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

We report here a structural study of RBaMn2O6 (R = 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 LaBaMn2O6 sample and the lattice parameters undergo anisotropic changes at T-C related to the orientation of the magnetic moments. Both PrBaMn2O6 and NdBaMn2O6 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 - a, and c with respect to the tetragonal phase. Two possible solutions belonging to the space groups Pmam and P21am 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, P2(1)am. The symmetry analysis of this transition unveils that the primary modes belong to the irreducible representations M5- and GM5- and the main distortions correspond to rotations of the MnO6 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) (3d(x2-y2)) 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.