Abstract
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 ${T}_{\mathrm{CO}1}\ensuremath{\approx}380\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {T}_{\mathrm{CO}2}\ensuremath{\approx}190\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and ${T}_{\mathrm{N}}\ensuremath{\approx}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. 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 $0.35{e}^{\ensuremath{-}}$ 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 $\ensuremath{\approx}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. 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.
Highlights
SmBaMn2O6 (SBMO) is a half doped manganite with intriguing properties [1,2,3] and promising technological applications [4,5]
We have investigated the different chargeordered and magnetic transitions developed by the A-site ordered SBMO perovskite as a function of temperature
Cooperative tilts of the MnO6 octahedra, which relieve the structural strain produced by the small size of Sm3+, stabilize the high temperature orthorhombic phase with Cmmm space group (SG)
Summary
SmBaMn2O6 (SBMO) is a half doped manganite with intriguing properties [1,2,3] and promising technological applications [4,5]. The combination of A-site ordering and MnO6 octahedral rotations can lift inversion symmetry through the trilinear coupling mechanism giving rise to a form of hybrid-improper ferroelectricity [8,9] In such a case, two primary distortions are needed to reach a ferroelectric phase and intermediate phases may appear as occurs in related Aurivillius compounds [10]. The simultaneous action of all of these distortions gives rise to a hybrid-improper mechanism that produces the polar ground state of SBMO [3] Another interesting result was the finding of competitive phases whose stability can be modified by epitaxial strain, which opens the possibility of growing metastable phases in epitaxial films for electronic applications [3].
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