Three charge-ordered phases in bilayered Pr(Sr0.1Ca0.9)2Mn2 O7: From antiferrodistortive to ferrodistortive structures

The structural phase transitions of Pr(Sr0.1Ca0.9)2Mn2O7 have been studied by synchrotron radiation x-ray powder diffraction. Three different charge order (CO) phases are identified as a function of temperature. The three phases have a checkerboard arrangement of compressed and expanded MnO6 octahedra in the perovskite bilayers. The structural changes associated with each transition are described in terms of the mode decomposition analysis with respect to the parent tetragonal structure. The high-temperature phase exhibits an orthorhombic symmetry (Amam) due to cooperative tilts of the MnO6 octahedra that can be ascribed to the irreducible representation (irrep) X3−. On cooling, the CO1 phase with Pbnm symmetry arises from the condensation of modes belonging to the irrep DT2 together with breathing (X1+) and antiferrodistortive (M5− and Y2) modes. The Mn site is split into two nonequivalent Mn atoms (Mn1 and Mn2) with a small charge segregation, and the asymmetric environment of Mn1 atoms form zigzag chains along the doubled b axis. Further cooling leads to the formation of the CO2 phase that comes from a relative change between the directions of previous X3− and DT2 distortions, producing the simultaneous condensation of new DT1 and ferrodistortive GM5− modes. In the CO2 structure (Am2m symmetry), Mn2 atoms are split into two nonequivalent sites but with similar oxidation states. Then the checkerboard arrangement between Mn1 and Mn2 atoms is preserved but with a reorientation of the zigzag chains along the a axis. At lower temperatures, the CO3 phase arises from a new direction change of irrep X3−, leading to the simultaneous condensation of new secondary modes. Its symmetry is Pn21m, and the Mn1 site is now split, which leads to a total of four nonequivalent Mn sites in the unit cell. In this phase, more tilts of the MnO6 octahedra are allowed following the a−b−c+ tilt schema.