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The main objective of the project was to develop a general theory of conductivity in oxides with wide band-gap (3 eV and above), namely, the oxides of main group metals such as post-transition metals Zn, Ga, In, Cd, Sn as well as light metals Mg, Ca, Al, Si. All these oxides share the same electronic configuration of cations ns0, yet, their electrical properties are very different ranging from classic insulators (MgO, CaO, Al2O3) to nearly-metallic conductors (doped In2O3, CdO, SnO2 and ZnO). Our goal was to understand the origin of electron localization.

Our comparative studies of post-transition and light-metal wide-bandgap oxides have revealed that the energy location of the cation(s) empty p-states with respect to the conduction band bottom plays the key role in determining the transport properties in these materials. Our results explain the trend in the observed conductivities/carrier mobilities in the conventional TCOs such as In2O3, ZnO and Ga2O3, and explain why light-metal oxides (Al2O3, CaO, MgO) remain classical insulators.

In accord with the results for single-cation oxides, our analysis of the nature of the conduction band in oxygen deficient 12CaO.7Al2O3 suggested that the conduction wave function is composed primarily of the Ca s and O p states (46 % and 48 % of the total charge in the cell, respectively). Because all Ca atoms in the cell give identical contributions to the conduction band, the resulting charge density distribution is uniform throughout the cage framework.The delocalization of the extra electrons in the oxygen deficient 12CaO.7Al2O3 manifests itself in a large electron velocity (5.57 x 10e-5 m/s in the [111] direction) to be compared to those calculated for oxygen deficient In2O3 (5.88 x 10e-5 m/s) and ZnO (3.90 x 10e-5 m/s) with similar carrier concentration.

To understand why the CaO and Al2O3 are classical insulators with a deep state for an oxygen vacancy defect (an F-center), whereas in 12CaO.7Al2O3, a conductivity of 1700 S/cm has been achieved, we compared the local oxygen coordination of Ca atoms in rocksalt CaO and in 12CaO.7Al2O3. It is found that Ca atoms in the latter have an unusual coordination with oxygen atoms: four planar oxygen atoms are at the same distance as in bulk CaO (2.4 Angstrom), whereas the other two O atoms are located at ~2.7 Angstrom. The electronic band structure calculation of hypothetical rocksalt CaO with elongated metal-oxygen distances (2.7 Angstrom) reveals that this results in an s-like bottom of the conduction band while the localized d-orbitals detrimental for charge transport are located at a higher energy. Therefore, our results reveal that the origin of the observed persistent conductivity in mayenite, Ca12Al14O33, is the unusual oxygen coordination of the Ca atoms which leads to an s-like conduction band.

Dr. Min Sik Park (former postdoctoral fellow in the PI's group working on the project, now at Samsung R&D) has performed first-principles calculations of transport coefficients for 12CaO.7Al2O3. This recently-developed capability was implemented within highly precise all-electron full-potential linearized augmented plane wave method (FLAPW). This code allows us to calculate temperature- and carrier-concentration-dependent electrical conductivity (sigma) and Seebeck coefficient (S) based on the distribution function given by Boltzmann equation in the constant relaxation time approximation. For the calculation of the group velocity which is included in the transport coefficients, we use full intra-band optical matrix elements defined within FLAPW method.

More specifically, Dr. Park calculated the electronic, optical and thermoelectric properties of undoped, stoichiometric and highly reduced Ca12Al14O33. The results of our detailed band structure investigations provide an insight into the observed insulator to metal transition in cage-structured mayenite. The calculated room temperature Seebeck coefficient -11 mV/K, and electrical conductivity, 634 S/cm, are both in excellent agreement with the experiment, -12.9 mV/K and 567 S/cm. Moreover, our calculated Seebeck coefficient and electrical conductivity as a function of the carrier concentration explain the observed change in the sign of the measured Seebeck coefficient – from -8 mV/K to +7 mV/K and to -13 mV/K as the electron concentration increases. We analyzed the calculated transport distribution (TD) function and associated the pronounced drop in TD with the reduction in the electron velocities in specific directions of the Brillouin zone.

Further, to study the optical properties of insulating and metallic mayenite, the dielectric function was calculated within FLAPW. The the imaginary part of the dielectric function is related to the optical absorption at a given frequency. We found an excellent agreement between the calculations and the experiment, and have identified the calculated absorption peaks with particular electronic transitions.

Dr. Min Sik Park, who joined the group in October 2008, has been 90% supported by this project.

The results of this project have been either published or prepared for publication:

1. J.E. Medvedeva, Combining optical transparency with electrical conductivity: challenges and prospects, in “Transparent Electronics: From Synthesis to Applications”, Editors: A. Facchetti and T. Marks, Publisher: John Wiley & Sons; April 2010

2. J.E. Medvedeva and C.L. Hettiarachchi, Complex transparent conducting oxides with tunable properties: role of crystal symmetry, chemical composition and carrier generation, Physical Review B, 81, 125116 (2010).

3. Min Sik Park, Jung-Hwan Song, J.E. Medvedeva, M. Kim, In Gee Kim, A.J. Freeman, Electronic structure and volume effect on thermoelectric transport in p-type bismuth and antimony tellurides, Physical Review B, 81, 155211 (2010)

4. M.I. Bertoni, J.E. Medvedeva, M.S Park, A.J. Freeman, K.R. Poeppelmeier, and T.O. Mason,Insulator to metal transition in Ca12Al14O33, submitted for publication in PRB

5. R.P.S.M. Lobo, F. Corre, N. Bontemps, M.I. Bertoni,T.O. Mason, K.R. Poeppelmeier, A.J. Freeman, Min Sik Park, and J.E. Medvedeva, Optical conductivity of mayenite: from insulator to metal, submitted for publication in JACS

Another 2 manuscripts are in a draft stage of preparation for publication. All necessary calculations have been finished for these manuscripts, the results have been analyzed and prepared for publication in the form of tables/figures, and the first drafts have been prepared.