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Cation Substitution (cation + substitution)
Selected AbstractsA Quantum-Chemical Study on Understanding the Dehydrogenation Mechanisms of Metal (Na, K, or Mg) Cation Substitution in Lithium Amide NanoclustersADVANCED FUNCTIONAL MATERIALS, Issue 12 2010Lanlan Li Abstract The hydrogen-releasing activity of (LiNH2)6,LiH nanoclusters and metal (Na, K, or Mg)-cation substituted nanoclusters (denoted as (NaNH2)(LiNH2)5, (KNH2)(LiNH2)5, and (MgNH)(LiNH2)5) are studied using ab initio molecular orbital theory. Kinetics results show that the rate-determining step for the dehydrogenation of the (LiNH2)6,LiH nanocluster is the ammonia liberation from the amide with a high activation energy of 167.0,kJ,mol,1 (at B3LYP/6-31,+,G(d,p) level). However, metal (Na, K, Mg)-cation substitution in amide,hydride nanosystems reduces the activation energies for the rate-determining step to 156.8, 149.6, and 144.1,kJ,mol,1 (at B3LYP/6-31,+,G(d,p) level) for (NaNH2)(LiNH2)5, (KNH2)(LiNH2)5, and (MgNH)(LiNH2)5, respectively. Furthermore, only the ,NH2 group bound to the Na/K cation is destabilized after Na/K cation substitution, indicating that the improving effect from Na/K-cation substitution is due to a short-range interaction. On the other hand, Mg-cation substitution affects all ,NH2 groups in the nanocluster, resulting in weakened N,H covalent bonding together with stronger ionic interactions between Li and the ,NH2 group. The present results shed light on the dehydrogenation mechanisms of metal-cation substitution in lithium amide,hydride nanoclusters and the application of (MgNH)(LiNH2)5 nanoclusters as promising hydrogen-storage media. [source] Composition influence on positron annihilation parameters in ZnO-based nanocrystal semiconductor powdersPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007L. C. Damonte Abstract Zn1,xMgxO powders at various compositions were obtained by mechanical milling from the binary oxides. The progressive incorporation of Mg atoms into the ZnO lattice was monitored by X-ray diffraction (XRD). The evolution of annihilation parameters with milling time and composition were analyzed and related to the possible types of mechanical and substitutional induced defect present. It was concluded that the average lifetime constitute a useful parameter to sense the complete cation substitution in the wurtzite structure. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Independent Effects of Nitrogen Substitution for Oxygen and Yttrium Substitution for Magnesium on the Properties of Mg-Y-Si-Al-O-N GlassesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2003Michael J. Pomeroy Glasses of composition (in equivalent percent) (28 ,x)Mg:xY:56Si:16Al:(100 ,y)O:yN, with x= 0, 14, or 28 for y= 0 and 15 and with x= 0, 7, 14, 21, or 28 for y= 10, were prepared by melting and casting. For glasses where the nitrogen content was varied for a constant cationic ratio, the glass molar volume (MV), compactness (C), Young's modulus (E), glass-transition temperature (Tg), and dilatometric-softening temperature (Tds) varied linearly as the nitrogen content increased, with MV decreasing and the other properties increasing. From the incremental changes in these properties with nitrogen content, for glasses with x= 0, 14, and 28, good linear fits (R2 > 0.99) were obtained, and best-fit slopes are reported here. The property changes and their linearity were consistent with the increased cross-linking of the glass network by tricoordinated nitrogen. The replacement of magnesium by yttrium led to a nonlinear decrease in glass compactness and to nonlinear increases in MV, Tg, and Tds. However, linear correlations were found for MV and ionic volume and for Tg, Tds, and the coordination of (Si,Al)(O,N) tetrahedra of the glass structural units to the modifier cations not involved in charge compensating aluminum ions in fourfold coordination. The replacement of magnesium by yttrium had little effect on Young's modulus, and this result was related to similar changes in the compactness, C. The present results showed that the effects of substituting nitrogen for oxygen and yttrium for magnesium are independent and additive; thus, no synergistic effects of anion and cation substitutions were observed. [source] Structural derivation and crystal chemistry of apatitesACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2003T. J. White The crystal structures of the [A(1)2][A(2)3](BO4)3X apatites and the related compounds [A(1)2][A(2)3](BO5)3X and [A(1)2][A(2)3](BO3)3X are collated and reviewed. The structural aristotype for this family is Mn5Si3 (D88 type, P63/mcm symmetry), whose cation array approximates that of all derivatives and from which related structures arise through the systematic insertion of anions into tetrahedral, triangular or linear interstices. The construction of a hierarchy of space-groups leads to three apatite families whose high-symmetry members are P63/m, Cmcm and P63cm. Alternatively, systematic crystallographic changes in apatite solid-solution series may be practically described as deviations from regular anion nets, with particular focus on the O(1),A(1),O(2) twist angle , projected on (001) of the A(1)O6 metaprism. For apatites that contain the same A cation, it is shown that , decreases linearly as a function of increasing average ionic radius of the formula unit. Large deviations from this simple relationship may indicate departures from P63/m symmetry or cation ordering. The inclusion of A(1)O6 metaprisms in structure drawings is useful for comparing apatites and condensed-apatites such as Sr5(BO3)3Br. The most common symmetry for the 74 chemically distinct [A(1)2][A(2)3](BO4)3X apatites that were surveyed was P63/m (57%), with progressively more complex chemistries adopting P63 (21%), P (9%), P (4.3%), P21/m (4.3%) and P21 (4.3%). In chemically complex apatites, charge balance is usually maintained through charge-coupled cation substitutions, or through appropriate mixing of monovalent and divalent X anions or X -site vacancies. More rarely, charge compensation is achieved through insertion/removal of oxygen to produce BO5 square pyramidal units (as in ReO5) or BO3 triangular coordination (as in AsO3). Polysomatism arises through the ordered filling of [001] BO4 tetrahedral strings to generate the apatite,nasonite family of structures. [source] | ||||||||||