The Experts below are selected from a list of 25851 Experts worldwide ranked by ideXlab platform
Andrea Peluso - One of the best experts on this subject based on the ideXlab platform.
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A series of compounds forming polar crystals and showing single-crystal-to-single-crystal transitions between polar phases
CrystEngComm, 2012Co-Authors: Roberto Centore, Mojca Jazbinsek, Angela Tuzi, Antonio Roviello, Amedeo Capobianco, Andrea PelusoAbstract:Condensation of 4-hydroxybenzohydrazide with a variety of non-chiral aliphatic ketones, methylethylketone, acetone, cyclohexanone and cyclobutanone, affords imine compounds crystallizing in polar space groups (Pna21 or Cc). For all the compounds synthesized, the crystal structure was determined by single crystal X-ray analysis and second harmonic generation experiments were performed on powder samples. The imine obtained using acetone shows solid state polymorphism with unusual behaviour; three different crystal phases have been identified (phases I, II and III), all having the same polar space group Pna21, and the transitions from I to II and from III to II are topotactic, i.e. single-crystal-to-single-crystal. During the irreversible transition from phase I to II, the polar axis undergoes a strong compression (about 15%) and single crystals of the parent phase are violently shattered into single crystal fragments of the new phase, while in the reversible transition from phase III to II, the polar axis expands (about 14%) and the integrity of single crystals is preserved.
Frank H. Herbstein - One of the best experts on this subject based on the ideXlab platform.
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Arrays with Local Centers of Symmetry in Space Groups Pca21 and Pna21
Acta Crystallographica Section B Structural Science, 1998Co-Authors: Richard E. Marsh, Verner Schomaker, Frank H. HerbsteinAbstract:Of the several hundred structures in the Cambridge Structural Database [version 4.6 (1992), Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, England] having space groups Pca2_1 or Pna2_1 and more than one molecule in the asymmetric unit (Z > 4), approximately three-quarters contain local centers of symmetry. These local centers, which are not crystallographic centers, occur predominantly near x = 1/8, y = ¼ in Pca2_1 or near x = 1/8, y = 0 in Pna2_1; this also holds for the limited number of examples we have examined of pseudo-centrosymmetric molecules with Z = 4. Local centers at these points create unusual correlations between corresponding atoms in the two molecules.
Tom Nilges - One of the best experts on this subject based on the ideXlab platform.
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Recent progress and developments in lithium cobalt phosphate chemistry- Syntheses, polymorphism and properties
Journal of Power Sources, 2018Co-Authors: Jennifer Ludwig, Tom NilgesAbstract:Abstract This review summarizes the development, investigation, and optimization of polymorphic lithium cobalt phosphate LiCoPO4. One of the three polymorphs known to date, olivine-type or Pnma-LiCoPO4, shows intriguing electrochemical properties as a high-voltage cathode material, which are of interest for next-generation lithium-ion batteries with higher energy density. Hence, scientists have developed optimization strategies to improve its performance for commercial applications. Herein, a number of procedures for the synthesis of Pnma-LiCoPO4 is presented, including thermodynamic as well as kinetically controlled approaches. The continuous improvement of its electrochemical performance is illustrated, which was realized by the development of solvothermal techniques that allow a precise particle size and morphology control. In the course of these investigations, two new polymorphs, Pna21-LiCoPO4 and Cmcm-LiCoPO4, have been discovered which show different physical and structural properties compared to Pnma-LiCoPO4. Despite their significantly poorer electrochemical performance, these polymorphs allow interesting insights into the variable structure chemistry of transition-metal phosphates, which canalizes in intriguing magnetic and thermal properties. The similarities and differences in the chemical and physical properties of Pnma-LiCoPO4, Pna21-LiCoPO4, and Cmcm-LiCoPO4 are discussed.
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Synthesis and characterization of metastable, 20 nm-sized Pna21-LiCoPO4 nanospheres
Journal of Solid State Chemistry, 2017Co-Authors: Jennifer Ludwig, Dennis Nordlund, Marca M. Doeff, Tom NilgesAbstract:Abstract The majority of research activities on LiCoPO 4 are focused on the phospho-olivine (space group Pnma ), which is a promising high-voltage cathode material for Li-ion batteries. In contrast, comparably little is known about its metastable Pna 2 1 modification. Herein, we present a comprehensive study on the structure–property relationships of 15–20 nm Pna 2 1 -LiCoPO 4 nanospheres prepared by a simple microwave-assisted solvothermal process. Unlike previous reports, the results indicate that the compound is non-stoichiometric and shows cation-mixing with Co ions on the Li sites, which provides an explanation for the poor electrochemical performance. Co L 2,3 -edge X-ray absorption spectroscopic data confirm the local tetrahedral symmetry of Co 2+ . Comprehensive studies on the thermal stability using thermogravimetric analysis, differential scanning calorimetry, and in situ powder X-ray diffraction show an exothermic phase transition to olivine Pnma -LiCoPO 4 at 527 °C. The influence of the atmosphere and the particle size on the thermal stability is also investigated.
Steven W. Rick - One of the best experts on this subject based on the ideXlab platform.
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Simulations of proton order and disorder in ice Ih
The Journal of chemical physics, 2005Co-Authors: Steven W. RickAbstract:Computer simulations of ice Ih with different proton orientations are presented. Simulations of proton disordered ice are carried out using a Monte Carlo method which samples over proton degree of freedom, allowing for the calculation of the dielectric constant and for the examination of the degree of proton disorder. Simulations are also presented for two proton ordered structures of ice Ih, the ferroelectric Cmc21 structure or ice XI and the antiferroelectric Pna21 structure. These simulations indicate that a transition to a proton ordered phase occurs at low temperatures (below 80 K). The symmetry of the ordered phase is found to be dependent on the water potential. The stability of the two proton ordered structures is due to a balance of short-ranged interactions which tend to stabilize the Pna21 structure and longer-range interactions which stabilize the Cmc21 structure.
Roberto Centore - One of the best experts on this subject based on the ideXlab platform.
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A series of compounds forming polar crystals and showing single-crystal-to-single-crystal transitions between polar phases
CrystEngComm, 2012Co-Authors: Roberto Centore, Mojca Jazbinsek, Angela Tuzi, Antonio Roviello, Amedeo Capobianco, Andrea PelusoAbstract:Condensation of 4-hydroxybenzohydrazide with a variety of non-chiral aliphatic ketones, methylethylketone, acetone, cyclohexanone and cyclobutanone, affords imine compounds crystallizing in polar space groups (Pna21 or Cc). For all the compounds synthesized, the crystal structure was determined by single crystal X-ray analysis and second harmonic generation experiments were performed on powder samples. The imine obtained using acetone shows solid state polymorphism with unusual behaviour; three different crystal phases have been identified (phases I, II and III), all having the same polar space group Pna21, and the transitions from I to II and from III to II are topotactic, i.e. single-crystal-to-single-crystal. During the irreversible transition from phase I to II, the polar axis undergoes a strong compression (about 15%) and single crystals of the parent phase are violently shattered into single crystal fragments of the new phase, while in the reversible transition from phase III to II, the polar axis expands (about 14%) and the integrity of single crystals is preserved.