Matthew R. Suchomel

Monoclinic crystal structure of polycrystalline Na0.5Bi0.5TiO3

Bismuth-based ferroelectric ceramics are currently under intense investigation for their potential as Pb-free alternatives to lead zirconate titanate-based piezoelectrics. Na0.5Bi0.5TiO3 (NBT), one of the widely studied compositions, has been assumed thus far to exhibit the rhombohedral space group R3c at room temperature. High-resolution powder x-ray diffraction patterns, however, reveal peak splitting in the room temperature phase that evidence the true structure as monoclinic with space group Cc. This peak splitting and Cc space group is only revealed in sintered powders; calcined powders are equally fit to an R3c model because microstructural contributions to peak broadening obscure the peak splitting.

High pressure bulk synthesis and characterization of the predicted multiferroic Bi(Fe1∕2Cr1∕2)O3

Bi(Fe1∕2Cr1∕2)O3, a recently proposed candidate multiferroic perovskite, is prepared in a bulk form by high pressure solid-state synthesis. The material is isostructural with polar BiFeO3 but is paramagnetic at room temperature due to disorder of the Fe3+ and Cr3+ cations on the B site. Mossbauer and magnetization measurements show a transition to a cooperative magnetic state below 130K.

Highly Transparent BaAl4O7 Polycrystalline Ceramic Obtained by Full Crystallization from Glass

Transparent polycrystalline ceramics are an emerging class of photonic quality materials competing with single crystal technology for a diverse range of applications including high-energy lasers, scintillating devices, optical lenses, and transparent armour. Polycrystalline ceramics offer several advantages, particularly in the fabrication of complex shapes and large-scale industrial production, and enable greater and more homogenous doping of optically active ions than is possible in single crystals. A limited number of either cubic or nanocrystalline transparent polycrystalline ceramics are known, but require complex and time-consuming synthetic approaches. Here, we show for the fi rst time that fully dense transparent polycrystalline ceramics can be simply obtained by direct and complete crystallization from glass. This is demonstrated for the previously unreported composition, BaAl 4 O 7 , which exhibits two orthorhombic polymorphs with micrometer grain size, both optically transparent in the visible range. This innovative synthetic route to transparent polycrystalline ceramics should facilitate the discovery of new, cost-effective chemical methods for transparent ceramic applications. Conventional optically transparent single crystal materials are widely used in numerous photonic applications. However, these materials face several technological and economical challenges, including a restricted list of appropriate single crystal compounds, limitations on the type and level of chemical doping, and mechanical and manufacturing requirements for large and complex physical shapes. Many of these obstacles can be avoided through the use of ceramic materials, which afford a wider range

Structure Resolution of Ba5Al3F19 and Investigation of Fluorine Ion Dynamics by Synchrotron Powder Diffraction, Variable-Temperature Solid-State NMR, and Quantum Computations

The room temperature structure of Ba(5)Al(3)F(19) has been solved using electron microscopy and synchrotron powder diffraction data. One-dimensional (1D) (27)Al and ultrafast magic-angle-spinning (MAS) (19)F NMR spectra have been recorded and are in agreement with the proposed structural model for Ba(5)Al(3)F(19). The (19)F isotropic chemical shift and (27)Al quadrupolar parameters have been calculated using the CASTEP code from the experimental and density functional theory geometry-optimized structures. After optimization, the calculated NMR parameters of both the (19)F and (27)Al nuclei show improved consistency with the experimental values, demonstrating that the geometry optimization step is necessary to obtain more accurate and reliable structural data. This also enables a complete and unambiguous assignment of the (19)F MAS NMR spectrum of Ba(5)Al(3)F(19). Variable-temperature 1D MAS (19)F NMR experiments have been carried out, showing the occurrence of fluorine ion mobility. Complementary insights were obtained from both two-dimensional (2D) exchange and 2D double-quantum dipolar recoupling NMR experiments, and a detailed analysis of the anionic motion in Ba(5)Al(3)F(19) is proposed, including the distinction between reorientational processes and chemical exchange involving bond breaking and re-formation.

An NMR‐Driven Crystallography Strategy to Overcome the Computability Limit of Powder Structure Determination: A Layered Aluminophosphate Case

Along with the growing complexity of many inorganic systems, structure determination from powders is getting more and more difficult, and represents a severe obstacle for the discovery of new materials. The lack of single crystals (or crystals of insufficient size or quality) unfortunately often rules out any structural determination. It is acknowledged that it is more difficult to get good-quality single crystals as soon as the cell volume of a structure increases. The challenge of structure elucidation of powders is very well reflected by the small number of new structures published per year that are determined from powder diffraction compared to the number of structures determined from single-crystal diffraction data. Aluminophosphates are no exceptions, with as little as eight structures in total determined from powder diffraction, out of the more than 250 structures reported in the database. For inorganic or hybrid powders, one of the key, but challenging, steps that prevents structure elucidation is the construction of an initial structural model. Strategies for enhancing the efficiency of powder-based structure determination have shown a continuous development, especially by incorporating direct space structural information. To assist this part, diffraction software products based on simulated annealing or Monte–Carlo global optimization in directspace like FOX, ESPOIR, TOPAS, and so on, have proven reliable strategies for structure determination of a wide range of solids. Alternative routes have been proposed for a drastic acceleration of the structure solution when prior knowledge of the constitutive building blocks is available, allowing to find the solution of extremely large cells. However, considering the frequent absence of such prior knowledge, these methods become unsuccessful due to the exponential explosion of computing time with the number of degrees of freedom in the unit cell. This constitutes a limit of computability causing a bottleneck for structure determination from powders. It is therefore necessary to overcome this limit to yield 100 % success by reducing the computational workload. Beyond the performance of computing facilities itself, a successful and rapid search is largely determined by the number of degrees of freedom involved in the crystal structure versus the amount of input (chemical, topological, geometric, and so on) injected in the search. Nuclear magnetic resonance spectroscopy associating state-of-the-art high magnetic fields, ultra-fast magic angle spinning (MAS), and tailored NMR pulse sequences has become an extremely powerful tool to provide information at the atomic level about the local environment of a given nucleus. In this contribution, we shall show with the example of a novel nano-perforated lamellar aluminophosphate, [Al5(OH) ACHTUNGTRENNUNG(PO4)3ACHTUNGTRENNUNG(PO3OH)4] [NH3ACHTUNGTRENNUNG(CH2)2NH3]2 [2H2O], hereafter referred to as AlPO4-(Al5P7)-DAE (DAE= diaminoethane), 1) how topological information extracted from NMR data, that is, an NMR-driven structure resolution, can be directly introduced in the structure-elucidation process, allowing the determination of an initial model, which was otherwise not possible despite the high quality of the synchrotron powder diffraction (SPD) data, 2) how the computation time necessary to search and converge to this model can be further drastically decreased by increasing the amount of NMR-based information up to the whole complex building blocks. This new methodology differs signifi[a] B. Bouchevreau, Dr. C. Martineau, Dr. F. Taulelle Institut Lavoisier de Versailles, UMR CNRS 8180 Universit de Versailles Saint Quentin en Yvelines 45 Avenue des Etats-Unis 78035 Versailles cedex (France) Fax: (+33) 139254277 E-mail : charlotte.martineau@chimie.uvsq.fr taulelle@chimie.uvsq.fr [b] Dr. C. Mellot-Draznieks Coll ge de France, 11 place Marcellin Berthelot Laboratoire de Chimie des Processus Biologiques FRE 34 88 CNRS, 75005, Paris (France) [c] Dr. A. Tuel IRCELYON, CNRS UMR 5256 Universit Lyon 1, 69626 Villeurbanne (France) [d] Dr. M. R. Suchomel Argonne National Laboratory Advanced Photon Source, Argonne, IL 60439 (USA) [e] Dr. J. Tr bosc, Prof. O. Lafon, Prof. J.-P. Amoureux Univ. de Lille Nord de France, UCCS USTL CNRS UMR 8181, 59652 Villeneuve d Ascq (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201203767.

Spin-induced symmetry breaking in orbitally ordered NiCr 2O 4 and CuCr 2O 4

At room temperature, the normal oxide spinels NiCr_2O_4 and CuCr_2O_4 are tetragonally distorted and crystallize in the I4_1/amd space group due to cooperative Jahn-Teller ordering driven by the orbital degeneracy of tetrahedral Ni

Synthesis and structure determination of CaSi1/3B2/3O8/3, a new calcium borosilicate

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Synthesis and structure resolution of RbLaF4.

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Frustration of Magnetic and Ferroelectric Long-Range Order in Bi2Mn4/3Ni2/3O6

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Charge disproportionation and the pressure-induced insulator–metal transition in cubic perovskite PbCrO3

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