Photoluminescence and cathodoluminescence ͑CL͒ spectra of stoichiometric and oxygen-deficient ZnO films grown on sapphire were examined. It was found that the intensities of the green and yellow emissions depend on the width of the free-carrier depletion region at the particle surface; the thinner the width, the larger the intensity. Experimental results and spectral analyses suggest that the mechanism responsible for the green ͑yellow͒ emission is the recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the single ionized oxygen vacancy V o ϩ ͑the single negatively charged interstitial oxygen ion O i Ϫ) center in the particle.

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First-principles calculations for the diffusion of transition metal solutes in nickel challenge the commonly accepted description of solute diffusion rates in metals. The traditional view is that larger atoms move slower than smaller atoms. Our calculation shows the opposite: larger atoms, in fact, can move much faster than smaller atoms. Conventional mechanisms involving the effect of misfit strain or the solute-vacancy binding interactions cannot explain this counterintuitive diffusion trend. Instead, the origin of this behavior stems from the bonding characteristics of the d electrons of solute atoms, suggesting that a similar diffusion trend also occurs in other types of host lattices.

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First-principles studies identify a vacancy mechanism underlying the unusually high O solubility and nucleation of stable O-enriched nanoclusters in defect-containing Fe. Oxygen, confined as an interstitial, shows an exceptionally high affinity for vacancies, an effect enhanced by spin polarization. If vacancies preexist, the O-vacancy pair formation energy essentially vanishes, allowing the O concentration to approach that of the vacancies. This O-vacancy mechanism enables the nucleation of O-enriched nanoclusters, that attract solutes with high O affinities (Ti and Y) and strengthen Fe-based alloys.

First-principles theory was used to investigate the roles of bond topology and covalency in the phase stability and elastic strength of 5d transition-metal diborides, focusing on elements (M=W, Re, Os) that have among the lowest compressibilities of all metals. Among the phases studied, the ReB(2)-type structure exhibits the largest incompressibility (c axis), comparable to that of diamond. This ReB(2) structure is predicted to be the ground-state phase for WB(2) and a pressure-induced phase (above 2.5 GPa) for OsB(2). Both strong covalency and a zigzag topology of interconnected bonds underlie these ultraincompressibilities. Interestingly, the Vickers hardness of WB(2) is estimated to be similar to that of superhard ReB(2).

Rapid Communications are intended for the accelerated publication of important new results and are therefore given priority treatment both in the editorial once and in production A.Rapid Communication in Physical Review 8 should be no longer than four printed pages and must be accompanied by an abstract Pa.ge proofs are sent to authors.Equilibrium point defects and their relation to the contrasting mechanical behavior of NiA1 and FeA1 are investigated. For NiA1, the defect structure is dominated by two types of defectsmonovacancies on the Ni sites and substitutional antisite defects on the Al sites. The defect structure of FeA1 differs from that of NiA1 in the occurrence of antisite defects at the transition-metal sites for Al-rich alloys and the tendency for vacancy clustering. The strong ordering (and brittleness) of NiA1 is attributed mainly to the difference in atomic size between constituent atoms.

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