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 800oC and its structure was characterized using powder X-ray diffraction (XRD) data (P63/m (no. 176), Z=2, a=6.2278(1) Å, and c=20.1467(3) Å). 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¯2c (no. 190), Z=2, a=6.2252(1) Å, and c=32.516(1) Å). 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.