Perovskites, crystal structure transitions

Figure 11.6 Views of perovskite crystal structure. Top—conventional cubic unit cell white circles = oxygen black circle = transition metal gray circles = alkali or alkaline earth metal. Bottom—extended unit cell to show the cage formed by the oxygen octa-hedra. Adapted from Bragg et al. (1965).

Fig. 12 Hexagonal perovskite crystal structure of the parent compound CsNiCfi. Black circles represent transition metal atoms, Ni in this case. White circles are ligands. Shaded circles are atoms of Cs . Face-sharing octahedrons [NiCle] are packed in linear chains (From [57])...

Fig. 8.3 Perovskite crystal structure of oxide ceramic materials used to fabricate composite membranes. A very large fraction of the metals in the periodic table can be substituted into the A and B lattice sites. A-sites contain larger cations such as alkaline earth and rare earths, including Ca, Sr and La, whereas the B-sites contain smaller transition metal cations such as H, Nb, V, Fe, Cr, Cu and Co. A near infinite variety of materials can be synthesized...

Figure 22. The octahedral fragment of the perovskite crystal structure ABO, with the transition metal atom B at the center and six oxygen atoms (shadowed) at the apexes of the octahedron. The letters a,b,c,. .. denote the off-center positions of the atom B in the eight wells of the APES induced by the PJTE.

When 0.4 < x < 0.53, an orthorhombic phase is observed in the AgxNb02+xFi.x system. This phase undergoes a phase transition at 900°C that leads to the formation of a tetragonal phase, which crystallizes in a tetragonal tungsten bronze-type structure with cell parameters a = 12.343 and c = 3.905 A. When 0.82 < x < 1, solid solutions based on AgNb03 were found, which crystallize in a perovskite-type structure. 

Alonso, J. A., Martinez-Lopez, M. J., Casais, M. T., Martinez, J. L., Demaseau, G., Largeteau, A., Garcia-Munoz, J. L., Munoz, A., Femandez-Diaz, M. T., High-pressure preparation, crystal structure, magnetic properties, and phase transitions in GdNi03 and DyNiOs perovskites, Chem. Mater. 11, 2463-2469 (1999). 

So far, the bonding and surface structure aspects of electrocatalysis have been presented in a somewhat abstract sort of way. In order to make electrocatalysis a little more real, it is helpful to go through an example—that of the catalysis of the evolution of oxygen from alkaline solutions onto substances called perovskites. Such materials are given by the general formula RT03, where R is a rare earth element such as lanthanum, and T is a transition metal such as nickel. In the electron catalysis studied, the lattice of the perovskite crystal was replicated with various transition metals, i.e., Ni, Co, Fe, Mn, and Cr, the R remaining always La. 

To account for these results for (Mg,Fe)Si03 perovskites, various charge transfer mechanisms have been proposed (Li and Jeanloz, 1990 Hirsch and Shankland, 1991 Sherman, 1991). Lattice defects permitting Fe3+ ions to exist in the perovskite structure give rise to oxygen — Fe and Fe2+ — Fe3+ charge transfer transitions, the latter being facilitated by the close proximity (279 pm) of the A sites (Fe2+) to the B sites (Fe3+) in the perovskite structure. The opacity of the hydrous phase D indicates that extensive electron delocalization may occur in its crystal structure. 

Surrounding the core, the mantle has a thickness of about 2900 km. Its mass is estimated at 4 x 1024 kg. It is composed mainly of high-density silicates of Mg and Fe. It is divided into three layers lower (2000 km), transition (500 km), and upper mantle (360 km). The lower mantle is predominantly formed by Mg-perovskite, Mg-wurstite, and Ca-perovskite, which contain water in their crystal structures. Incredibly as it may seem, because of this water content the lower mantle is believed to contain more water than the oceans. 

The compound I YCujOy shows an even higher superconducting transition ( 93 K) and crystallizes as a defect perovskite. The structure of I YCh Oy has been determined by neutron diffraction analysis. oThe space group is Pmmm with a = 3.8198, b = 3.8849 and c = 11.6762 A. Barium and yttrium are ordered on the A site to give a tripled cell along c and the oxygens occupy 7/9 of the anion sites. One third of the copper is in 4-fold coordination and 2/3 are five-fold coordinated (Gallagher, P.K. O Bryan, H.M. Sunshine,... 

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