Structural investigation of oxygen non-stoichiometry and cation doping in misfit-layered thermoelectric (Ca 2CoO 3−x)(CoO 2) δ, δ≈1.61

Abstract

Astract The chemical and crystallographic natures of oxygen non-stoichiometry and cation doping in the ‘misfit-layered’ cobaltate [Ca2CoO3−x][CoO2]δ, δ≈1.61, were investigated. Single-phase polycrystalline samples were prepared with the help of a high-energy planetary ball mill, and single crystals grown from a K2CO3/KCl flux. Polycrystalline undoped, Ti4+-doped, and Nd3+-doped samples were annealed under both air and under argon in order to modify the oxygen non-stoichiometry. The incommensurately modulated composite crystal structures of undoped and Ti4+-doped single crystals were refined against X-ray diffraction data using the superspace group X2/m(0,δ,0)s0. For the as-made (air-annealed) undoped crystal, 14(2)% oxygen vacancies were found on the disordered O2 site in the central layer of the rock-salt-type subsystem, giving an overall stoichiometry [Ca2CoO2.86][CoO2]1.61. For the Ti4+-doped crystal this site was found to be almost fully occupied, while Ti4+-dopant cations were located on the disordered Co1 site in the central layer of the rock-salt-type subsystem, giving an overall formula [Ca2Co0.74Ti0.26O2.98][CoO2]1.61. In both cases, the average cobalt oxidation state is 3.13+. The single-crystal refinements, thermogravimetric analyses and variations in the lattice parameters of polycrystalline samples refined against synchrotron X-ray diffraction (XRD) data indicate that for both undoped and Ti4+-doped samples, substantial numbers of additional oxygen vacancies (15–20%) can be introduced on this site by annealing under an inert atmosphere. Nd3+-doped samples have a much lower tolerance (