Influence of Cation Size on the Structural Features of Ln_1/2A_1/2MnO₃ Perovskites at Room Temperature

Polycrystalline samples of Ln1/2Sr1/2MnO3 (Ln = La, Pr, La0.33Nd0.67, Nd, Nd0.5Sm0.5, Sm, and Gd) and Ln1/2Ca1/2MnO3 (Ln = La, Pr, La0.5Nd0.5, Nd, Sm, and Y0.5Sm0.5) 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, 〈rA〉, decreases. The two crystallographically distinct Mn−O−Mn bonds [Mn−O(eq)−Mn and Mn−O(ax)−Mn] are almost identical for Ln0.5Ca0.5MnO3 compounds, with the exception of La0.5Ca0.5MnO3. The La0.5Ca0.5MnO3 compound and the entire Ln0.5Sr0.5MnO3 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 Ln0.5Ca0.5MnO3 compounds have Pnma symmetry, whereas across the Ln0.5Sr0.5MnO3 series with increasing 〈rA〉, 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 (R3̄c) 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 Ln0.5Sr0.5MnO3 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 Sm0.5Ca0.5MnO3 and Sm0.25Y0.25Ca0.5MnO3.