[en] It has been shown recently that Au labeling [V. C. Venezia, D. J. Eaglesham, T. E. Haynes, A. Agarwal, D. C. Jacobson, H.-J. Gossmann, and F. H. Baumann, Appl. Phys. Lett. 73, 2980 (1998)] can be used to profile vacancy-type defects located near half the projected range ((1/2) R_p) in MeV-implanted Si. In this letter, we have determined the ratio of vacancies annihilated to Au atoms trapped (calibration factor ''k'') for the Au-labeling technique. The calibration experiment consisted of three steps: (1) a 2 MeV Si⁺ implant into Si(100) followed by annealing at 815 degree sign C to form stable excess vacancy defects; (2) controlled injection of interstitials in the (1/2) R_p region of the above implant via 600 keV Si⁺ ions followed by annealing to dissolve the {311} defects; and (3) Au labeling. The reduction in Au concentration in the near-surface region (0.1-1.6 μm) with increasing interstitial injection provides the most direct evidence so far that Au labeling detects the vacancy-type defects. By correlating this reduction in Au with the known number of interstitials injected, it was determined that k=1.2±0.2 vacancies per trapped Au atom. (c) 2000 American Institute of Physics

[en] Highlights: • Unfolding supercells with vacancies is discussed. • Fully vacant supercells reduce all PC band probabilities uniformly. • Net fully vacant supercells also reduce all PC band probabilities uniformly. - Abstract: We study how vacancies alter the effective primitive cell bands projected out of supercell eigenstates via Brillouin zone unfolding. Two types of vacant primitive cells are of particular interest: Fully vacant, in which all atoms in a single cell are missing; and net fully vacant, in which the atoms comprising a full set for a single cell are missing from more than one cell. We find that a fully vacant primitive cell and a net fully vacant primitive cell have the same effect on the primitive cell bands. We show that the probability reduction for any primitive cell band is the same, regardless of band or wavevector in the primitive cell Brillouin zone, for both fully and net fully vacant primitive cells. We illustrate these results with a two-band model.

[en] The authors compare the properties of vacancies in ⁴He implied by X-ray, NMR, and acoustic measurements with specific heat, pressure, and melting curve data. Taken together, these different measurements indicate that the model of vacancies as occupying a narrow band of states cannot be correct. Better agreement can be obtained by assuming that vacancies occupy a wide band but inconsistencies remain. An unambiguous determination of the properties of vacancies requires additional experimental and theoretical work

[en] Conclusions of this presentation are: (1) np-Au foams were successfully synthesized by de-alloying process; (2) np-Au foams remain porous structure after Ne ion irradiation to 1 dpa; (3) SFTs were observed in irradiated np-Au foams with highest and intermediate flux, while no SFTs were observed with lowest flux; (4) SFTs were observed in irradiated np-Au foams at RT, whereas no SFTs were observed at LNT irradiation; (5) The diffusivity of vacancies in Au at RT is high enough so that the vacancies have enough time to agglomerate and thus collapse. As a result, SFTs were formed; (6) The high flux created much more damage/time, vacancies don't have enough time to diffuse or recombine. As a result, SFTs were formed.

[en] The properties of the lattice vacancy in graphite are not well understand. Quenching of vacancies into graphite is very difficult because of the very high temperatures necessary to introduce significant equilibrium concentrations (> 3000degC) and the relatively low temperatures at which they are significantly mobile (∼ 1200degC). The interpretation of most irradiation damage experiments is uncertain because of the presence of complex (and interstitial) defects. This note proposes a possible method of studying vacancy properties in isolation using ¹⁴C₆ decay. (author)

[en] We propose that hydrogen-passivated multivacancies which appear to be fully saturated with H can actually capture additional H in electrically inactive sites. In silicon, first-principles total energy calculations show that splitting an (m≥2) multivacancy into a mono- and an (m-1) vacancy provides a low-strain pairing site for H, 0.4eV per H lower than any known bulk pairing site. This monovacancy ejection mechanism is an excellent candidate for the H reservoir found both in crystalline and amorphous Si. A distinct H pairing on the fully saturated m vacancies, by forming an internal surface Si-Si dimer, provides the final state of light-induced metastable degradation of hydrogenated amorphous silicon

[en] Tungsten (W) with a body-centered cubic (bcc) structure and W alloys are believed to be the most promising alternatives to plasma facing materials in ITER. In this work, the interaction between helium (He) atom and vacancy defect in tungsten (W) has been investigated by using first-principles simulations. We have obtained that the most stable site for He in tungsten is the substitutional position because He can keep its own electronic structure at this position. In the studied tungsten system, vacancy can act as a trapping center for surrounding He atom with negative trapping energy. The migration behaviors of He atom at tetrahedral interstitial site in W, which can be trapped by vacancy but the final position is almost unchanged comparing with its initial position through structural relaxation, have been predicted and discussed. It is also found that single He atom prefers to go through an octahedral site rather than through a direct path to the vacancy, and the stronger the interaction between He atom and vacancy is, the lower the migration barrier will be

[en] A model calculation based on the static dielectric screening theory has been performed to estimate the probable number of positron-trapping levels in metal mono-vacancies and it is shown that there cannot be more than one. (author). 8 refs, 1 tab

[en] We investigated the Raman spectra of thermally reduced and untreated Y- and Ca-stabilized cubic zirconia. A substantial decrease in Raman activity was observed in the acoustic mode region upon reduction. Analyses of these spectra as a phonon density of states and of the observed spectral changes indicate that the reduced states are more ordered than the untreated one. This is consistent with a nonrandom arrangement of vacancies, which produces the superposition of periodic sequences of vacancies within domains. This, in turn, causes the increment in coherence length of phonons that is manifested as a decrease in acoustic mode activity. copyright 1997 American Institute of Physics

[en] The method of molecular dynamics has been used to investigate the defects produced by the collapse of vacancy clusters in model f.c.c., and b.c.c. metals. The results are pertinent to the geometry of defects formed by the collapse of displacement cascades, although the nature of the problem was simplified here to considerate of almost spherical voids containing about 40 vacancies. The collapsed structure is dependent on the form of the interatomic potential and is sometimes too disordered to be described as a conventional defect. However, in a non-equilibrium f.c.c. model representing copper, the void transformed to a stacking-fault tetrahedron, the nature of which is described in section 3. In a non-equilibrium model of a b.c.c. crystal, the void collapsed directly to a loop with Burgers vector <100>: the relevance of this result to the crystallography of loops in b.c.c. metals is discussed in sections 4 and 5. (orig.)