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Title: Polaron formation, native defects, and electronic conduction in metal tungstates

Publication Date:
Sponsoring Org.:
OSTI Identifier:
Grant/Contract Number:
SC0001717; AC02-05CH11231
Resource Type:
Journal Article: Publisher's Accepted Manuscript
Journal Name:
Physical Review Materials
Additional Journal Information:
Journal Volume: 1; Journal Issue: 2; Related Information: CHORUS Timestamp: 2017-07-12 22:09:56; Journal ID: ISSN 2475-9953
American Physical Society
Country of Publication:
United States

Citation Formats

Hoang, Khang. Polaron formation, native defects, and electronic conduction in metal tungstates. United States: N. p., 2017. Web. doi:10.1103/PhysRevMaterials.1.024603.
Hoang, Khang. Polaron formation, native defects, and electronic conduction in metal tungstates. United States. doi:10.1103/PhysRevMaterials.1.024603.
Hoang, Khang. 2017. "Polaron formation, native defects, and electronic conduction in metal tungstates". United States. doi:10.1103/PhysRevMaterials.1.024603.
title = {Polaron formation, native defects, and electronic conduction in metal tungstates},
author = {Hoang, Khang},
abstractNote = {},
doi = {10.1103/PhysRevMaterials.1.024603},
journal = {Physical Review Materials},
number = 2,
volume = 1,
place = {United States},
year = 2017,
month = 7

Journal Article:
Free Publicly Available Full Text
This content will become publicly available on July 12, 2018
Publisher's Accepted Manuscript

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  • A correlation between the Fermi level pinning deduced from Schottky barrier heights and from electrical properties of irradiated III--V semiconductors is found. The correlation indicates that similar defects are responsible for the Fermi level stabilization in both cases. It is proposed that amphoteric native defects, i.e., the defects which change their electrical characteristics depending on the Fermi level position, play a dominant role in the processes leading to a Schottky barrier formation. A detailed analysis of metal--GaAs contacts shows that in this case the amphoteric defects responsible for the barrier heights are V/sub Ga/ (acceptor) and a donor complex As/submore » Ga/+V/sub As/. It is shown that for thick metal coverages two barriers are formed. A surface barrier determined by the charge associated with a native defect and the bulk barrier controlled by the bulk doping. The sum of the two barrier heights satisfies the Schottky condition for the interface, but it is the bulk barrier that determines the macroscopic electrical properties of the contact. The model explains the evolution of the Fermi level position at the interface observed for metal coverages varying in a broad range of thicknesses. The relationship of the present proposal to previous models of Schottky barriers is discussed.« less
  • Mixed valence O-doped UO 2+x and photoexcited UO 2 containing transitory U 3+ and U 5+ host a coherent polaronic quantum phase (CPQP) that exhibits the characteristics of a Fröhlich-type, nonequilibrium, phonon-coupled Bose-Einstein condensate whose stability and coherence are amplified by collective, anharmonic motions of atoms and charges. Complementary to the available, detailed, real space information from scattering and EXAFS, an outstanding question is the electronic structure. Mapping the Mott gap in UO 2, U 4O 9, and U 3O 7 with O XAS and NIXS and UM5 RIXS shows that O doping raises the peak of the U5f statesmore » of the valence band by ~0.4 eV relative to a calculated value of 0.25 eV. However, it lowers the edge of the conduction band by 1.5 eV vs the calculated 0.6 eV, a difference much larger than the experimental error. This 1.9 eV reduction in the gap width constitutes most of the 2–2.2 eV gap measured by optical absorption. In addition, the XAS spectra show a tail that will intersect the occupied U5f states and give a continuous density-of-states that increases rapidly above its constricted intersection. Femtosecond-resolved photoemission measurements of UO 2, coincident with the excitation pulse with 4.7 eV excitation, show the unoccupied U5f states of UO 2 and no hot electrons. 3.1 eV excitation, however, complements the O-doping results by giving a continuous population of electrons for several eV above the Fermi level. The CPQP in photoexcited UO 2 therefore fulfills the criteria for a nonequilibrium condensate. The electron distributions resulting from both excitations persist for 5–10 ps, indicating that they are the final state that therefore forms without passing through the initial continuous distribution of nonthermal electrons observed for other materials. Three exceptional findings are: (1) the direct formation of both of these long lived (>3–10 ps) excited states without the short lived nonthermal intermediate; (2) the superthermal metallic state is as or more stable than typical photoinduced metallic phases; and (3) the absence of hot electrons accompanying the insulating UO 2 excited state. This heterogeneous, nonequilibrium, Fröhlich BEC stabilized by a Fano-Feshbach resonance therefore continues to exhibit unique properties.« less
  • To realize a trivalent ion conduction in solids, the Sc{sub 2}(WO{sub 4}){sub 3}-type structure was chosen on the basis of the mobile trivalent ions and the structure which reduces the electrostatic interaction between the framework and the mobile trivalent ionic species as much as possible. The typical conductivity of the rare earth tungstates R{sub 2}(WO{sub 4}){sub 3} (R = Sc, Y, and Er-Lu) with the Sc{sub 2}(WO{sub 4}){sub 3}-type structure was found to be on the order of 10{sup {minus}5} S cm{sup {minus}1} at 600 C. Among the rare earth tungstates, Sc{sub 2}(WO{sub 4}){sub 3} ({sigma}{sub 600 C} = 6.5more » {times} 10{sup {minus}5} S cm{sup {minus}1}, E{sub a} = 44.1 kJ mol{sup {minus}1}) was found to be the most suitable size for the ionic conduction with regard to the relation between the mobile ion radius and the lattice size. The rare earth ion conducting characteristics were investigated by means of the rare earth concentration cell measurements and dc electrolysis. The electromotive force measurements with the Sc-Y binary alloy and the yttrium tungsten bronze as the electrodes strongly suggest the possibility of the trivalent ion conduction of rare earths such as Sc{sup 3+} and Y{sup 3+}. Furthermore, by the dc electrolysis, the mobile species was clarified to be the rare earth ions Sc{sup 3+} and Y{sup 3+} in the rare earth tungstates with the Sc{sub 2}(WO{sub 4}){sub 3}-type structure.« less
  • Monoclinic potassium rare-earth double tungstates [KRE(WO{sub 4}){sub 2}, RE = Y, Lu, Yb; KREW] are well suited as hosts for active lanthanide ion (Ln{sup 3+}) dopants for diode-pumped solid-state lasers, with particular interest in thin-disk laser configurations when they are grown as thin films. Using synchrotron white-beam x-ray topography, we have imaged defects and strain in top-seeded solution-grown (TSSG) bulk substrates of different rare-earth tungstates as well as within Yb{sup 3+}- and Tm{sup 3+}-doped epitaxies for thin-disk laser applications grown on these substrates by liquid-phase epitaxy. Higher structural stress in Yb:KYW/KYW epitaxies compared with Yb:KLuW/KLuW epitaxies is found to lowermore » efficiency in laser operation. The quality of Tm:KLuW/KLuW epitaxial films is sensitive to doping level, film thickness, and growth rate. Inhomogeneous stresses within the layers are dominated by lattice-mismatch effects rather than by crystallographic anisotropy.« less