Crystal and Magnetic Structure of the Orthorhombic Perovskite YbMnO₃

The perovskite polymorph of YbMnO 3 has been revisited. The orthorhombic phase is obtained through a high-pressure (HP) reconstructive transformation from the hexagonal sol−gel-synthesized YbMnO 3 by means of HP annealing at 5 GPa and 1100 °C. Neutron powder diffraction shows that the HP form crystallizes in the orthorhombic space group Pbnm . At 298 K, the refined values for lattice parameters a, b, and c are respectively 5.2208(3), 5.8033(3), and 7.3053(4) Å. The average Mn−O−Mn angle is 141.9°, indicative of a strong tilting of the MnO 6 octahedra. Magnetic susceptibility measurements indicate antiferromagnetic order of Mn 3+ spins at T N = 43 K and of Yb 3+ spins at 4 K. The magnetic structure is commensurate A y with an ordered moment 3.45(5) μ B at 9 K. The magnetic susceptibility obeys the Curie−Weiss law, χ = C /( T − θ ), in two temperature regions, that is, above 240 K and 6−30 K. The obtained effective paramagnetic moments μ eff are 6.37 μ B and 4.83 μ B for the two temperature ranges, respectively. The former is attributed to a superposition of paramagnetic Mn 3+ and Yb 3+ spins while the latter reflects primarily the paramagnetic Yb 3+ moments. Re-entry of A y magnetic ordering requires a reconsideration of the factors that contribute to the competitive spin−spin interactions on the MnO 3 array.