Symmetry mode analysis of distorted polar/nonpolar structures in A -site ordered SmBaMn2O6 perovskite

Abstract

We present a comprehensive structural study of the charge-orbital ordering and magnetic phase transitions observed in the A-site ordered SmBaMn 2 O 6 perovskite combining synchrotron radiation x-ray powder diffraction and symmetry-adapted modes analysis. In SmBaMn 2 O 6 , successive phase transitions in charge, spin, and lattice degrees of freedom take place with decreasing temperature at T CO1 ≈ 380 K, T CO2 ≈ 190 K, and T N ≈ 250 K. The main difference between the two charge-ordered phases concerns the stacking sequence along the c axis, which is double for the high temperature charge-ordered phase and has led to controversy in the literature. We show that both charge-ordered phases are pseudosymmetric with respect to the ideal undistorted tetragonal structure of A-site ordered RBaMn 2 O 6 perovskites and lead to two nonequivalent Mn sites. However, the charge segregation stabilizes at about 0.35e − in the low temperature charge-ordered phase, clearly below the nominal separation of one charge unit between Mn 3+ and Mn 4+ and undergoes a prominent increase in the high temperature charge-ordered phase when warming above ≈250 K. The two Mn sites are anisotropic in both charge-ordered phases but the analysis of the active modes discloses that only the low temperature charge-ordered phase displays a Jahn-Teller-like distortion for one of the Mn sites. In addition, this low temperature charge-ordered phase has polar symmetry compatible with ferroelectricity along the a axis.

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

“…Finally, the c -axis was divided by 2 in the P 2 1 / n cell. Figure a shows the temperature dependence of the resulting lattice parameters for the YBMO compound between 100 and 603 K. The temperature evolution of these parameters is reminiscent of those observed in isostructural compounds. , It is characterized by the strong changes at T 1 that coincide with the corresponding anomaly indicated in the magnetic measurements (see Figure ). In this transition, there is a huge expansion of the b -axis, while a strong shrinkage is produced for the a - and c- axes.…”

“…The Mn atoms are shifted along the y -direction, while an asymmetric stretching mode of the O2 atoms leads to a net component along the b-axis in the opposite direction to the Mn displacement. Calculation of the theoretical displacive-type ferroelectric polarization yield a value of 4.3 μC cm –2 for this phase. This value is somewhat greater than that calculated in the related SmBaMn 2 O 6 , which suggests an increase of the polarization as the distortion of the crystal structure increases in agreement with theoretical predictions …”

“…Calculation of the theoretical displacive-type ferroelectric polarization yield a value of 4.3 μC cm –2 for this phase. This value is somewhat greater than that calculated in the related SmBaMn 2 O 6 , which suggests an increase of the polarization as the distortion of the crystal structure increases in agreement with theoretical predictions …”

“…14, standard setting P 2 1 / c ). The reason is that its symmetry is closely related to an analogous phase of the SmBaMn 2 O 6 compound . Both compounds exhibit related structural phase transitions with the similar temperature dependence of the superstructure peaks.…”

“…Our recent symmetry-adapted mode analysis of SmBaMn 2 O 6 has revealed that charge segregation between the nonequivalent Mn sites is indeed below the nominal separation of one charge unit. The local environments of the two types of Mn atoms are distorted, showing anisotropy in both CO phases, but only in the low-temperature CO phase one of the distorted MnO 6 octahedra resembles a Jahn–Teller like distortion …”