Pyrosilicate
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A pyrosilicate is a type of chemical compound; either an ionic compound that contains the pyrosilicate anion Si2O6− 7, or an organic compound with the hexavalent ≡O3Si-O-SiO3≡ group. The anion is also called disilicate or diorthosilicate.
Ionic pyrosilicates can be considered salts of the unstable pyrosilicic acid, H6Si2O7. Unlike the acid, the salts can be stable. Indeed, pyrosilicates occur widely in nature as a class of silicate minerals, specifically the sorosilicates - though some sorosilicate minerals, such as gehlenite, replace one of the silicon atoms with tetracoordinated aluminium or boron, giving the isostructural anions AlSiO7−7 and BSiO7−7.
Some notable synthetic pyrosilicates include
- sodium pyrosilicate Na6Si2O7, a possible component of water glass.
- sodium iron(II) pyrosilicate Na2Fe2Si2O7, a potential cathode material for batteries.
- sodium manganese(II) pyrosilicate Na2Mn2Si2O7, another potential cathode material.
Structure
The pyrosilicate anion can be described as two SiO4 tetrahedra that share a vertex (an oxygen atom). The vertices that are not shared carry a negative charge each.
The structure of solid sodium pyrosilicate was described by Volker Kahlenberg and others in 2010.
Yuri Smolin and Yuri Shepelev determined in 1970 the crystal structures of pyrosilicates of rare earth elements with generic formula Ln2Si2O7, where "Ln" stands for either one of lanthanum, cerium, neodymium, samarium, europium, gadolinium, dysprosium, holmium, yttrium, erbium, thulium, or ytterbium. They were found to belong to four distinct crystallographic classes, determined by the size of the cation. Other researchers also studied yttrium pyrosilicate Y2Si2O7., gadolinium pyrosilicate Gd2Si2O7., and scandium pyrosilicate Sc2Si2O7.
Preparation
Rare earth pyrosilicates Ln2Si2O7 can be obtained by fusing the corresponding oxide Ln2O3 with silica in 1:2 molar ratio, Single crystals can be grown by the Verneuil process or the Czochralski method.
Industrial pyrosilicate can be produced by the depolymerisation of metasilicate by alkali, which releases water on breaking the Si−O−Si bond. This proceeds according to the idealised equation
2 SiO2−3 + 2 OH− → Si2O6−7 + H2O
Additional alkali will degrade the bonds further, instead yielding orthosilicate.