Rhombohedral perovskites

The effects of firing conditions on samples whose La Mn ratio is fixed at 1 is illustrated by the following data reported by Hauback and coworkers.If the sample is fired in Ar at 1000 °C, it has the composition LaMnOs.oo and a structure that is an orthorhombic distortion of the cubic perovskite. If the sample is heated in air at 750 °C, a sample with a rhombohedral perovskite structure results that becomes ferromagnetic at low temperatures before transforming at 9 K to the ORTH 1 variant of the GdFeOs structure. On the other hand if the firing temperature is lowered to 900 °C, 5 increases to 0.09 and a phase with a rhombohedral modification of perovskite is formed. If the La/Mn ratio is lowered to 0.88, firing at 750 °C results in a rhombohedral phase with S = -0.08 that... 

IR spectroscopy can be used to distinguish several different phases characterized by the stoichiometry ABO3 (Table 3.4), such as cubic, tetragonal, orthorombic and rhombohedral perovskites (such as SrTiOs, BaTiOs, LaFeOs and LaMnOs, respectively [56, 64, 65]), from ilmenites and lithium niobate structures. In Figure 3.10 the spectrum of LaFeOs is reported. It shows some of the 26 IR active modes expected. 

For the orthorhombic Fhnm structure, mutual displacements of the rare earth and oxygen atoms from their ideal positions are observed. The maximum shift of R-cation is observed along the [010] direction, whereas the displacement along [100] is considerably smaller and does not exceed 0.07—0.08 A (Figure 40). The magnitude of the R-cation displacement decreases with increasing cationic radius and approaches zero when the transition to rhombohedral perovskites occurs for r(R +)>1.155 A (Figure 40). 

In contrast to orthorhombic aluminates, RAIO3 compounds with rhombohe-dral structures display no anomalies both in the low- and high-temperature expansion (Figure 50). For rhombohedral perovskites, the thermal expansion in the fl-direction is smaller compared with the c-direction (0.75% and 1% at 1200 K, respectively). The Pa and pc values are practically identical for all rhombohedral rare earth aluminates (Figure 50B and D). The analysis of some representatives of Ri xR x AIO3 solid solutions shows that the character of their thermal expansion is... 

Conclusion (2) has been confirmed by a study of CO dissociation (first step of the F-T hydrocarbons synthesis) as shown on the following figure (Figure 28.5), where the less efficient catalysts are the rhombohedral perovskites and the maximal CO conversion of 12% is obtained for the reduced LaCoo.4oFeo,6o03> which contains 2.1 wt% of Co°. For the same perovsldte calcined at 600 °C (8.6 wt% of Co°), 28% of CO dissociation is reached (not shown here). 

It shows increasing lattice parameters and crystallite Loj2 with increasing temperature up to 300 °C. From the cell parameters obtained after calcination and at room temperature, it can be inferred that the stmcture is rhombohedral perovskite LaCoOa or during the synthesis step was deformation or distortion diverting structure of cubic ideality. 

The crystallite sizes and the lattice parameters of the perovskite structure are shown in Table 2. All products were nanocrystalline with sizes of 21-22 nm. The lattiee parameters calculated from XRD data have proved that rhombohedral perovskite structure was obtained, with values very close to those of ICSD 473444 (a= 5.4953 A and c= 13.3422 A). 

Localized and collective ti-electron configurations can be identified from magnetic data since collective electrons usually exhibit no spontaneous atomic moment. As an example, isolated Ni ions in oxides are paramagnetic with a spin moment of 1 /ig, often showing Jahn-Teller distortions of the local site symmetry at low temperatures. The rhombohedral perovskite LaNiOj, on the other hand, is metallic and exhibits temperature-independent Pauli paramagnetism. 

---