Comparative study of rutile and anatase SnO2 and TiO2 : Band-edge structures, dielectric functions, and polaron effects

• SnO2 and TiO2 polymorphs (rutile and anatase) are oxides with similar crystal structures, comparable bond lengths, and electronic band-gap energies, but different optical and electronic properties. In this work, we have studied the origin of these differences from the band-edge structures and electron-phonon coupling. The band-edge structures, dielectric functions, and effective masses were calculated by means of a first-principles approach with the exchange-correlation described by a hybrid functional. The phonon frequencies were calculated using a finite displacement method with non-analytic correction, and the phonon contribution to the dielectric functions was modeled using a multi-phonon Lorentz model. The calculated band-edge structures show that the bottommost conduction bands are highly dispersive for SnO2 polymorphs but flat dispersive for TiO2 polymorphs because of the strongly localized Ti-3d states. Consequently, SnO2 polymorphs present small effective electron masses and a weak optical absorption, whereas the TiO2 polymorphs present a strong optical absorption and larger effective electron masses. Due to the strong ionic bonds, TiO2 have larger Born effective charges than that of SnO2, result in stronger polaron effect and larger average static dielectric constant epsilon(0). For example, epsilon(0) = 115 for rutile TiO2 whereas epsilon(0) = 9.5 for rutile SnO2. Moreover, it is interesting to note that the epsilon(0) in rutile TiO2 is much larger than in anatase TiO2 (epsilon(0) = 28) although they have the same chemical compositions, which related to the local structure distortion of the phases.

Ämnesord

NATURVETENSKAP  -- Fysik -- Annan fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Other Physics Topics (hsv//eng)

Nyckelord

Born effective charge

Chemical compositions

Electron phonon couplings

Electronic band gaps

Exchange-correlations

First-principles approaches

Optical and electronic properties

Static dielectric constants

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