Site occupancy and cation binding states in reduced polycrystalline SrxBa1-xNb2O6

Site occupancy and cation binding states in the proposed thermoelectric n-type oxide SrxBa1-xNb2O6 (SBN100x) were investigated using X-ray photoelectron spectroscopy (XPS). Sr 3d XPS spectra from unreduced polycrystalline SBN100x with various compositions contained two distinct spin-orbit doublets corresponding to Sr occupying either A1 or A2 positions in the SBN lattice; the higher binding energy state was associated with Sr ions at A2 sites, presumably due to their increased coordination over Sr at A1 sites. To gain insight into optimizing the thermoelectric properties of reduced SBN, sintered SBN50 specimens were reduced in Ar/H-2 or N-2/H-2 ambient. A decrease in the average Nb valence was observed in Nb 3d photoemission through the growth of low-binding energy components after reduction in either environment; evidence of surface NbN formation was apparent with longer reducing times in N-2/H-2. Both the single-component Ba 3d emission and the A2 component of the Sr 3d spectra show shifting to lower binding energy as the reduction time is increased, supporting the hypothesis of preferential oxygen vacancy formation adjacent to A2 sites. X-ray diffraction patterns revealed the formation of NbO2 in both reducing environments; in the case of extended reduction in N-2/H-2, NbO2 is gradually converted to NbN phases. Given the known properties of metallic NbN and semiconducting NbO2, the findings obtained here may be used to maximize the thermoelectric performance of SBN via the fabrication of composite structures containing both NbO2 and NbN. (C) 2014 AIP Publishing LLC.