New intergrowth compounds in the La–Co–Al system, La₃Co₅Al₂, La₆Co₇Al₇, and La₄Co_9.6Al_3.4: Crystal structures, bonding, and magnetism

Three new intergrowth compounds in the La–Co–Al system have been synthesized using arc-melting with subsequent annealing. La₃Co₅Al₂ and La₆Co₇Al₇ crystallize in the La₆Co₁₁Ga₃-type (I4/mcm, tI80) with stoichiometry defined by different filling of the two Co sites: a = 8.1959 (8), c = 23.082 (7) Å (La₃Co₅Al₂) and a = 8.2426 (13), c = 23.768 (10) Å (La₆Co₇Al₇). The structures present a stacking variant of the YNi₉In₂-, U₃Si₂-, and CuAl₂-type layers along the c-direction. La₄Co_9.6Al_3.4 compound crystallizes with its own type (P4/mbm, tP34, a = 8.2060 (13), c = 8.989 (2) Å) and represents an intergrowth structure composed of YNi₉In₂- and U₃Si₂-type slabs stacked along the c-axis. La₃Co₅Al₂ and La₆Co₇Al₇ compounds do not form a homogeneous region whereas for La₄Co_9.6Al_3.4 compound a narrow solid solubility range has been detected. DFT calculations predict all compounds to be metallic, with Co d-states playing a major role near the Fermi level. Bonding analysis reveals a large number of bonding contacts for each compound and indicates complex systems of delocalized bonding. Combined specific heat and magnetic susceptibility measurement data indicate low-temperature transition to a magnetic state for the La₄Co_9.6Al_3.4 compound.