Syntheses, optical properties and electronic structures of copper(I) tantalates: Cu5Ta11O30 and Cu3Ta7O19

Two copper tantalates, Cu5Ta11O30 (1) and Cu3Ta7O19 (2), were synthesized by solid-state and flux synthetic methods, respectively. A synthetic route yielding 2 in high purity was found using a CuCl flux at 800deg. C and its structure was characterized using powder X-ray diffraction (XRD) data (P63/m (no. 176), Z=2, a=6.2278(1) A, and c=20.1467(3) A). The solid-state synthesis of 1 was performed using excess Cu2O that helped to facilitate the growth of single crystals and their characterization by XRD (P6-bar2c (no. 190), Z=2, a=6.2252(1) A, and c=32.516(1) A). The atomic structures of both copper tantalates consist of alternating single and double layers of TaO7 pentagonal bipyramids that are bridged by a single layer of isolated TaO6 octahedra and linearly-coordinated Cu+. Measured optical bandgap sizes of ∼2.59 and ∼2.47 eV for 1 and 2 were located well within visible-light energies and were consistent with their orange-yellow colours. Each also exhibits optical absorption coefficients at the band edge of ∼700 and ∼275 cm-1, respectively, and which were significantly smaller than that for NaTaO3 of ∼1450 cm-1. Results of LMTO calculations indicate that their visible-light absorption is attributable mainly to indirect bandgap transitions between Cu 3d10 and Ta 5d0 orbitals within the TaO7 pentagonal bipyramids. - Graphical abstract: The copper(I) tantalates, Cu5Ta11O30 and Cu3Ta7O19, were synthesized by high-temperature solid-state reactions and by a new CuCl flux method, respectively. Their structures consist of single and double layers of TaO7 pentagonal bipyramids separated by isolated TaO6 octahedra and Cu atoms. UV-vis spectra show visible-light bandgap sizes of ∼2.5-2.6 eV, which LMTO calculations show arise primarily from indirect transitions between the filled Cu 3d10 and the empty Ta 5d0 orbitals in the TaO7 pentagonal bipyramids.