Crystal and magnetic structure of the orthorhombic perovskite YbMnO3

The perovskite polymorph of YbMnO3 has been revisited. The orthorhombic phase is obtained through a high-pressure (HP) reconstructive transformation from the hexagonal sol-gel-synthesized YbMnO3 by means of HP annealing at 5 GPa and 1100 degrees 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) angstrom. The average Mn-O-Mn angle is 141.9 degrees, indicative of a strong tilting of the MnO6 octahedra. Magnetic susceptibility measurements indicate antiferromagnetic order of Mn3+ spins at T-N = 43 K and of Yb3+ spins at 4 K. The magnetic structure is commensurate A, with an ordered moment 3.45(5) mu(B) at 9 K. The magnetic susceptibility obeys the Curie-Weiss law, chi = C/(T-theta), in two temperature regions, that is, above 240 K and 6-30 K. The obtained effective paramagnetic moments mu(eff) are 6.37 mu(B) and 4.83 mu(B) for the two temperature ranges, respectively. The former is attributed to a superposition of paramagnetic Mn3+ and Yb3+ spins while the latter reflects primarily the paramagnetic Yb3+ moments. Re-entry of A(y) magnetic ordering requires a reconsideration of the factors that contribute to the competitive spin-spin interactions on the MnO3 array.