Synthesis, Structure, and Thermal Instability of the Cu2Ta4O11 Phase

The Cu­(I)-tantalate, Cu2Ta4O11, has been synthesized by flux methods in high purity and characterized by single-crystal X-ray diffraction (space group R3̅c (167), a = 6.219(2) Å, c = 37.107(1) Å). The compound is a new n = 1 member of the Cu­(I)-tantalate CuxTa3n+1O8n+3 series of structures and can be prepared in a molten CuCl flux within a relatively low temperature range of ∼625–700 °C, in comparison to the synthesis of Cu5Ta11O30 (n = 1.5) and Cu3Ta7O19 (n = 2) at ∼800 to 1000 °C. The structure consists of layers of TaO7 pentagonal bipyramids that alternate with layers of isolated TaO6 octahedra and linearly coordinated Cu­(I) cations. An increasing Cu-site vacancy across this series from Cu3Ta7O19 (100%), to Cu5Ta11O30 (83.3%), to Cu2Ta4O11 (66.7%) leads to an increasing fraction of O atoms that are not locally charge balanced by the Ta­(V)/Cu­(I) cations and thus yields decreased stability of Cu2Ta4O11. Thermal analysis shows that Cu2Ta4O11 decomposes in air or under flowing nitrogen at temperatures above ∼550 °C (in the absence of the CuCl flux) into a mixture of known tantalates and Cu­(II)-tantalate phases. The compound exhibits a bandgap size of ∼2.55 eV (indirect), with higher-energy direct transitions starting at ∼2.73 eV. Electronic structure calculations confirm the indirect nature of the lowest-energy bandgap transition, which arises from valence and conduction band states that are primarily composed of Cu 3d10 and Ta 5d0 orbital contributions, respectively.