Transport and magnetic properties of polycrystalline La2CoMnO6-delta, 0less than or equal todeltaless than or equal to0.05, have revealed the existence of two distinguishable monoclinic ferromagnetic phases separated by a two-phase domain 0.02less than or equal todeltaless than or equal to0.05 and a pseudotetragonal (c/a<root2) phase with deltagreater than or equal to0.05 that was prepared at 600 degreesC. A nearly oxygen-stoichiometric sample having a magnetization M(5 K, 50 kOe)=5.78mu(B)/f.u. with a Curie temperature T(c)approximate to226 K is identified as atomically ordered La2Co2+Mn4+O6 containing about 1.8% antiferromagnetic spins at antisites. This ferromagnetic phase is an n-type polaronic conductor that progressively traps mobile electrons at the oxygen vacancies that introduced them on lowering the temperature. Although the x-ray-diffraction pattern can be indexed in orthorhombic (Pbnm) or monoclinic (P2(1)/n) symmetry with betaapproximate to90degrees, atomic order identifies the space group as P2(1)/n. A second monoclinic, ferromagnetic phase with T-c<150 K and deltaapproximate to0.05 has a large, positive thermoelectric power that increases progressively with decreasing temperature. Quenching a deltaapproximate to0.02 sample from 1350 degreesC into liquid N-2 gave a single phase with T-c=134 K and deltaapproximate to0.05. A sample with deltagreater than or equal to0.05 that was synthesized at 600 degreesC was pseudotetragonal (c/a<root2) and had a paramagnetic Weiss constant theta<T(c)approximate to225 K as well as a significantly smaller magnetization, but its magnetization curve M(T) showed no evidence of spin-glass behavior; its large, positive thermoelectric power was characteristic of polaronic conduction without trapping of mobile charge carriers at lower temperatures. Interpretation of the two phases with deltaapproximate to0.05 is based on the hypothesis that introduction of high-spin Mn3+ by the oxygen vacancies creates around it additional Mn3+ and intermediate-spin Co3+ at neighboring sites; the resulting gain in elastic energy from cooperative, dynamic Jahn-Teller deformations at these ions must be sufficient to overcome the cost of about 0.2 eV for the electron transfer from a Co2+ ion to a Mn4+ ion.
