Crystal growth and structural refinement of NaMn7O12

Centrosymmetry Breaking and Ferroelectricity Driven by Short-Range Magnetic Order in the Quadruple Perovskite (YMn3)Mn4O12

By means of single-crystal X-ray diffraction, we give direct crystallographic evidence of a centrosymmetry breaking below T_S = 200 K, concomitant with the onset of a commensurate structural modulation in the quadruple perovskite YMn₃Mn₄O₁₂. This result, which explains the anomalously large thermal coefficient of the Y³⁺ ion in previously reported structural models, is attributed to the small size of the Y³⁺ ion, which causes its underbonding within the dodecahedral coordination polyhedron. The present data are consistent with a commensurate superstructure described by an I-centered pseudo-orthorhombic cell with polar Ia symmetry and a ≈ a_F√2 = 10.4352(7) Å, b ≈ 2b_F = 14.6049(9) Å, c ≈ c_F√2 = 10.6961(7) Å, and β = 90.110(3)°, where a_F ≈ c_F ≈ 7.45 Å, b_F ≈ 7.34 Å, and β ≈ 91° are the unit cell parameters of the I2/m structure observed at room temperature. Consistent with the above polar structure, at lower temperature, T* = 70 K, we observe in polycrystalline samples an anomaly of the direct current (DC) and alternating current (AC) magnetization, concomitant with the appearance of a net electric polarization, as indicated by pyrocurrent and dielectric constant measurements. These results, complemented by electrical transport measurements, suggest a magnetic ferroelectricity driven by short-range magnetic order in YMn₃Mn₄O₁₂.