Determination of the Crystal Structures in the A-Site-Ordered YBaMn2O6 Perovskite

Joaquín García,Gloria Subías,Javier Blasco,François Fauth,José Luis García-Muñoz

doi:10.1021/acs.jpcc.1c04697

Abstract

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 T1 ≈ 512 K from a C2/m cell with a single Mn site to a P21/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 T2 ≈ 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 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 at TN ≈ 200 K and the c-axis is no longer duplicated. The low-temperature phase is polar with SG P21. 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 MnO6 octahedra that could be associated with an ordering of eg (3dx2–z2/3dy2–z2) orbitals. Our refinements disclose that this phase is ferroelectric with significant polar displacements of the Mn and Obasal 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.

Highlights

Manganese perovskites have been the subject of intense research due to the close interplay between magnetism and current transport properties that gives rise to exotic phenomena such as giant magnetoresistance or charge ordering (CO) phases.[1−3] The rich phase diagram of these compounds promises a wide variety of applications that include, for example, suitable materials for solid oxide fuel cell,[4] oxygen storage,[5] multifunctional spintronic,[6] or solid-state refrigeration.[7]
The origin of these fascinating properties is the competitive interactions among spin, charge, orbital, and lattice degrees of freedom, which are especially strong in half-doped manganites where different CO and orbital ordering (OO) phases are stabilized.[8−11] More recently, the strong impact that the A-site order has on these properties has been found.[12−14]
The magnetization shows a sudden drop in the temperature range between 495 and 512 K, depending on whether the sample is heated or cooled

Summary

Introduction

Manganese perovskites have been the subject of intense research due to the close interplay between magnetism and current transport properties that gives rise to exotic phenomena such as giant magnetoresistance or charge ordering (CO) phases.[1−3] The rich phase diagram of these compounds promises a wide variety of applications that include, for example, suitable materials for solid oxide fuel cell,[4] oxygen storage,[5] multifunctional spintronic,[6] or solid-state refrigeration.[7] The origin of these fascinating properties is the competitive interactions among spin, charge, orbital, and lattice degrees of freedom, which are especially strong in half-doped manganites where different CO and orbital ordering (OO) phases are stabilized.[8−11] More recently, the strong impact that the A-site order has on these properties has been found.[12−14] In these compounds with the nominal formula RBaMn2O6 (R = rare earth or Y), the A-site ordering makes the MnO2 sublattice sandwiched between the RO and BaO sublattices of very different sizes inducing asymmetric distortions in the MnO6 octahedron.[12] the combination of A-site ordering and MnO6 octahedral tilt can break inversion symmetry through the trilinear coupling mechanism giving rise to a new type of hybrid-improper ferroelectricity.[15] As manganites usually exhibit strong magnetic interactions, this mechanism opens the possibility of developing new multiferroic materials whose application is more promising due to the success in the epitaxial growth of thin films.[16]. In the case of large R atoms (La, Pr, and Nd), the ferromagnetic (FM) correlations are enhanced in A-site ordered compounds with respect to the parent simple perovskites.[12,13,17] The ground state of LaBaMn2O6 exhibits a long-range ferromagnetic order with the typical tetragonal cell of an undistorted A-site ordered perovskite.[17,18] In the case of Pr- and Nd-based compounds, the FM transition is followed

Methods

Results

Conclusion