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 a0a0c-, a-b0a-, and a-b+a- octahedral tilting. The stabilities of competing structures have been evaluated using density functional theory (DFT) calculations, which show that cis-ordered models are more stable than competing trans-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 Pnma symmetry (Ce: a = 5.69666(8), b = 8.03272(9), and c = 5.70893(7) Å; Pr: a = 5.6868(1), b = 8.0153(1), and c = 5.68057(8) Å) as a result of a-b+a- tilting of the octahedra. The structure of LaTaN2O is re-examined and found to possess orthorhombic Imma symmetry (a = 5.7093(1), b = 8.0591(2), and c = 5.7386(2) Å) as a result of a-b0a- 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.