EELS studies of MgB2 superconductor obtained under high pressure

Yu, Rui; Li, S.C.; Wang, Y.Q.; Kong, Xiao; Zhu, Jianglong; Li, F.Y.; Liu, Z.X.; Duan, Xiaofeng; Zhang, Z.; Jin, C. Q.

doi:10.1016/s0921-4534(01)00959-5

Cited by 21 publications

(19 citation statements)

References 25 publications

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“…The elements Mg and Oa re attributed to the oxy-functionalized nanosheets.T he Ca nd Cu peaks arise from the lacey carbon-coated copperT EM grid. The minor Si peak is likely from an impurity.T he boron EEL spectrum shows the presence of two boron peaks (K-edges) at about 194 and2 02 eV.T he obtained EEL spectruml ooks identicalt ot he boron K-edge in MgB 4 O 7 (from WEELS, web source for electron energy-loss spectra)a nd matches well with the boride and borate compounds, [52][53][54][55] which suggests that the first K-edge is attributed to the absorption by p * states and the other K-edge is attributed to the s * states. The presenceo fp * and s * states implies the existence of sp 2 hybridization,w hich indicates that these nanostructures contain ab oron honeycombn etwork.…”

Section: Resultssupporting

confidence: 53%

Simple, Green, and High‐Yield Production of Boron‐Based Nanostructures with Diverse Morphologies by Dissolution and Recrystallization of Layered Magnesium Diboride Crystals in Water

Gunda

Das

2018

ChemPhysChem

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Layered metal diborides that contain metal atoms sandwiched between boron honeycomb planes offer a rich opportunity to access graphenic forms of boron. We recently demonstrated that magnesium diboride (MgB ) could be exfoliated by ultrasonication in water to yield boron-based nanosheets. However, knowledge of the fate of metal boride crystals in aqueous phases is still in its incipient stages. This work presents our preliminary findings on the discovery that MgB crystals can undergo dissolution in water under ambient conditions to result in precursors (prenucleation clusters) that, upon aging, undergo nonclassical crystallization preferentially growing in lateral directions by two-dimensional (2D) oriented attachment. We show that this recrystallization can be utilized as an avenue to obtain a high yield (≈92 %) of boron-based nanostructures, including nanodots, nanograins, nanoflakes, and nanosheets. These nanostructures comprise boron honeycomb planes chemically modified with hydride and oxy functional groups, which results in an overall negative charge on their surfaces. This ability of MgB crystals to yield prenucleation clusters that can self-seed to form nanostructures comprising chemically modified boron honeycomb planes presents a new facet to the physicochemical interaction of MgB with water. These findings also open newer avenues to obtain boron-based nanostructures with tunable morphologies by varying the chemical milieu during recrystallization.

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Section: Resultssupporting

confidence: 53%

Simple, Green, and High‐Yield Production of Boron‐Based Nanostructures with Diverse Morphologies by Dissolution and Recrystallization of Layered Magnesium Diboride Crystals in Water

Gunda

Das

2018

ChemPhysChem

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“…Details are given elsewhere. 20 TEM thin specimens were prepared by both tripod polishing, with short-time ͑5-min͒ and low-energy ͑2.5-kV͒ ion milling ͑for cleaning surfaces͒, and grinding into small pieces in methanol. The absence of radiation damage due to ion milling was confirmed by comparing two samples from the two different thinning techniques.…”

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confidence: 99%

Anisotropic excitons inMgB2from orientation-dependent electron-energy-loss spectroscopy

Jiang

Spence

et al. 2002

Phys. Rev. B

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The momentum transfer in electron-energy-loss spectra ͑EELS͒ recorded in an electron microscope can be controlled in a manner akin to polarization-dependent soft-x-ray absorption spectroscopy, however with much greater spatial resolution. Here we report orientation-dependent EELS from fine-grained MgB 2 for both B K and Mg L 2,3 edges. Calculations based on an excited final state are used to suggest an anisotropic exciton for the peak at 55 eV at the Mg L 23 edge.Since the discovery of the superconductivity in layered metal MgB 2 at a superconducting transition temperature T c ϳ39 K, 1 much attention has been focused on understanding the superconductivity mechanism. Evidence of the B isotope shift of T c suggests a phonon-mediated BCS superconductivity. 2 Further evidence from high-resolution x-ray photoemission spectroscopy ͑XES͒ of multiple superconducting gaps in MgB 2 indicates a deviation from the conventional isotropic s-wave mechanism. 3 Band structure calculations show that the valence band of MgB 2 consists mainly of two distinct bands of two-dimensional covalent in-plane (B 2p x,y ) and three-dimensional metallic-type interlayer (B 2p z ). 4 -9 Anisotropic features of the density of states ͑DOS͒ approaching the Fermi energy in singe-crystal MgB 2 have been reported using angle-resolved XES. 10 Recently, the orientation-dependent B DOS has been reported, 11 but the results are different from our experiments and calculations. In addition, measurements of the orientation dependence of the unoccupied Mg DOS have not been reported. The grain size is generally less than a micron in sintered MgB 2 . Therefore spatially resolved spectroscopic methods become necessary for this anisotropic polycrystalline material.Electron-energy-loss spectroscopy ͑EELS͒ has also been used to study the orientation dependence of anisotropic single crystals. 12 When combined with a transmission electron microscope ͑TEM͒ this becomes a powerful technique to probe the electronic structure in the near atomic level and therefore to probe the anisotropic DOS in fine-grained solids.Here we report experimental EELS evidence for anisotropic features in both the B K and Mg L 23 edges. Under the assumption of the dipole selection rule ͑small momentum transfer, q•rӶ1), the B K and Mg L 23 edges approximately probe the unoccupied B p and Mg sϩd states, respectively. The orientation dependence of the B K edge reflects its anisotropic features in the DOS, which have been predicted by band structure calculations. However, the q dependence of the Mg DOS has not been reported before.In a one-electron approximation, EELS gives the fraction of beam electrons that, in traversing a thin slab, have lost energy E to the excitation of a core electron from an initial state to unoccupied final states. The transition probabilities can be calculated using Fermi's golden rule. For small angle scattering, the dipole approximation applies; therefore, the EELS intensities can be simplified as the product of the DOS and an atomic transition matrix: 13where ͉i͘ and...

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“…Несмотря на то, что при большом увеличении наблюдается плотная, почти без пор и рыхлых областей керамика, при меньшем увеличении видно, что поваренная соль,являющаяся средой, передающей давление в камере «Тороид», при нагреве под давлением проникла в керамику и заполнила все промежутки между плотными областями. Плотность полученной керамики составила 1,7 г/см 3 .…”

Section: результаты и обсуждениеunclassified

“…Однако ни один из них не является достаточно эффективным для получения массивного образца MgB 2 с высокой плотностью. Многочисленные исследования показали, что несмотря на практические трудности использование высоких давлений и повышенных температур приводит к хорошим результатам при синтезе керамики MgB 2 [1][2][3][4]. Несколько реже используется применение высоких давлений в качестве дополнительной обработки соединения, например, в [5] разработана методика получения керамики MgB 2 , включающая две стадии одноосной деформации с последующим отжигом после каждой деформации.…”

Section: Introductionunclassified