Polymorphism and Structural Distortions of Mixed-Metal Oxide Photocatalysts Constructed with α-U₃O₈ Types of Layers

A series of mixed-metal oxide structures based on the stacking of α-U₃O₈ type pentagonal bipyramid layers have been investigated for symmetry lowering distortions and photocatalytic activity. The family of structures contains the general composition ${{\text{A}}^{m+}}_{\left(\right(n+1)/\text{m})}{\text{B}}_{(3n+1)}{\text{O}}_{(8n+3)}$ (e.g., A = Ag, Bi, Ca, Cu, Ce, Dy, Eu, Gd K, La, Nd, Pb, Pr, Sr, Y; B = Nb, Ta; $m=1\text{-}3$ ; $n=1$ , 1.5, 2), and the edge-shared BO₇ pentagonal pyramid single, double, and/or triple layers are differentiated by the average thickness, (i.e., 1 ≤ n ≤ 2), of the BO₇ layers and the local coordination environment of the "A" site cations. Temperature dependent polymorphism has been investigated for structures containing single layered ( $n=1$ ) monovalent ( $m=1$ ) "A" site cations (e.g., Ag₂Nb₄O₁₁, Na₂Nb₄O₁₁, and Cu₂Ta₄O₁₁). Furthermore, symmetry lowering distortions were observed for the Pb ion-exchange synthesis of Ag₂Ta₄O₁₁ to yield PbTa₄O₁₁. Several members within the subset of the family have been constructed with optical and electronic properties that are suitable for the conversion of solar energy to chemical fuels via water splitting.