Ba 4 RuMn 2 O 10 : A Noncentrosymmetric Polar Crystal Structure with Disordered Trimers.

Phase-pure polycrystalline Ba 4 RuMn 2 O 10 was prepared and determined to adopt the noncentrosymmetric polar crystal structure (space group Cmc 2 1 ) based on results of second harmonic generation, convergent beam electron diffraction, and Rietveld refinements using powder neutron diffraction data. The crystal structure features zigzag chains of corner-shared trimers, which contain three distorted face-sharing octahedra. The three metal sites in the trimers are occupied by disordered Ru/Mn with three different ratios: Ru1:Mn1 = 0.202(8):0.798(8), Ru2:Mn2 = 0.27(1):0.73(1), and Ru3:Mn3 = 0.40(1):0.60(1), successfully lowering the symmetry and inducing the polar crystal structure from the centrosymmetric parent compounds Ba 4 T 3 O 10 (T = Mn, Ru; space group Cmca ). The valence state of Ru/Mn is confirmed to be +4 according to X-ray absorption near-edge spectroscopy. Ba 4 RuMn 2 O 10 is a narrow bandgap (∼0.6 eV) semiconductor exhibiting spin-glass behavior with strong magnetic frustration and antiferromagnetic interactions.