroborating the hypothesis deduced previously for the incorporation of copper in the meneghinite structure. The very small departure (∼0.54 versus 0.50) from the commensurate value of the modulation raises the question of whether other sulfosalts considered superstructures have been properly described, and, in this light, if incommensurate modulation in sulfosalts could be much more common than thought.



Ordered vacancy distribution in 2/1 mullite: a superspace model

2017-05-12

A mullite single crystal with composition Al4.84Si1.16O9.58 (2) exhibiting sharp satellite reflections was investigated by means of X-ray diffraction. For the refinement of a superspace model in the superspace group Pbam(α0½)0ss different scale factors for main and satellite reflections were used in order to describe an ordered mullite structure embedded in a disordered polymorph. The ordered fraction of the mullite sample exhibits a completely ordered vacancy distribution and can be described as a block structure of vacancy blocks (VBs) that alternate with vacancy-free blocks (VFBs) along a and c. The incommensurate nature of mullite originates from a modulation of the block size, which depends on the composition. The displacive modulation is analyzed with respect to the vacancy distribution and a possible Al/Si ordering scheme is derived, although the measurement itself is not sensitive to the Al/Si distribution. An idealized, commensurate approximation for 2/1 mullite is also presented. Comparison of the ordered superspace model with different preceding models reconciles many key investigations of the last decades with partly contradicting conclusions, where mullite was usually treated as either ordered or disordered instead of considering simultaneously different states of order.