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

Abstract

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 ab + a-) over Imma (tilt system ab 0 a-) to I4/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 (R3 hc) aaatilt 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 .

Pressure-induced tuning of a magnetic phase separation in Nd0.53Sr0.47MnO3<

“…This uniaxial compression of the c-axis is in good agreement with a well established physical pic- ture. Indeed, the contraction of the c parameter is consistent with the stabilization of an A-type AFM phase and d x 2 −y 2 orbital ordering at high pressure 7,8,20,23 . In this framework, a simple way to quantitatively determine if further distortions are induced by pressure is to calculate the orthorhombic strain.…”

Role ofA-site cations in the metal-insulator transition inPr0.5Ca0.5CoOBarón-González1, Frontera2, García-Muñoz3 et al. 2010Phys. Rev. B395600View full textAdd to dashboardCite

“…To illustrate the importance of this bond bending, we mention that it is comparable to that produced in R 0.5 Ca 0.5 MnO 3 perovskites series when going from R =Pr to the much smaller Sm ͑in this case, the change in V is −1.3%, at RT, and the change in the average Mn-O distance is 0.002 Å͒. 28 This comparison also allows to foresee that an important reduction in the space available for the cations at the A site of the perovskite must take place. This is confirmed by the abrupt reduction ͑on cooling͒ in the average A-O bond distance displayed in Fig.…”

Electronic phase separation and other novel phenomena and properties exhibited by mixed-valent rare-earth manganites and related materials

“…increasing hr A i is equivalent to increasing the hydrostatic pressure, which increases the Mn-O-Mn bond angle and consequently the bandwidth. Detailed studies of the effect of hr A i on the electronic order have been reported by Woodward et al (1998). Systems with very large hr A i tend to be FM and metallic, with a Curie temperature T c that increases with hr A i.…”