The bond ionicity of MB<sub>2</sub>(M = Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al and Y)

Chen, X L; Tu, Qiansi; He, Meng; Dai, Li‐Xin; Wu, Li‐Ming

doi:10.1088/0953-8984/13/29/105

Cited by 16 publications

(18 citation statements)

References 26 publications

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“…The displacements are also restricted to in-plane neighbours only. This is consistent with the anisotropic elastic constant matrix (Table 1), dielectric tensor, and metallic-covalent nature of the material, which was reported to have covalent bond in plane and metallic bonds out of plane [9,13,38] (also evident in the charge density distribution of Figure 5). Conveniently, this enables the study of reasonably large defect clusters (e.g.…”

Section: Defect Reactionsupporting

confidence: 88%

Defect evolution in burnable absorber candidate material: Uranium diboride, UB2

Burr¹,

Kardoulaki²,

Holmes³

et al. 2019

Journal of Nuclear Materials

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The stability, diffusivity and clustering behaviour of defects in uranium diboride (UB 2) was investigated in light of the potential application as a burnable absorber in nuclear fuel. UB 2 was found to accommodate limited deviations from stoichiometry, which should be a consideration when manufacturing and operating the material. Self-diffusivity of both U and B was found to be sluggish (10 −14 cm 2 /s for B and 10 −19 cm 2 /s for U at 2000 K) and highly anisotropic, with migration along the basal planes being orders of magnitude faster than c-axis migration. The anisotropy of defect migration (both interstitials and vacancies) is predicted to hinder recombination of defects produced by collision cascades, thus limiting the radiation tolerance of the material. Boron and uranium vacancies exhibit a drive to cluster. Boron vacancies in particular, which are mobile on basal planes, are predicted to cluster into strongly bound di-vacancy, which in turn are less mobile. These are then predicted to grow into larger two-dimensional vacancy clusters on the B plane, leading to anisotropic swelling. We provide an analytical expression to predict the stability of these clusters based on purely geometrical considerations. Finally, the accommodation of Li, He and Xe onto vacancy clusters was considered. Li appears to stabilise the structure upon U depletion, while the retention of He and Xe appears to rise with increasing B depletion, through the formation of vacancy clusters.

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Section: Defect Reactionsupporting

confidence: 88%

Defect evolution in burnable absorber candidate material: Uranium diboride, UB2

Burr¹,

Kardoulaki²,

Holmes³

et al. 2019

Journal of Nuclear Materials

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“…We begin with the electronic structure of the bulk, explain how the unique layered structure is stabilized, and show that the electrophilic B-terminated surface suffers a spontaneous reconstruction under the influence of adsorbed oxygen. The calculated ionicity of bulk MgB 2 coincides with a previous study based on Phillips-van Vechten-Levine (PVL) dielectric theory, 17 where 96.8% ionicity of Mg-B bonding was reported. We compute the oxygen adsorption potential energy surface map for both Mg-and B-terminated surfaces of MgB 2 (0001) and further detail their electronic structures as well as oxygen diffusion transition state geometries.…”

Section: Introductionsupporting

confidence: 88%

“…The nearly complete charge transfer of valence electrons from Mg to B indicates ionic bonding character, as reflected by the Bader charge analysis, charge density differences, and band structure decompositions presented in this work, which is further consistent with previous PVL dielectric theory analysis. 17 However, the in-plane chemical interaction of the B layer is found to be covalent, while the out-of-plane interaction is mildly metallic. 49 These unique properties of bulk MgB 2 present important fundamental issues related to the surface properties of the material.…”

Section: Resultsmentioning

confidence: 97%

Electronic structure and surface properties ofMgB2(0001) upon oxygen adsorption

et al. 2018

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We use density-functional theory to investigate the bulk and surface properties of MgB2. The unique bonding structure of MgB2 is investigated by Bader's atoms-in-molecules, charge density difference, and occupancy projected band structure analyses. Oxygen adsorption on the chargedepleted surfaces of MgB2 is studied by a surface potential energy mapping method, reporting a complete map including low-symmetry binding sites. The B-terminated MgB2(0001) demonstrates reconstruction of the graphene-like B layer and the reconstructed geometry exposes a threefold site of the sub-surface Mg, making it accessible from the surface. Detailed reconstruction mechanisms are studied by simulated annealing method based on ab-initio molecular dynamics and nudged elastic band calculations. The surface clustering of B atoms significantly modifies the B 2p states to occupy low energy valence states. The present work emphasizes that a thorough understanding of the surface phase may explain an apparent inconsistency in the experimental surface characterization of MgB2. Furthermore, these results suggest that the surface passivation can be an important technical challenge when it comes to development of a superconducting device using MgB2.

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“…First, the number of effective valence electrons (( Z μ ) * ) must be calculated. When considering atoms without d‐electrons, ( Z μ ) * can be replaced by the number of valence electrons ( Z μ ) 20 . With the knowledge of ( Z μ ) * and N C (nearest coordination number) for both atom A and atom B, the number of effective valence electrons per μ bond (( n e μ ) * ) can be determined by 10 …”

Section: Theorymentioning

confidence: 99%

Theoretical Calculation and Measurement of the Hardness of Diopside

Smedskjær¹,

Jensen²,

Yue³

2008

Journal of the American Ceramic Society

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In the present paper, we test the suitability of a previously proposed model by Gao et al.9 for describing the hardness of more complicated, multicomponent crystals by both calculating and measuring the hardness of a diopside crystal. We analyze the limitations of the model and suggest that factors like load and temperature should be considered in the model. The calculated value of hardness of the diopside crystal is compared with the Vickers hardness values measured at different loads. A good agreement is found between the calculated hardness and the value measured at 25°C at a load of 0.5 N. Crystal orientation and measurement temperature have an impact on the experimental value of hardness of the crystals.

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The bond ionicity of MB₂(M = Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al and Y)

Cited by 16 publications

References 26 publications

Defect evolution in burnable absorber candidate material: Uranium diboride, UB2

Defect evolution in burnable absorber candidate material: Uranium diboride, UB2

Electronic structure and surface properties ofMgB2(0001) upon oxygen adsorption

Theoretical Calculation and Measurement of the Hardness of Diopside

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The bond ionicity of MB2(M = Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al and Y)

Cited by 16 publications

References 26 publications

Defect evolution in burnable absorber candidate material: Uranium diboride, UB2

Defect evolution in burnable absorber candidate material: Uranium diboride, UB2

Electronic structure and surface properties ofMgB2(0001) upon oxygen adsorption

Theoretical Calculation and Measurement of the Hardness of Diopside

Contact Info

Product

Resources

About

The bond ionicity of MB₂(M = Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al and Y)