Two-Dimensional Fluorine Distribution in a Heavily Distorted Perovskite Nickel Oxyfluoride Revealed by First-Principles Calculation

Abstract

Most perovskite oxyfluorides synthesized to date have cubic structures, wherein fluorine atoms tend to reside at every oxygen site randomly. In this theoretical study, we show a twodimensional fluorine arrangement in a perovskite nickel oxyfluoride (NdNiO 2 F) with a large orthorhombic distortion. The site selectivity in the perovskite lattice is due to the orthorhombicity, which stabilizes the two-dimensional cis configurations with shorter Ni−O and longer Ni−F bonds to minimize the electrostatic energy. The electronic structure of NdNiO 2 F is characterized by its large octahedral rotation and the higher electronegativity of fluorine than oxygen. We also observed how the anion arrangement was affected by the biaxial strain by modeling epitaxial strained thin film on substrate and found that the 2D cis structure remains the most stable. However, the orientation of the two-dimensional structure containing F depends on the magnitude of the biaxial strain. Our findings suggest that fluorine doping in orthorhombic perovskite oxides effectively yields oxyfluorides with anisotropic anion ordering.

Symmetry and the Role of the Anion Sublattice in Aurivillius Oxyfluoride Bi2TiO4F2

“…Nevertheless, by using penalties for both cation and anion sites, most structures refined to give Ti–O bonds shorter than Ti–F bonds. This is consistent with structures reported for NdNiO 2 F, 75 α- A 3 MoO 3 F 3 ( A = Rb and K), 1 , 4 and other oxyfluorides. 19 For both Γ 3 – and M 3 – apical ordering, the GII results in Table 2 and Figure 7 suggest that, in general, the fac structures are less strained than the equivalent mer structures, which is again consistent with the importance of pπ dπ bonding between oxygen and d 0 cations, favoring cis and fac isomers with shorter Ti–O bonds.…”

O/F anion order in Pb$_2$Ti$_4$O$_9$F$_2$ stabilized by the 6$s^2$ lone pair electrons of Pb$^{2+}$

“…5. The total energies are roughly proportional to the electrostatic potentials, consistent with a previous systematic study on NdNiO 2 F by simulations [46]. Meanwhile, the F-in-site6 cannot be fully explained by the Pauling's second law (structural stabilization via electrostatic gain), requiring us to investigate further the origin of the F-in-site6 stabilization.…”

Anionic ordering in Pb2Ti4O9F2 revisited by nuclear magnetic resonance and density functional theory

“…The total energies are roughly proportional to the electrostatic energies, consistent with a previous systematic study on NdNiO 2 F by simulations. 49 However, F-in-site6, which had the lowest total energy, is an exception to Pauling's second rule 28 because the lowest electrostatic energy was predicted for the F-in-site5. The reason of the small deviation (∼0.1 eV per f.u.)…”