Contents and Chemical Science

The substitution of cobalt and nickel for manganese in the perovskite manganate LaBiMn 2 O 6+δ has been realized, leading to the perovskites LaBiMn 2- M x O 6+δ , with M = Co, Ni and 0 ≤ x ≤ 2/3.In contrast to the literature those phases are found to be orthorhombic with Pnma symmetry.More importantly, it is shown that ferromagnetism is enhanced, T C being increased from 80 K for the parent compound (x = 0) to 97 K for Ni-phase, and to 130 K for the Co-phase.Moreover, a strong competition between ferromagnetism and a glassy-ferromagnetic state in the case of nickel or a spin-glass behaviour in the case of cobalt is observed.These phenomena are interpreted in the frame of a phase separation scenario, where the ferromagnetic Mn 4+ /Ni 2+ and Mn 4+ /Co 2+ interactions reinforce the Mn 3+ /Mn 4+ interactions.These compounds are found to be insulating with a relatively large positive value of thermoelectric power.