Determination of the crystal structures in the a-site-ordered YBaMn2O6 perovskite

Abstract

We present a complete structural study of the successive phase transitions observed in the YBaMn2O6compound 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 YBaMn2O6compound shows three consecutive phase transitions on cooling from 603 K down to 100 K. It undergoes a first-order structural transition atT1≈ 512 K from aC2/mcell with a single Mn site to aP21/ccell with two nonequivalent Mn sites. No checkerboard ordering of the two types of MnO6octahedra is revealed, and there is no significant charge segregation. A second transition is observed belowT2≈ 460 K giving rise to a duplication of thec-axis and the occurrence of four nonequivalent Mn sites. These sites are grouped in two pairs, producing, in this case, a checkerboard arrangement in theab-plane with an average charge segregation of Δq≈ 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 atTN≈ 200 K and thec-axis is no longer duplicated. The low-temperature phase is polar with SGP21. It also contains four nonequivalent Mn sites grouped in two pairs. The charge difference between these pairs is increased, achieving a value of Δq≈ 0.7 e-. In this phase, an asymmetric stretching mode favors a Jahn-Teller-like distortion in the expanded MnO6octahedra that could be associated with an ordering ofeg(3dx2-z2/3dy2-z2) orbitals. Our refinements disclose that this phase is ferroelectric with significant polar displacements of the Mn and Obasalatoms along theb-axis. The simultaneous occurrence of ferroelectricity and magnetic ordering indicates that YBaMn2O6can be considered as a type II multiferroic compound and can present magnetoelectric coupling.