Syntheses, optical properties and electronic structures of copper(I) tantalates: Cu 5Ta 11O 30 and Cu 3Ta 7O 19

Abstract

Two copper tantalates, Cu 5Ta 11O 30 ( 1) and Cu 3Ta 7O 19 ( 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 800 oC and its structure was characterized using powder X-ray diffraction (XRD) data ( P6 3 /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 Cu 2O that helped to facilitate the growth of single crystals and their characterization by XRD ( P6¯2 c (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 TaO 7 pentagonal bipyramids that are bridged by a single layer of isolated TaO 6 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 NaTaO 3 of ∼1450 cm −1. Results of LMTO calculations indicate that their visible-light absorption is attributable mainly to indirect bandgap transitions between Cu 3 d 10 and Ta 5 d 0 orbitals within the TaO 7 pentagonal bipyramids.