The impact of anion ordering on octahedra distortion and phase transitions in SrTaO₂N and BaTaO₂N

Abstract

In this work we synthesized BaTaON and SrTaON using a two-step high-temperature solid-state reaction method and analysed the structural distortions, relative to the ideal cubic perovskite structure, according to group theory. From a complete distortion analysis/refinement using high-resolution neutron diffraction data in the temperature range 8 to 613 K, we identified tetragonal structures for BaTaON [P4/mmm (No. 123)] and SrTaON [I4/mcm (No. 140)]. In contrast to an anion-disordered cubic perovskite (Pm \overline{3}m No. 221) with Ta at the cell center, both systems show a site preference for oxygen anions in the two opposite corners (along the c axis) of the Ta-O/N octahedra rather than the four square corners in the ab plane (Γ occupancy distortion), which induces a tetragonal elongation of the unit cell with the c axis being longer than the a axis. A further Ta-O/N octahedra displacement [R(a,0,0), rotation about the c axis] distortion was observed in SrTaON. This distortion mode is accompanied by an increased unit-cell distortion that decreases as the temperature increases. Ultimately a second-order phase transition caused by the loss of the R(a,0,0) mode was observed at 400-450 K.

Probing the Site-Selective Doping in SrSnO3:Eu Oxides and Its Impact on the Crystal and Electronic Structures Using Synchrotron Radiation and DFT Simulations

“…Even after the DFT calculations, it was not possible to definitely determine the type of band gap because defects, such as oxygen vacancies and reduced Sn, were not considered. Thus, the y axes in the spectra were plotted as (α h ν) 2 to derive direct band gaps (Figure a) and as (α hv ) 1/2 to obtain indirect band gaps (Figure b) …”

“…Thus, the y axes in the spectra were plotted as (αhν) 2 to derive direct band gaps (Figure 8a) and as (αhv) 1/2 to obtain indirect band gaps (Figure 8b). 63 Slightly different direct E g values were observed for the Eudoped samples (E g(exp) = 4.07 eV for the (Sr 0.99 Eu 0.01 )SnO 3 and E g(exp) = 4.14 eV for the Sr(Sn 0.99 Eu 0.01 )O 3 ) compared to that for undoped SrSnO 3 (E g(exp) = 4.08 eV) perovskite, which may be related to the different amount of induced structural defects due to the local lattice distortions, as indicated by the S-XRD results. This evidence is in accordance with the DFT data presented before.…”

Ammonothermal Synthesis of EAMO2N (EA = Sr, Ba; M = Nb, Ta) Perovskites and 14N Solid‐State NMR Spectroscopic Investigations of AM(O,N)3 (A = Ca, Sr, Ba, La)

“…It should be noted that during ammonothermal synthesis employing two mineralizers (NaOH and NaNH 2 ) local concentration variations may be possible resulting in slight differences of oxygen and nitrogen contents of the products . However, the true crystal structure of BaTaO 2 N is still controversially discussed in literature . The local symmetry in the crystals could be broken and space group Pm 3 m used in our Rietveld refinement represents the statistically averaged structure , …”

Perovskite BaTaO2N: From Materials Synthesis to Solar Water Splitting

“…123)) shows a site preference for oxide anions in the two opposite corners (along the c ‐axis) of the TaO 4 N 2 octahedra rather than the four‐square corners in the ab plane, leading to a distortion of the unit cell with the c ‐axis being slightly longer than the a ‐axis. [ 88 ] Figure 4c,d shows a schematic representation of the band structure and DFT‐calculated band structure of BaTaO 2 N. The top of the valence band (highest occupied molecular orbital – HOMO) shifts upward due to the presence of hybridized N 2p and O 2p orbitals, while the bottom of the conduction band (lowest unoccupied molecular orbital – LUMO), which is composed of empty Ta 5d orbitals, remains unaffected. [ 62,89 ] By comparing the three different crystal structures of BaTaO 2 N, Bettine et al.…”