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.

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.Figure optionsDownload as PowerPoint slide