Site-Differentiated Solid Solution in (Na_1−xCu_x)₂Ta₄O₁₁ and Its Electronic Structure and Optical Properties

Abstract

The (Na(1-x)Cu(x))(2)Ta(4)O(11) (0 ≤ x ≤ 0.78) solid-solution was synthesized within evacuated fused-silica vessels and characterized by powder X-ray diffraction techniques (space group: R3c (#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 TaO(7) pentagonal bipyramids as well as layers of isolated TaO(6) 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 "Cu(2)Ta(4)O(11)", 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 3d(10) orbitals and the formation of a new higher-energy valence band. A direct bandgap transition emerges at k = Γ that is derived from the filled Cu 3d(10) and the empty Ta 5d(0) 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.

Structural phase transitions in the Ag2Nb4O11–Na2Nb4O11 solid solution

“…These materials are not based solely on corner-sharing octahedra, but are comprised of alternating layers of distorted edge-sharing TaO 7 or NbO 7 polyhedra which are corner-connected to layers containing regular TaO 6 or NbO 6 octahedra. The larger, lower valancy cations are incorporated into the layers with the regular octahedra and, in addition to Ag + , Na + and Ca 2+ , the cations Cu + [10,11], Sr 2+ [12], and La 3+ [13] are compatible with this structure, although the presence of La 3+ requires charge-balancing by replacing some Ta 5+ with Zr 4+ . The majority of known phases with the natrotantite structure are formed with Ta 5+ .…”

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

“…The solid solutions did not display any phase transformation; however, Wyckoff site substitution of Na cations and Cu cations in the R3c crystal structure of the solid solution were observed together with a significant decrease in bandgap size from ≈4.0 eV to ≈2.65 eV [71]. The origin of the differential site occupancy of the Ag and Cu cations is driven by the preferred coordination number of the Na/Cu cation to the surrounding O atoms [71].…”

“…The Na 2−x Cu x Ta 4 O 11 solid solutions were also investigated by the Maggard group with compositional limits between 0 ≤ x ≤ 0.78 at a reaction temperature of 950 • C [71]. The solid solutions did not display any phase transformation; however, Wyckoff site substitution of Na cations and Cu cations in the R3c crystal structure of the solid solution were observed together with a significant decrease in bandgap size from ≈4.0 eV to ≈2.65 eV [71].…”

“…Improved charge separation and photocatalytic/photoelectrochemical redox reactions have been proposed to be enhanced in these structures due to the preferential anisotropic diffusion of charge carriers along the BO 7 pentagonal bipyramid layers. The anisotropic charge diffusion is a result of the delocalization of the Nb 4d and the Ta 5d lowest-energy unoccupied crystal orbitals across the BO 7 pentagonal bipyramid layers, as confirmed by electronic structure calculations which show the largest band dispersion in these crystallographic directions [35,47,64,65,71,[122][123][124][125][126][127][128].…”

Enhanced Photocatalytic Property of Cu Doped Sodium Niobate

“…It has been demonstrated that doping Cu in oxides and sulfides can modify their electronic structures [25][26][27][28][29][30][31]. For example, the replacement of Cu for Na in Na 2 Ta 4 O 11 can create visible light absorption due to the introduction of Cu d 10 orbital in the top of valence band [25].…”