| Popis: |
Oxide nitrides are an emerging class of compounds. Perovskite RETaN2O [RE = La (Imma), Ce (Pnma), Pr (Pnma)] as well as ATaO2N [A = Ca (Pnma), Sr (I4/mcm), Ba (Pm-3m)] and pyrochlore RE2Ta2O5N2 where RE = Ce, Pr (both: Fd-3m) have been synthesized by solid state and solution-based methods. Crystal structures, solved by powder XRD and NPD for all the rare earth analogs (La, Ce, Pr), are reported for the first time. Studies on the preparation techniques of oxide nitrides in both, bulk powder and film format, show that solution based precipitation techniques decrease crystallite size, increase reactivity, and enable isomorphic films by sedimentation processes. Computational studies on anion ordering generated a library of ordering models herein and finds that, like -O2N compounds, an ordered cis orientation and out-of-center tantalum displacement provide the most stable model for perovskites with an -N2O anion stoichiometry. UV-Vis diffuse reflectance reveals band gaps for CeTaN2O (2.0 eV), PrTaN2O (2.0 eV), Ce2Ta2N2O5(3.0 eV) and Pr2Ta2N2O5(3.3 eV). Structure-property relations from calculations elucidate that valence band maximum positions within the compound series is affected by bond lengths and f-orbital contributions. Calculated band gaps decrease as dispersion at the conduction band minimum decreases due to octahedral tilting. These insights shed light on why photocatalytic studies on these perovskite oxide nitrides do not yield appreciable oxygen evolution rates. |
