Formation of anion-vacancy clusters and nanocavities in thermochemically reduced MgO single crystals

Monge, M.A.; Попов, А. И.; Ballesteros, C.; González, R.; Chen, Y.; Kotomin, E. A.

doi:10.1103/physrevb.62.9299

Cited by 56 publications

(22 citation statements)

References 29 publications

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“…17,18,20,37 Recently, some of these assignments have been confirmed through comparison with ab initio calculations on the optical properties of these centers. [38][39][40] Oxygen vacancies are not the only point defects observed in MgO; under neutron irradiation, other bands appear, some of which have been attributed to the presence of aggregates of F centers, 12,41 and again this has been confirmed by accurate ab initio cluster model calculations. 42 The occurrence of oxygen vacancies is not exclusive of simple oxides: they are present in more complex materials such as SiO 2 , Li 2 O, ZrO 2 , TiO 2 , and Al 2 O 3 .…”

Section: Introductionmentioning

confidence: 72%

Theoretical study of bulk and surface oxygen and aluminum vacancies inα−Al2O3

2004

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The formation energy, geometry, and electronic structure of isolated oxygen and aluminum vacancies in bulk and on the ͑0001͒ surface of corundum (␣-Al 2 O 3 ) have been investigated by means of periodic calculations in the framework of density functional theory within the generalized gradient approximation and large supercells. The energy cost to form an oxygen vacancy in the bulk is estimated to be of the order of 10 eV, whereas that corresponding to the formation of Al vacancies is found to be at least a 30% larger. The relaxation of the material is rather small for both defects. The removal of an oxygen atom in bulk ␣-Al 2 O 3 is accompanied by the appearance of an impurity level in the gap, which is a strong indication of electron localization. This has been further confirmed by integration of the density of states in the energy interval corresponding to the impurity level and by several other theoretical analyses. For the ␣-Al 2 O 3 (0001) surface, the formation of oxygen and aluminum vacancies exhibits many similarities with the bulk; the energy cost to form Al vacancies is much larger than for O vacancies and, in both cases, it is accompanied by rather small atomic displacements. However, there are also significant differences between bulk and surface oxygen and aluminum vacancies. Thus the formation energy of one of these point defects in the surface is rather smaller as expected and, more importantly, the degree of electronic delocalization is also larger.

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Section: Introductionmentioning

confidence: 72%

Theoretical study of bulk and surface oxygen and aluminum vacancies inα−Al2O3

2004

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“…The first band is consistent with the appearance of a signal at 3.6 eV upon annealing of MgO samples containing a high concentration of these point defects and also assigned to F-center aggregates. 4 This is supported by model studies of the kinetics of F-center aggregation. 5 However, while some author has reported similar bands at 3.5 and 2.1 eV in additive coloring experiments, 6 Chen et al were not able to observe these transitions in electron irradiated additive colored samples.…”

Section: Introductionmentioning

confidence: 76%

Ground- and excited-state properties ofM-center oxygen vacancy aggregates in the bulk and surface of MgO

et al. 2003

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Aggregates of oxygen vacancies ͑F centers͒ represent a particular form of point defects in ionic crystals. In this study we have considered the combination of two oxygen vacancies, the M center, in the bulk and on the surface of MgO by means of cluster model calculations. Both neutral and charged forms of the defect M and M ϩ have been taken into account. The ground state of the M center is characterized by the presence of two doubly occupied impurity levels in the gap of the material; in M ϩ centers the highest level is singly occupied.For the ground-state properties we used a gradient corrected density functional theory approach. The dipoleallowed singlet-to-singlet and doublet-to-doublet electronic transitions have been determined by means of explicitly correlated multireference second-order perturbation theory calculations. These have been compared with optical transitions determined with the time-dependent density functional theory formalism. The results show that bulk M and M ϩ centers give rise to intense absorptions at about 4.4 and 4.0 eV, respectively. Another less intense transition at 1.3 eV has also been found for the M ϩ center. On the surface the transitions occur at 1.6 eV (M ϩ ) and 2 eV ͑M͒. The results are compared with recently reported electron energy loss spectroscopy spectra on MgO thin films.

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“…6,12,[17][18][19][20][21] The most intense absorption occurs at Ϸ5.0 eV ͑250 nm͒, which has been identified as due to both the F ϩ and the F centers. 3,6,12,22 Two absorption bands at 3.49 and 1.27 eV ͑355 and 975 nm, respectively͒ are correlated with anion divacancies, F 2 .…”

Section: A Background Of Point Defectsmentioning

confidence: 99%

“…2͒, the 2.16 eV band was absent. 17 One would expect that if the 2.16 eV band was due to an anion divacancy, this band would emerge. Indeed, the 3.49 and 1.27 eV bands were observed, confirming that these two bands are due to anion divacancies.…”

Section: A Background Of Point Defectsmentioning

confidence: 99%

“…Second, optical absorption permits defect characterization previously identified in neutron-irradiated crystals. 4,6,12,[17][18][19][20][21][22]37 Third, the resulting hardening is determined with the nanoindentation technique, which is very powerful for the measurement of the mechanical properties of the implanted region. This technique can measure the hardness and Young's modulus of layers as thin as 100 nm and especially the variation of these parameters with the indentation contact depth.…”

Section: B Background On Ion Implantationmentioning

confidence: 99%

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Optical and mechanical properties of MgO crystals implanted with lithium ions

Savoini

Cáceres

Vergara

et al. 2004

Journal of Applied Physics

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Defect profile induced by implantation of Li+ ions with an energy of 175 keV and a fluence of 1×1017 ions/cm2 in MgO single crystals was characterized by Rutherford backscattering and optical absorption measurements. Several absorption bands at 5.0, 3.49, 2.16, and 1.27 eV, identical to those found in neutron irradiated crystals, were observed and have been previously associated with oxygen vacancies and higher-order point defects involving oxygen vacancies. Despite the high fluence of Li+ ions, no evidence was found for the formation of Li nanocolloids during implantation. Nanoindentation experiments demonstrated that both the hardness and Young’s modulus were higher in the implanted layer than in the sample before implantation. The maximum values were H=(17.4±0.4) and E=(358±9) GPa, respectively, at a contact depth of ≈165 nm. Thermal annealings in flowing argon at increasing temperatures improved the crystalline quality of the implanted layer. After annealing at 500 K, two extinction bands at ≈2.75 and 3.80 eV emerged. These bands are attributed to Mie scattering from metallic lithium nanocolloids with either a face-centered- or a body-centered-cubic structure. The latter band was almost absent by 950 K. The former reached a maximum intensity after the thermal treatment at 1050 K and disappeared by 1250 K. The behavior of these bands can be satisfactorily explained by the Maxwell–Garnett theory. The decrease in hardening cannot be correlated with the thermal destruction of the absorption bands at 5.0, 3.49, 2.16, and 1.27 eV, but rather with the annihilation of both lithium and oxygen interstitials. Lithium outdiffusion from the implanted region takes place at temperatures of ≈1100 K. It is concluded that the hardening observed in the implanted region was primarily due to the extraordinarily large concentration of both lithium and oxygen interstitials.

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Formation of anion-vacancy clusters and nanocavities in thermochemically reduced MgO single crystals

Cited by 56 publications

References 29 publications

Theoretical study of bulk and surface oxygen and aluminum vacancies inα−Al2O3

Theoretical study of bulk and surface oxygen and aluminum vacancies inα−Al2O3

Ground- and excited-state properties ofM-center oxygen vacancy aggregates in the bulk and surface of MgO

Optical and mechanical properties of MgO crystals implanted with lithium ions

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