Multiple magnetic phases of La 2 CoMnO 6 − δ ( 0 δ 0 . 0 5 )

TLDR: In this paper, the existence of two distinguishable monoclinic ferromagnetic phases separated by a two-phase domain was revealed, one of which is pseudotetragonal and the other is an n-type polaronic conductor.

Abstract: Transport and magnetic properties of polycrystalline ${\mathrm{La}}_{2}{\mathrm{CoMnO}}_{6\ensuremath{-}\ensuremath{\delta}},$ $0l~\ensuremath{\delta}l~0.05,$ have revealed the existence of two distinguishable monoclinic ferromagnetic phases separated by a two-phase domain $0.02l~\ensuremath{\delta}l~0.05$ and a pseudotetragonal $(c/al\sqrt{2})$ phase with $\ensuremath{\delta}g~0.05$ that was prepared at 600 \ifmmode^\circ\else\textdegree\fi{}C. A nearly oxygen-stoichiometric sample having a magnetization $M(5\mathrm{K},50\mathrm{kOe})=5.78{\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.$ with a Curie temperature ${T}_{c}\ensuremath{\approx}226\mathrm{K}$ is identified as atomically ordered ${\mathrm{La}}_{2}{\mathrm{Co}}^{2+}{\mathrm{Mn}}^{4+}{\mathrm{O}}_{6}$ 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 $\ensuremath{\beta}\ensuremath{\approx}90\ifmmode^\circ\else\textdegree\fi{},$ atomic order identifies the space group as ${P2}_{1}/n.$ A second monoclinic, ferromagnetic phase with ${T}_{c}l150\mathrm{K}$ and $\ensuremath{\delta}\ensuremath{\approx}0.05$ has a large, positive thermoelectric power that increases progressively with decreasing temperature. Quenching a $\ensuremath{\delta}\ensuremath{\approx}0.02$ sample from 1350 \ifmmode^\circ\else\textdegree\fi{}C into liquid ${\mathrm{N}}_{2}$ gave a single phase with ${T}_{c}=134\mathrm{K}$ and $\ensuremath{\delta}\ensuremath{\approx}0.05.$ A sample with $\ensuremath{\delta}g~0.05$ that was synthesized at 600 \ifmmode^\circ\else\textdegree\fi{}C was pseudotetragonal $(c/al\sqrt{2})$ and had a paramagnetic Weiss constant $\ensuremath{\theta}l{T}_{c}\ensuremath{\approx}225\mathrm{K}$ 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 $\ensuremath{\delta}\ensuremath{\approx}0.05$ is based on the hypothesis that introduction of high-spin ${\mathrm{Mn}}^{3+}$ by the oxygen vacancies creates around it additional ${\mathrm{Mn}}^{3+}$ and intermediate-spin ${\mathrm{Co}}^{3+}$ 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 ${\mathrm{Co}}^{2+}\mathrm{ion}$ to a ${\mathrm{Mn}}^{4+}\mathrm{ion}.$