Partial molecular orbitals in face-sharing 3d manganese trimer: Comparative studies on Ba_{4}TaMn_{3}O_{12} and Ba_{4}NbMn_{3}O_{12}

We present a molecular orbital candidate Ba_{4}TaMn_{3}O_{12} with a face-sharing octahedra trimer, by comparing it with a related compound Ba_{4}NbMn_{3}O_{12}. The synthesis of the polycrystalline powder is optimized by suppressing the secondary impurity phase via x-ray diffraction. Magnetic susceptibility measurements on the optimized samples reveal a weak magnetic hysteresis with magnetic transitions consistent with heat capacity results. The effective magnetic moments from susceptibility indicate a strongly coupled S=2 antiferromagnetic trimer at around room temperature, whereas the estimated magnetic entropy from heat capacity suggests the localized S=3/2 timer. These results can be explainable by a partial molecular orbital state, in which three t_{2g} electrons are localized in each Mn ion and one e_{g} electron is delocalized over two-end Mn ions of the trimer based on density functional theory calculations. This unconventional 3d orbital state is comprehended as a consequence of competition between the hybrid interatomic orbitals within the Mn trimer and the local moment formation by on-site Coulomb correlations.