INFLUENCE OF CATION SIZE ON THE STRUCTURAL FEATURES OF LU(1 2)A(1/2)MUO(3) PEROVSKITES AT ROOM-TEMPERATURE/

Polycrystalline samples of Ln(1/2)Sr(1/2)MnO(3) (Ln = La, Pr, La0.33Nd 0.67, Nd, Nd0.5Sm0.5, Sm, and Gd) and Ln(1/2)Ca(1/2)MnO(3) (Ln = La, P r, La0.5Nd0.5, Nd, Sm, and Y0.5Sm0.5) have been prepared, and structur e determinations have been carried out at room temperature using high- resolution synchrotron X-ray powder diffraction data. The octahedral t ilting distortion increases as the average ionic radius of the Ln/A ca tions, [rA], 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 La0.5Ca0.5MnO3. The La0.5 Ca0.5MnO3 compound and the entire Ln(0.5)Sr(0.5)MnO(3) series adopt st ructures where the Mn-O(eq)-Mn bond angle is consistently and signific antly larger (2-6 degrees) 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 (rA), an evolution from P nma (tilt system a(-)b(+)a(-)) over Imma (tilt system a(-)b(0)a(-)) to I4/mcm (tilt system a(0)a(0)c(-)) symmetry is observed. It appears th at the latter two tilt systems are stabilized with respect to the rhom bohedral (R (3) over bar c) a(-)a(-)a(-) tilt system, by short-range l ayered ordering of A-site cations. Changes in the octahedral tilt syst em at room temperature are linked to changes in the low-temperature ma gnetic structure. In particular, the simultaneous onset of charge orde ring 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, b ut individual Mn-O bond distances show the presence of a cooperative J ahn-Teller effect (orbital ordering) at room temperature in Sm0.5Ca0.5 MnO3 and Sm0.25Y0.25Ca0.5MnO3.