Pnma perovskite structure

For example, the family of perovskite minerals and high-temperature ceramic superconductors exhibits this descent of symmetry, from the cubic "ideal" perovskite structure (space group Pm3m, the real mineral perovskite is orthorhombic, space group Pnma, with a fourfold larger unit cell than the ideal cubic one) to orthorhombic structures for the highest-critical... 

The rare earth titanates crystallize in an orthorhombically distorted form of the perovskite structure, commonly known as the GdFeOs type, found widely for other LnT03 materials and described in Pnma (Pbnm in earlier literature) (Figure 16). As the Ln + ions are too small for 12-fold coordination, the TOe octahedral tilt to accommodate a lower coordination number, which is near 9-fold. 

The manganites with general formula Laj vSrvMn03 have structures of distorted perovskite. Depending on the types of distortions, the structure could be called orthorhombic O with space group Pnma, orthorhombic O with space group Pnma and rhombohedral phase with the space group R3c. The first structure... 

It is also possible that the coupling coefficients can be temperature-dependent, in which case the apparent thermodynamic character of a phase transition might appear to vary with temperature. This effect would be detectable as an unusual evolution pattern for the order parameter, as shown, perhaps, by NaMgFs perovskite. In this perovskite, the orthorhombic structure appears to develop directly from the cubic structure according to a transition Pm3m < Pnma (Zhao et al. 1993a,b, 1994 Topor et al. 1997). The orthorhombic structure has q2t qA = and qi = q , = q = 0. Equation (32) yields... 

The structure of AI2O3 is considerably more complicated than that of MgO and CaO. Corundum is the equilibrium phase of AI2O3 at ambient conditions. The symmetry of this structure is I. 5c it has 6 formula units in the unit cell and four independent structural parameters. Although no other phase of AI2O3 has ever been observed, several structures are possible at high pressure conditions. Of these, only the perovskite (Pnma)... 

The room temperature X-ray powder diffraction patterns of these compounds are shown in Fig. 2a. A structural transition from orthorhombic symmetry (space group Pnma) for samples with x O.4 to rhombohedral symmetry (space group R-3c) for 0.5perovskite phases for the x = 0.8 composition was observed. Representative Rietveld fits to the X-ray diffraction data for the samples LPS20, LNSC50 and LNSC80 are shown in Fig. 2b, 2c and 2d, respectively. 

---