We present a comprehensive structural study of the charge-orbital ordering and magnetic phase transitions observed in the $A$-site ordered $\mathrm{SmBa}{\mathrm{Mn}}_2{\mathrm{O}}_6$ perovskite combining synchrotron radiation x-ray powder diffraction and symmetry-adapted modes analysis. In $\mathrm{SmBa}{\mathrm{Mn}}_2{\mathrm{O}}_6$, successive phase transitions in charge, spin, and lattice degrees of freedom take place with decreasing temperature at $T_{\mathrm{CO}1}\approx 380\mathrm{K}, T_{\mathrm{CO}2}\approx 190\mathrm{K}$, and $T_{\mathrm{N}}\approx 250\mathrm{K}$. 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 $R\mathrm{Ba}{\mathrm{Mn}}_2{\mathrm{O}}_6$ 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 ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ and undergoes a prominent increase in the high temperature charge-ordered phase when warming above $\approx 250\mathrm{K}$. 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.

• Received 24 March 2021
• Accepted 1 June 2021

DOI:https://doi.org/10.1103/PhysRevB.103.214110