Perovskite structure silicate

Neither element shows any simple aqueous chemistry in the M(IV) state, as the oxides M02 are insoluble in water at all pH values. Reaction of Sn02 in molten KOH gives the octahedral hydroxanion [Sn(OH)6]2-, in contrast to the normal tetrahedral silicates and germinates, but in parallel with isoelectronic compounds such as Te(OH)6 also found in period 5. Other stannates are mixed oxides without discrete oxoanions (e.g. CaSn03 with the perovskite structure). 

Following the widespread acceptance of the view that silicate perovskites may be major components of the lower mantle (see Jeanloz and Thompson, 1983), there have been a number of attempts to calculate the structure, elastic properties, and equations of state of these materials (Wolf and Jeanloz, 1985 Wolf and Bukowinski, 1985, 1987 Matsui et al., 1987 Hemley et al., 1987). A great deal of interest has also been generated in the crystal chemistry of perovskite-structure phases because of their high-temperature superconducting properties. 

The lower mantle is made predominantly of magnesium perovskite, and thus a solid understanding of element partitioning between perovskite and silicate melt is relevant to the early history of the Earth. Much of the early work with perovskite was done on aluminum-free or low-aluminum materials. However, magnesium perovskite can accommodate AI2O3 into its structure (Wood, 2000 Stebbins et ai, 2001),... 

Miigge and others found that the only minerals that could easily be deformed under ambient conditions were the alkali halides and a few sulfides and carbonates. An exception to this was periclase (MgO), which deformed by 110 (110) dodecahedral glide in the same way as halite (NaCl). A more recently discovered exception is SrTiOs with the cubic perovskite structure, which can be deformed plastically at ambient and high temperatures but is brittle at intermediate temperatures (see Section 9.4.7). Other oxides and silicate minerals either cleaved or twinned when attempts were made to deform them at normal temperatures and pressures [1]. 

The hosts for ACT and REE immobilization are phases with a fluorite-derived structure (cubic zirconia-based solid solutions, pyrochlore, zirco-nolite, murataite), and zircon. The REEs and minor ACTs may be incorporated in perovskite, monazite, apatite-britholite, and titanite. Perovskite and titanite are also hosts for Sr, whereas hollandite is a host phase for Cs and corrosion products. None of these ceramics is truly a single-phase material, and other phases such as silicates (pyroxene, nepheliiie, plagioclase), oxides (spinel, hibonite/loveringite, crichtonite), or phosphates may be present and incorporate some radionuclides and process contaminants. A brief description of the most important phases suitable for immobilization of ACTs and REEs is given below. 

As noted in 2.11, ligands forming high-symmetry coordination polyhedra (i.e., regular octahedra, tetrahedra, cubes and dodecahedra) about central transition metal ions are rare. Such highly idealized coordinations, nevertheless, do exist in the periclase (octahedra), cubic perovskite (octahedra, dodecahedra) and spinel (tetrahedra) structures. The more important rock-forming oxide and silicate minerals provide, instead, low-symmetry coordination environments. These include trigonally distorted octahedra in the corundum, spinel and gar-... 

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 authors [34] proposed to use perovskites ABO3, where A are calcium cations, or a mixture of calcium and lanthanum, and B are iron, cobalt, nickel or manganese cations, or their mixtures. Besides, aluminates, silicates, aluminium sihcates, zirconates and chromates of different types are added as structure-forming components providing strength and stability to thermal shocks [34]. 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

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