Complementary ab initio and X-ray nanodiffraction studies of Ta2O5

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 = 6.425 Å, b = 3.769 Å and c = 7.706 Å. 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.

Mesoporous Tantalum Oxide Photocatalyst: Structure and Activity Evaluation

“…In agreement with previous observations, these values are higher than those determined on the basis of XRD data (∼2.11 eV, value close to βTa 2 O 5 and oxygen deficient βTa 2 O 5 with E g =2.5 eV) and 665 nm (∼1.9 eV, value close to 11 LTa 2 O 5 with E g =1.5 eV) respectively …”

“…Ta 2 O 5 is known to adopt a large number of crystal structures, classified according to the thermal history of the annealed samples as low (LTa 2 O 5 ) or high temperature (HTa 2 O 5 ‐ above 1300 °C) polymorphs . Amongst the orthorhombic LTa 2 O 5 forms, simpler or more complex crystalline structures were reported: the so‐called β‐Ta 2 O 5 whose unit cell contains two formula units (Ta 4 O 10 ) and larger periodic superstructures along the b ‐axis, m LTa 2 O 5 (where m =5‐25 is the periodic number) . Several structural differences between these orthorhombic LTa 2 O 5 forms are to be noted: β‐Ta 2 O 5 is composed by an arrangement of only TaO 6 octahedrons without any defects, whereas m LTa 2 O 5 includes TaO 6 octahedrons and TaO 7 pentagonal bipyramids with mandatory defects such as oxygen vacancies or (less energetically favored) interstitial cations .…”

Effect of high-flux, low-energy He+ ion irradiation on Ta as a plasma-facing material

“…The majority of Ta 2 O 5 is in orthorhombic phase, commonly known as β-Ta 2 O 5 . Observed β-Ta 2 O 5 XRD planes are (1 29 0), (0 2 0), and (4 0 0) corresponding to 2θ values of 33.3, 50.1, and 59.3°, respectively363738. We also notice evidence of hexagonal metastable δ-Ta 2 O 5 phase with a (1 1 2) XRD plane corresponding to a 2θ value of 72.8° 39.…”

Local atomic order of the amorphous TaOx thin films in relation to their chemical resistivity

“…ref. [22][23][24][25][26]. However, the signicance of local Ta 2 O 5 structure rather than long-range order and crystal symmetry, for the properties of this material was postulated.…”

“…There is still ongoing discussion about the crystal structure of stoichiometric Ta 2 O 5 . [21][22][23][24][25]28 To generate the scattering paths for EXAFS analysis in FEFF6 program we have considered several structures and the best agreement with experiment was obtained for the orthorhombic one, referred in literature as b A , 24,28 aer optimization at DFT level. This result is somewhat intriguing, as b A phase was predicted to be the least stable Ta 2 O 5 polymorph in DFT calculation.…”