Determination Of The Crystal Structures In The A-Site-Ordered Ybamn2o6 Perovskite

We present a complete structural study of the successive phase transitions observed in the YBaMn2O6 compound with the layered ordering of cations on the perovskite A-site. We have combined synchrotron radiation X-ray powder diffraction and symmetry-adapted mode analysis to describe the distorted structures as pseudosymmetric with respect to the parent tetragonal structure. The YBaMn2O6 compound shows three consecutive phase transitions on cooling from 603 K down to 100 K. It undergoes a first-order structural transition at T-1 approximate to 512 K from a C2/m cell with a single Mn site to a P2(1)/c cell with two nonequivalent Mn sites. No checkerboard ordering of the two types of MnO6 octahedra is revealed, and there is no significant charge segregation. A second transition is observed below T-2 approximate to 460 K giving rise to a duplication of the c-axis and the occurrence of four nonequivalent Mn sites. These sites are grouped in two pairs, producing, in this case, a checkerboard arrangement in the ab-plane with an average charge segregation of Delta q approximate to 0.4 e(-). The observed distortions in this phase disagree with the formation of an orbital-ordered phase. Finally, another structural transition is observed coupled to the magnetic transition at TN approximate to 200 K and the c-axis is no longer duplicated. The low-temperature phase is polar with SG P2(1). It also contains four nonequivalent Mn sites grouped in two pairs. The charge difference between these pairs is increased, achieving a value of Delta q approximate to 0.7 e(-). In this phase, an asymmetric stretching mode favors a Jahn-Teller-like distortion in the expanded MnO6 octahedra that could be associated with an ordering of eg (3d(x)(-z)(2)(2)/3d(y)(-z)(2)(2)) orbitals. Our refinements disclose that this phase is ferroelectric with significant polar displacements of the Mn and O-basal atoms along the b-axis. The simultaneous occurrence of ferroelectricity and magnetic ordering indicates that YBaMn2O6 can be considered as a type II multiferroic compound and can present magnetoelectric coupling.