Magnetic and Transport Properties of Ba2Co9O14 and Ba1.9A0.1Co9O14 (A = La or Na)

The valence and spin-state distributions of Co ions and the complex structure of antiferromagnetic Ba2Co9O14 have led to the suggestion that doped Ba2Co9O14 compounds may be good thermoelectric materials. We have checked this suggestion by measuring the magnetic properties as well as the transport properties of nominal Ba(1.9)A(0.1)Co(9)O(14) (A = La or Na). We show that although all compounds are indicated to be single phase by powder X-ray diffraction analysis, they are all p-type polaronic conductors with low mobile-hole concentrations. Magnetic-susceptibility data of the parent and La-doped compounds give evidence of a second magnetic phase with ferromagnetic order setting in below 215 K; but this second phase is not seen in the Na-doped sample. We conclude that the structure is stabilized by oxidation and that cation exolution from the Ba2Co9O14 structure creates cation vacancies that oxidize the high-spin (HS) Co(II) to the intermediate-spin Co(III) where there is no hole doping; the low-spin (LS) Co(III) of the parent structure is not reduced by the substitution of La for Ba. Disordering of the HS and LS sites does not occur, so the doped Ba2Co9O14 compositions are not candidates for thermoelectric materials.