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81.
We present a fundamental method to assess the doping limits of hetero-polar materials; applied to the case of ZnO, we show clearly that electrons are stable and holes are unstable under the limits of thermodynamic control. 相似文献
82.
French SA Di Tommaso D Zanotti-Gerosa A Hancock F Catlow CR 《Chemical communications (Cambridge, England)》2007,(23):2381-2383
The high enantioselectivity in the hydrogenation of acetophenone catalysed by trans-Ru(H)2(S,S-dpen)(S-xylbinap) is explained in terms of the existence of a stable intermediate formed when the reactant enters the catalyst pocket fixing the molecular orientation. 相似文献
83.
Dewi W. Lewis Robin W. Grimes C. Richard A. Catlow 《Journal of molecular catalysis. A, Chemical》1995,100(1-3):103-114
Atomistic simulation techniques have been used to study the bulk and surface defect chemistry of MgO and Li/MgO catalysts. The energetics and stability of defects, particularly those which are thought to influence the activity of the catalysts, have been investigated. Of particular note is the enhanced stability of hole and substitutional defects at low-coordinate sites, both at steps and small protosteps. From the results of our calculations we comment generally on the factors controlling the overall activity of these catalysts. 相似文献
84.
Association energies of nearest-neighbour and next-nearest-neighbour associates between substitutional, trivalent rare-earth ions and interstitial fluoride ions CaF2, SrF2 and BaF2 are obtained by lattice simulation calculations. The dopant ion-fluoride ion interaction is described (i) with a set of potentials obtained with electron gas methods, and (ii) with a set of potentials derived semi-empirically from the host lattice cation-anion interaction potentials. The calculations successfully simulate the experimentally observed variations of the dopant-interstitial binding energies with the radius of the dopant ion, and with the lattice parameter of the host. The better quantitative agreement is obtained with the semi-empirical potentials. The variations are explained by an evaluation of the displacements of the ions constituting the associates. 相似文献
85.
Chen HY Di Tommaso D Hogarth G Catlow CR 《Dalton transactions (Cambridge, England : 2003)》2012,41(6):1867-1877
Density functional theory calculations have been used to investigate the hydrogenation of acetophenone (ACP) catalysed by the RuH(2)(diphosphine)(diamine) complexes with emphasis on the effect of the structure of the diphosphine and diamine ligands on the enantioselectivity. The computed reaction coordinate diagrams of RuH(2)(diphosphine)[(S,S)-DPEN] catalysed reactions with different (S)-diphosphine ligands (XylBINAP, TolBINAP, and BINAP) show that the presence of two methyl groups in the meta position is critical to obtaining a high difference in activation energy for the reaction pathways associated with the (R)- and (S)-alcohols, and consequently high enantioselectivity. The effect of the diamine structure while keeping the TolBINAP and XylBINAP fixed has also been analysed. To enhance the enantioselectivity of the TolBINAP system, the addition of two methyl groups and the removal of a phenyl group of the diamine (DMAPEN) offer the necessary steric interactions. We conclude by reporting a correlation between the enantiomeric excess and the difference in the computed activation energies of the two most favourable (S) and (R) reaction pathways, which shows that the computational procedure adopted could be used to predict the enantiomeric excess of ketone hydrogenation reactions catalysed by the Noyori-type catalysts, and assist in the choice of ligand when optimising the enantiomeric excess. 相似文献
86.
Theoretical techniques have been used to examine the two models proposed for the defect structure of stabilized zirconias. The first model is based on simple clusters of dopant ions and the charge compensating oxygen vacancies, while the second suggests that fluorite-related microdomains form within the host lattice. We show that the energies of formation of the clusters and microdomains are almost equal, the microdomain being favoured by 0.09 eV per dopant ion. Thus the calculations suggest that both point defects and microdomains may be present, with point defects predominating at low dopant concentration and higher temperatures. 相似文献
87.
An explanation is suggested for the observation that in fluorites the temperature of transition to the superionic state is depressed by rare-earth dopants. 相似文献
88.
P.J. Bendall C.R.A. Catlow J. Corish P.W.M. Jacobs 《Journal of solid state chemistry》1984,51(2):159-169
A theoretical study has been made of defect clustering in fluorite crystals. The emphasis is on the aggregation processes that can occur beyond the dimer stage which was discussed previously (J. Corish, C. R. A. Catlow, P. W. M. Jacobs, and S. H. Ong,Phys. Rev. B25, 6425 (1982)). The systems studied comprise CaF2, SrF2, and BaF2 doped withMF3whereM =Y, Lu, Yb, Er, Tb, Gd, Eu, or La, and also SrCl2 doped with GdCl3, LaCl3, or PrCl3. Two tetramers, one containing a single anion vacancy and two 〈111〉 relaxed F? ions and the other with a planar arrangement of the dopant ions, are stable for certain systems while a trimer and hexamers, with an octahedral arrangement of substitutionalM3+, also show considerable stability. A notable feature is the increase in the relative stability of larger complexes as the ratio of the radius of the dopant ion to that of the host lattice cation is decreased. The results are discussed in relation to recent EXAFS and neutron scattering work on these systems and should provide a useful guide in future experimental studies. 相似文献
89.
French SA Sokol AA Catlow CR Kornherr A Zifferer G 《Chemical communications (Cambridge, England)》2004,(1):20-21
For polysiloxanes to be used as a protective coating it is important that proton transfer, a trigger to polymerisation, is a facile process. Here we investigate the initial stages of polycondensation and compare different silanol tail groups and the effect of solvent (isopropanol). In the case of (3-aminopropyl)trihydroxysilane we see the potential for self catalysis as the tail group is a proton acceptor, while thiolpropyltrihydroxysilane and isopropanol do not promote proton transfer. 相似文献
90.
C.R.A. Catlow J. Corish J.M. Quigley P.W.M. Jacobs 《Journal of Physics and Chemistry of Solids》1980,41(3):231-240
Theoretical methods, based on Mott-Littleton techniques are used to investigate the defect structures of alkali-halide crystals doped with divalent ions. The systems studied are those having Mg2+, Ca2+, Sr2+ and Ba2+ as impurity ions in NaCl, KCl and KBr crystals. Our calculations find comparable stability of the (110) nearest-neighbour and (200) next-nearest-neighbour complexes whereas the (211) complex is found to be less stable. Significant trends in the variation of binding energy with dopant ion radius are predicted. We also consider the activation energies for the w1, w2, w3 and w4 type jumps which occur in the vicinity of the impurity ion. The results are, where possible, compared with experimentally determined defect energies and their implications in diffusion processes of M2+ impurities in the alkali halides are discussed. 相似文献