Publications in Scientific Journals:

R. Hollerweger, D. Holec, J. Paulitsch, M. Bartosik, R. Daniel, R. Rachbauer, P. Polcik, J. Keckes, C. Krywka, H. Euchner, P.H. Mayrhofer:
"Complementary ab initio and X-ray nanodiffraction studies of Ta2O5";
Acta Materialia, 83 (2015), 276 - 284.

English abstract:
The complex structure of Ta2O5 led to the development of various structural models. Among them, superstructures represent the most
stable configurations. However, their formation requires kinetic activity and long-range ordering processes, which are hardly present during physical
vapor deposition. Based on nano-beam X-ray diffraction and concomitant ab initio studies, a new metastable orthorhombic basic structure is introduced
for Ta2O5 with lattice parameters a = 6.425A ˚ , b = 3.769A ˚ and c = 7.706A ˚ . The unit cell containing only 14 atoms, i.e. two formula unit
blocks in the c direction, is characterized by periodically alternating the occupied oxygen site between two possible positions in succeeding 002-
planes. This structure can be described by the space group 53 (Pncm) with four Wyckoff positions, and exhibits an energy of formation of
3.209 eV atom 1. Among all the reported basic structures, its energy of formation is closest to those of superstructures. Furthermore, this model
exhibits a 2.5 eV band gap, which is closer to experimental data than the band gap of any other basic-structure model. The sputtered Ta2O5 films
develop only a superstructure if annealed at temperatures >800 C in air or vacuum. Based on these results and the conveniently small unit cell size, it
is proposed that the basic-structure model described here is an ideal candidate for both structure and electronic state descriptions of orthorhombic
Ta2O5 materials.

Ta2O5; Structure; Nanobeam diffraction; Ab initio; DOS

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