Study of Anion Order/Disorder in RTaN₂O (R = La, Ce, Pr) Perovskite Nitride Oxides

Symmetries and model structures are given for ABN₂O (and ABNO₂) perovskites that possess long-range ordering of anions in combination with <i>a</i>⁰<i>a</i>⁰<i>c</i>^–, <i>a</i>^–<i>b</i>⁰<i>a</i>^–, and <i>a</i>^–<i>b</i>⁺<i>a</i>^– octahedral tilting. The stabilities of competing structures have been evaluated using density functional theory (DFT) calculations, which show that <i>cis</i>-ordered models are more stable than competing <i>trans</i>-ordered polymorphs. To test the validity of these predictions, the perovskite nitride oxides LaTaN₂O, CeTaN₂O, and PrTaN₂O have been synthesized and characterized using neutron powder diffraction. CeTaN₂O and PrTaN₂O crystallize with orthorhombic <i>Pnma</i> symmetry (Ce: <i>a</i> = 5.69666(8), <i>b</i> = 8.03272(9), and <i>c</i> = 5.70893(7) Å; Pr: <i>a</i> = 5.6868(1), <i>b</i> = 8.0153(1), and <i>c</i> = 5.68057(8) Å) as a result of <i>a</i>^–<i>b</i>⁺<i>a</i>^– tilting of the octahedra. The structure of LaTaN₂O is re-examined and found to possess orthorhombic <i>Imma</i> symmetry (<i>a</i> = 5.7093(1), <i>b</i> = 8.0591(2), and <i>c</i> = 5.7386(2) Å) as a result of <i>a</i>^–<i>b</i>⁰<i>a</i>^– tilting. No evidence for long-range anion order is found in any of the three compounds. Optical band gaps for these compounds are measured to be 2.0 eV (LaTaN₂O), 1.9 eV (CeTaN₂O), and 2.0 eV (PrTaN₂O). These values are 0.6–0.7 eV smaller than CaTaNO₂ where the Ta-centered octahedra tilt by a similar amount. As the nitrogen content increases, there is an increase in the overlap of the anion 2p orbitals, which increases the energy of the valence band maximum and narrows the band gap