Spectroscopic characterization of a multiband complex oxide: Insulating and conducting cement 12CaO$\ifmmode\cdot\else\textperiodcentered\fi{}$7Al${}_{2}$O${}_{3}$

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Spectroscopic characterization of a multiband complex oxide: Insulating and conducting cement 12CaO $\cdot$ 7Al $_{2}$ O $_{3}$

J. A. McLeod, A. Buling, E. Z. Kurmaev, P. V. Sushko, M. Neumann, L. D. Finkelstein, S.-W. Kim, H. Hosono, and A. Moewes

Phys. Rev. B 85, 045204 – Published 13 January 2012

Abstract

Natural 12CaO $\cdot$ 7Al $_{2}$ O $_{3}$ (C12A7) is a wide band gap 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, x-ray 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.

Received 15 July 2011

DOI:https://doi.org/10.1103/PhysRevB.85.045204

Authors & Affiliations

J. A. McLeod^*

Department of Physics and Engineering Physics, University of Saskatchewan, 116 Science Place, Saskatoon, Saskatchewan, Canada S7N 5E2

A. Buling

Department of Physics, University of Osnabrück, Barbarastraße 7, D-49069 Osnabrück, Germany

E. Z. Kurmaev

Institute of Metal Physics, Russian Academy of Sciences–Ural Division, 620990 Yekaterinburg, Russia

P. V. Sushko

Department of Physics and Astronomy and London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, United Kingdom and WPI–Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

M. Neumann

Department of Physics, University of Osnabrück, Barbarastraße 7, D-49069 Osnabrück, Germany

L. D. Finkelstein

Institute of Metal Physics, Russian Academy of Sciences–Ural Division, 620219 Yekaterinburg, Russia

S.-W. Kim

Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

H. Hosono

Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

A. Moewes

Department of Physics and Engineering Physics, University of Saskatchewan, 116 Science Place, Saskatoon, Saskatchewan, Canada S7N 5E2

^*john.mcleod@usask.ca

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Vol. 85, Iss. 4 — 15 January 2012

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Figure 1
The lattice structure of C12A7:e $^{-}$ (the location of the electrons is given for visualization purpose only). The lattice is a body centered cubic (in the $I \bar{4} 3 d$ space group) with a lattice constant of 11.989 Å (Ref. 12).Reuse & Permissions
Figure 2
The survey XPS spectrum and the O K XES spectrum of sample B (conducting). The O K XES spectrum of sample B is compared to the O K XES spectrum of several reference materials.Reuse & Permissions
Figure 3
The XPS spectra collected for samples A and B. (a) The O $1 s$ spectra; the inset offers a more direct comparison of the O $1 s$ peak. (b) The Ca $2 p_{3 / 2}$ and $2 p_{1 / 2}$ spectra. (c) The Al $2 p_{3 / 2, 1 / 2}$ spectra. (d) The valence band spectra, as well as the semicore O $2 s$ and Ca $3 p$ peaks.Reuse & Permissions
Figure 4
The XES and XAS spectra. (a) The Ca L $_{2, 3}$ XES and $2 p$ XAS. (b) The O K XES and $1 s$ XAS; note that two different XES spectra corresponding to different excitation energies (535 and 550 eV) are plotted for each sample. The second derivative of the XES (excited at 535 eV) and XAS for sample A are shown in inset in the middle, on a consistent energy scale. (c) The Al L $_{2, 3}$ XES and $2 p$ XAS; the second derivative of the XES and XAS for sample A is inset in the middle, again on a consistent same energy scale. In all panels, vertical lines emphasize the hybridization between the spectra.Reuse & Permissions
Figure 5
The calculated DOS (lines) compared to the XES, XAS, and valence XPS measurements (circles). The DOS projected on the atomic orbitals of each atomic species are plotted along with the corresponding XES and XAS spectra. (a) The Ca states and spectra; note that the Ca L $_{2}$ emission has been cut off at $\sim$ $- 0.5$ eV due to the onset of the reemission lines shown in Fig. 4, and the spectral weight in the Ca L $_{2, 3}$ XES between $- 3$ and 0 eV is due to the L $_{2}$ emission line, not background counts. (b) The O states and spectra (note that the non-resonantly-excited O K XES for sample A and the resonantly excited O K XES for sample B are shown). (c) The Al states and spectra. (d) The total valence states and spectra. The difference between the O $1 s$ XAS of samples B and A is plotted in (e) in green in the middle of the figure, while a magnification of the valence XPS near the Fermi level is plotted in (f) at the bottom of the figure. The calculated highest occupied state in C12A7:O $^{2 -}$ and the Fermi level in C12A7:e $^{-}$ are labeled as E $_{F}$ (A) and E $_{F}$ (B), respectively. The energy scale is aligned with the valence XPS measurements, and is accurate with respect to the core level energies of Fig. 3.Reuse & Permissions
Figure 6
A stylized schematic of the C12A7 electronic structure. Occupied states are shaded while unoccupied states are clear. The Fermi level of C12A7:e $^{-}$ is noted, and the gaps between regions that can be determined by our spectroscopy techniques are labeled. The figure is to scale in terms of energy; each tick mark on the central energy axis represents 1 eV. (See Table for numerical values.)Reuse & Permissions

Physical Review B

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Spectroscopic characterization of a multiband complex oxide: Insulating and conducting cement 12CaO $\cdot$ 7Al $_{2}$ O $_{3}$

J. A. McLeod, A. Buling, E. Z. Kurmaev, P. V. Sushko, M. Neumann, L. D. Finkelstein, S.-W. Kim, H. Hosono, and A. Moewes

Phys. Rev. B 85, 045204 – Published 13 January 2012

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Physical Review B

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Spectroscopic characterization of a multiband complex oxide: Insulating and conducting cement 12CaO·7Al2O3

J. A. McLeod, A. Buling, E. Z. Kurmaev, P. V. Sushko, M. Neumann, L. D. Finkelstein, S.-W. Kim, H. Hosono, and A. Moewes

Phys. Rev. B 85, 045204 – Published 13 January 2012

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Spectroscopic characterization of a multiband complex oxide: Insulating and conducting cement 12CaO $\cdot$ 7Al $_{2}$ O $_{3}$