Thermoelectric misfit-layered cobalt oxides with interlayers of hydroxide and peroxide species

Abstract

Among the thermoelectric misfit-layered cobalt oxides, [M(m)A(2)O(m+2)]qCoO(2), the parent m=0 phases exhibit divergent chemical features but are less understood than the more common m>0 members of the series. Here we synthesize Sr-for-Ca substituted [(Ca(1-x)Sr(x)c)(z)(O,OH)(2)](q)CoO2 zero phases up to x=0.2 through low-temperature hydrothermal conversion of precursor powders of the m=1 misfit system, [Co(Ca1-xSrx)(2)O-3](q)CoO2. In the zero-phase [(Ca1-xSrx)(z)(O,OH)(2)](q)CoO2 system, as the Sr content x increases the lattice expands anisotropically along the c axis such that the ab-plane dimension and the misfit parameter q remain essentially constant. X-ray absorption spectroscopy data suggest the presence of peroxide-type oxygen species in the (Ca1-xSrx)(z)(O,OH)(2) rock-salt block and together with infrared spectroscopy, thermogravimetric and low-temperature resistivity and thermopower measurements evidence that the isovalent Sr-for-Ca substitution controls the balance between the peroxide and hydroxide species in the (Ca1-x,Sr-x)(z)(O,OH)(2) block but leaves the valence of Co essentially intact in the CoO2 block. The higher electrical conductivity of the Sr-substituted phases is explained as a consequence of increased carrier mobility. (C) 2013 Elsevier Inc. All rights reserved.