Influence of cation size on the structural features of Ln_½A_½MnO₃ perovskites at room temperature

Abstract

Polycrystalline samples of Ln_½Sr_½MnO₃ (Ln=La, Pr, La_0.33Nd_0.67, Nd, Nd_0.5Sm_0.5, Sm, and Gd) and Ln_½Ca_½MnO₃ (Ln=La, Pr, La_0.5Nd_0.5, Nd, Sm, and Y_0.5Sm_0.5) have been prepared, and structure determinations have been carried out at room temperature using high-resolution synchrotron X-ray powder diffraction data. The octahedral tilting distortion increases as the average ionic radius of the Ln/A cations, <r_A>, decreases. The two crystallographically distinct Mn-O-Mn bonds [Mn-O(eq)-Mn and Mn-O(ax)-Mn] are almost identical for Ln_0.5Ca_0.5MnO₃ compounds, with the exception of La_0.5Ca_0.5MnO₃. The La_0.5Ca_0.5MnO₃ compound and the entire Ln_0.5Sr_0.5MnO₃ series adopt structures where the Mn-O(eq)-Mn bond angle is consistently and significantly larger (2-6°) than the Mn-O(ax)-Mn bond angle. All of the Ln_0.5Ca_0.5MnO₃ compounds have Pnma symmetry, whereas across the Ln_0.5Sr_0.5MnO₃ series with increasing <r_A>, an evolution from Pnma (tilt system a⁻b⁺a⁻) over Imma (tilt system a⁻b⁰a⁻) to I4/mcm (tilt system a⁰a⁰c⁻) symmetry is observed. It appears that the latter two tilt systems are stabilized with respect to the rhombohedral (R3c) a⁻a⁻a⁻ tilt system, by short-range layered ordering of A-site cations. Changes in the octahedral tilt system at room temperature are linked to changes in the low-temperature magnetic structure. In particular, the simultaneous onset of charge ordering and CE-type antiferromagnetism in the Ln_0.5Sr_0.5MnO₃ series appears to be closely associated with the Imma structure. The average Mn-O bond distance is relatively constant across the entire series, but individual Mn-O bond distances show the presence of a cooperative Jahn-Teller effect (orbital ordering) at room temperature in Sm_0.5Ca_0.5MnO₃ and Sm_0.25Y_0.25Ca_0.5MnO₃.