Direct Evidence for Cage Conduction Band in Superconducting Cement 12CaO·7Al<sub>2</sub>O<sub>3</sub> by Low-Energy High-Resolution Photoemission Spectroscopy

Souma, S.; Arakane, T.; Sato, Takafumi; Takahashi, Takashi; Kim, Sung Wng; Matsuishi, Satoru; Hosono, Hideo

doi:10.1143/jpsj.79.103704

Cited by 14 publications

(19 citation statements)

References 27 publications

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“…42 Recent low temperature (T ∼ 10 K) Xe-sourced XPS measurements of C12A7:e − produced a clearer picture of the 2p states of extra-framework O 2− ions at about 5 eV above the edge of the framework valence band. 43 Unfortunately the O 2− valence states at ∼ −2.5 eV in sample A can not be unambiguously resolved in the valence XPS due to sample charging.…”

Section: Resultsmentioning

confidence: 99%

Experimental Evidence of Cage Conduction Bands in Superconducting Cement 12CaO$\cdot$7Al$_2$O$_3$

McLeod,

Buling,

Kurmaev

et al. 2012

Preprint

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Natural 12CaO•7Al 2 O 3 (C12A7) is a wide bandgap insulator, but conductivity can be realized by introducing oxygen deficiency. Currently, there are two competing models explaining conductivity in oxygen-deficient C12A7, one involving the electron transfer via a "cage conduction band" inside the nominal band gap, the other involving electron hopping along framework lattice sites. To help resolve this debate, we probe insulating and conducting C12A7 with X-ray emission, Xray absorption, and X-ray photoemission spectroscopy, which provide a full picture of both the valence and conduction band edges in these materials. These measurements suggest the existence of a narrow conduction band between the main conduction and valence bands common in both conducting and insulating C12A7 and support the theory that free electrons in oxygen-deficient C12A7 occupy the low-energy states of this narrow band. Our measurements are corroborated with density functional theory calculations.

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Section: Resultsmentioning

confidence: 99%

Experimental Evidence of Cage Conduction Bands in Superconducting Cement 12CaO$\cdot$7Al$_2$O$_3$

McLeod,

Buling,

Kurmaev

et al. 2012

Preprint

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“…In pristine mayenite, CCB is unoccupied while free O 2− -2p levels form the occupied state located at 2 eV above the frame valence band (FVB). Slight electron doping divides the CCB to an empty band and fully occupied cage valence band (CVB, similar to the F + center observed in CaO [29]) due to the polaron formation [25][26][27]. By further electron doping, the kinetic energy gain surpasses the lattice distortion to induce the insulator-to-metal transition [8].…”

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confidence: 88%

“…The discrepancy is tentatively attributed to the metallic environment of Mu − that is largely different for H − in fully hydrogenated (insulating) C12A7:H − . Since the wTF-µSR spectra in the pristine sample exhibit sizable δ n (primarily due to the nearest neighboring 27 Al nuclei), the local environment of muon can be further examined by comparing δ n with that estimated for the presumed muon sites. For narrowing down the candidate muon sites, the first principle calculation using the Vienna Ab-initio Simulation Package (VASP) [21] was performed for pristine mayenite.…”

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confidence: 99%

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Cage electron-hydroxyl complex state as electron donor in mayenite

et al. 2016

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It is inferred from the chemical shift of muon spin rotation (µSR) spectra that muons implanted in pristine (fully oxidized) mayenite, [Ca 12 Al 14 O 32 ] 2+ [ 5 O 2− ] (C12A7, with referring to the vacant cage), are bound to O 2− at the cage center to form OMu − (where Mu represents muonium, a muonic analog of the H atom). However, an isolated negatively charged state (Mu − , an analog of H − ) becomes dominant when the compound approaches the state of electride [Ca 12 Al 14 O 32 ] 2+ [ 4 2e − ] as a result of the reduction process. Moreover, the OMu − state in the pristine specimen exhibits depolarization of paramagnetic origin at low temperatures (below ∼30 K), indicating that OMu − accompanies a loosely bound electron in the cage that can be thermally activated. This suggests that interstitial muons (and hence H) forming a "cage electron-hydroxyl" complex can serve as electron donors in C12A7.

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“…Processing C12A7 in a reducing environment leads to replacement of clathrated oxygens with electrons (12,13) (5,14,15). In crystalline C12A7:e -, the electrons occupy the cage conduction band (CCB) states associated with the lattice cages (16,17), leading to polaron-type conduction at x < ∼0.5 and metallic conduction at x > 0.5 (13,16). The exceptionally low work function of crystalline C12A7:e -(2.4 eV) (18) is used in electron field emitters (19,20), fieldeffect transistors (21), resistive memory (22), and enhancement of catalytic activity for ammonia synthesis (23).…”

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confidence: 99%

Electron anions and the glass transition temperature

Johnson

Sushko

Tomota

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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Properties of glasses are typically controlled by judicious selection of the glass-forming and glass-modifying constituents. Through an experimental and computational study of the crystalline, molten, and amorphous [Ca 12 Al 14 O 32 ] 2+ · (e -) 2 , we demonstrate that electron anions in this system behave as glass modifiers that strongly affect solidification dynamics, the glass transition temperature, and spectroscopic properties of the resultant amorphous material. The concentration of such electron anions is a consequential control parameter: It invokes materials evolution pathways and properties not available in conventional glasses, which opens a unique avenue in rational materials design.amorphous materials | glass transition | electrides | molecular dynamics | density functional theory O ne of the key challenges in materials design is identification of degrees of freedom that ensure robust control of the properties of interest. Small changes in composition of crystalline materials can lead to large changes in their electronic properties, such as optical absorption and electrical conductivity (1). Similarly, a small concentration of (co)dopants in amorphous materials can be used to control their optical properties (2, 3). Glass properties are typically controlled by varying composition or processing conditions (4). However, delicate control of viscosity and glass transition temperature in multicomponent materials (T g ) has remained elusive due to the collective origin of these properties. Substitution of atomic anions with electron anions in materials to form electrides (5, 6) introduces an additional degree of freedom. Here we demonstrate that electron anions dramatically change dynamics of atoms in an inorganic amorphous material, which strongly affects its T g . Extending the concept of electron anions to other amorphous materials would provide a powerful instrument for the design of glasses with finely tuned characteristics.Calcium aluminates (CA), composed of CaO and Al 2 O 3 , represent a common family of oxide glasses. Whereas pure Al 2 O 3 is a poor glass former, blending it with CaO leads to the formation of stable glasses composed of AlO 4 tetrahedra with strong and directional covalent bonds instead of nondirectional ionic bonds in bulk Al 2 O 3 (7). In contrast, Ca-O bonds retain their ionic character; they are weaker than Al-O bonds and lack a preferred orientation, which enables motion of the AlO 4 tetrahedra relative to each other. Hence, the T g of stoichiometric CA systems decreases by ∼50 K as the CaO content increases from 57 mol% to 70 mol% (Fig. 1). This relatively narrow interval of T g variation with composition is typical of multicomponent glasses (8, 9). The T g for stoichiometric [Ca 12 (5,14,15). In crystalline C12A7:e -, the electrons occupy the cage conduction band (CCB) states associated with the lattice cages (16, 17), leading to polaron-type conduction at x < ∼0.5 and metallic conduction at x > 0.5 (13, 16). The exceptionally low work function of crystalline C12A7:e -...

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Direct Evidence for Cage Conduction Band in Superconducting Cement 12CaO·7Al₂O₃ by Low-Energy High-Resolution Photoemission Spectroscopy

Cited by 14 publications

References 27 publications

Experimental Evidence of Cage Conduction Bands in Superconducting Cement 12CaO$\cdot$7Al$_2$O$_3$

Experimental Evidence of Cage Conduction Bands in Superconducting Cement 12CaO$\cdot$7Al$_2$O$_3$

Cage electron-hydroxyl complex state as electron donor in mayenite

Electron anions and the glass transition temperature

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Direct Evidence for Cage Conduction Band in Superconducting Cement 12CaO·7Al2O3 by Low-Energy High-Resolution Photoemission Spectroscopy

Cited by 14 publications

References 27 publications

Experimental Evidence of Cage Conduction Bands in Superconducting Cement 12CaO$\cdot$7Al$_2$O$_3$

Experimental Evidence of Cage Conduction Bands in Superconducting Cement 12CaO$\cdot$7Al$_2$O$_3$

Cage electron-hydroxyl complex state as electron donor in mayenite

Electron anions and the glass transition temperature

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Direct Evidence for Cage Conduction Band in Superconducting Cement 12CaO·7Al₂O₃ by Low-Energy High-Resolution Photoemission Spectroscopy