Structure and phase transitions in A -site ordered RBaMn2O6 ( R=Pr,Nd ) perovskites with a polar ground state

Abstract

We report here a structural study of $R\mathrm{Ba}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ ($R=\mathrm{La}$, Pr, and Nd) compounds by means of synchrotron radiation x-ray powder diffraction and Raman spectroscopy. The three compounds are $A$-site ordered perovskites adopting the prototypical tetragonal structure at high temperature. A ferromagnetic transition is observed in the $\mathrm{LaBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ sample and the lattice parameters undergo anisotropic changes at ${T}_{C}$ related to the orientation of the magnetic moments. Both $\mathrm{PrBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ and $\mathrm{NdBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ have a structural transition coupled to an electronic localization and an antiferromagnetic transition. In both cases, the x-ray diffraction patterns reveal that the low-temperature phase is orthorhombic with lattice parameters $a+b,b\ensuremath{-}a$, and $c$ with respect to the tetragonal phase. Two possible solutions belonging to the space groups Pmam and $P{2}_{1}am$ can yield accurate refinements of the x-ray patterns. However, the active modes in the low-temperature phase disclosed by the Raman spectroscopy clearly point to the noncentrosymmetric space group, $P{2}_{1}am$. The symmetry analysis of this transition unveils that the primary modes belong to the irreducible representations M5\ensuremath{-} and GM5\ensuremath{-} and the main distortions correspond to rotations of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra and an asymmetric combination of stretching and scissoring modes of the basal oxygens in these octahedra. We conclude that the low-temperature phase is polar and the main contribution comes from the displacement of oxygen atoms from their centrosymmetric positions. However, negligible contribution from the asymmetric stretching associated with a Jahn-Teller distortion is found in this structural transition, suggesting the lack of ferroic orbital ordering of ${e}_{g}$ ($3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$) orbitals in the orthorhombic $ab$ plane. There is only one inequivalent site for the Mn atom in the low-temperature polar phase so charge ordering cannot account for the electronic localization having a structural origin.