Structural and magnetic properties of the cobaltate series (BaSr)4−xLa2xCo4O15

We report the structural and magnetic properties of a new class of cobaltates with the chemical formula (BaSr)${}_{4\ensuremath{-}x}$La${}_{2x}$Co${}_{4}$O${}_{15}$ ($x=0$, 0.5, and 1). These compounds crystallize in a hexagonal structure in which cobalt ions are distributed among two distinct crystallographic sites with different oxygen coordination. Three Co-O tetrahedra and one octahedron are linked by shared oxygen atoms to form Co${}_{4}$O${}_{15}$ clusters, which are packed together into a honeycomblike network. Partial substitution of Sr and/or Ba atoms by La allows one to adjust the degree of Co valence mixing, but all compositions remain subject to a random distribution of charge. Magnetic susceptibility together with neutron scattering measurements reveal that all studied specimens are characterized by competing ferro- and antiferro-magnetic exchange interactions that give rise to a three-dimensional Heisenberg spin-glass state. Neutron spectroscopy shows a clear trend of slowing down of spin-dynamics upon increasing La concentration, suggesting a reduction in charge randomness in the doped samples.