Site-Differentiated Solid Solution in (Na_1−<i>x</i>Cu_<i>x</i>)₂Ta₄O₁₁ and Its Electronic Structure and Optical Properties

The (Na1−xCux)2Ta4O11 (0 ≤ x ≤ 0.78) solid-solution was synthesized within evacuated fused-silica vessels and characterized by powder X-ray diffraction techniques (space group: R3̅c (#167), Z = 6, a = 6.2061(2)−6.2131(2) Å, c = 36.712(1)−36.861(1) Å, for x = 0.37, 0.57, and 0.78). The structure consists of single layers of TaO7 pentagonal bipyramids as well as layers of isolated TaO6 octahedra surrounded by Na+ and Cu+ cations. Full-profile Rietveld refinements revealed a site-differentiated substitution of Na+ cations located in the 12c (Wyckoff) crystallographic site for Cu+ cations in the 18d crystallographic site. This site differentiation is driven by the linear coordination geometry afforded at the Cu+ site compared to the distorted seven-coordinate geometry of the Na+ site. Compositions more Cu-rich than x ∼ 0.78, that is, closer to “Cu2Ta4O11”, could not be synthesized owing to the destabilizing Na+/Cu+ vacancies that increase with x up to the highest attainable value of ∼26%. The UV−vis diffuse reflectance spectra show a significant red-shift of the bandgap size from ∼4.0 eV to ∼2.65 eV with increasing Cu+ content across the series. Electronic structure calculations using the TB-LMTO-ASA approach show that the reduction in bandgap size arises from the introduction of Cu 3d10 orbitals and the formation of a new higher-energy valence band. A direct bandgap transition emerges at k⃗ = Γ that is derived from the filled Cu 3d10 and the empty Ta 5d0 orbitals, including a small amount of mixing with the O 2p orbitals. The resulting conduction and valence band energies are determined to favorably bracket the redox potentials for water reduction and oxidation, meeting the thermodynamic requirement for photocatalytic water-splitting reactions.