  Ba4NbMn3O12 is reported, synthesized by a solid state method in air. The
crystal structure, determined by performing refinements on room temperature
powder X-ray diffraction data by the Rietveld method, consists of Mn3O12
trimers in the configuration of three face-sharing MnO6 octahedra, with the
trimers arranged in triangular planes. An effective moment of 4.82 {\mu}B/f.u
is observed and competing antiferromagnetic and ferromagnetic interactions
between Mn ions are inferred from the Weiss temperature of -4 K and the
ferrimagnetic ordering transition of approximately 42 K. Ba4NbMn3O12 is a
semiconductor with a transport activation energy of 0.37 eV.
