Influence of Cation Size on the Structural Features of Ln1/2A1/2MnO3 Perovskites at Room Temperature

Polycrystalline samples of Ln 1/2 Sr 1/2 MnO 3 (Ln = La, Pr, La 0.33 Nd 0.67, Nd, Nd 0.5 Sm 0.5, Sm, and Gd) and Ln 1/2 Ca 1/2 MnO 3 (Ln = La, Pr, La 0.5 Nd 0.5, Nd, Sm, and Y 0.5 Sm 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 0.5 Ca 0.5 MnO 3 compounds, with the exception of La 0.5 Ca 0.5 MnO 3 . The La 0.5 Ca 0.5 MnO 3 compound and the entire Ln 0.5 Sr 0.5 MnO 3 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 0.5 Ca 0.5 MnO 3 compounds have Pnma symmetry, whereas across the Ln 0.5 Sr 0.5 MnO 3 series with increasing 〈 r A 〉, an evolution from Pnma (tilt system a - b + a - ) over Imma (tilt system a - b 0 a - ) to I 4/ mcm (tilt system a 0 a 0 c - ) 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 Ln 0.5 Sr 0.5 MnO 3 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 0.5 Ca 0.5 MnO 3 and Sm 0.25 Y 0.25 Ca 0.5 MnO 3 .