Pyrosilicate
{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid =
| Name = Pyrosilicate anion
| IUPACName = Pyrosilicate
| OtherNames = disilicate, diorthosilicate
| ImageFile = Silicate-double-tetrahedra-3D-balls.png
| ImageClass = bg-transparent
| ImageSize = 200px
| ImageName =
|Section1={{Chembox Identifiers
| InChI1 =
| InChIKey1 =
| CASNo_Ref = {{cascite|changed|CAS}}
| CASNo =
| PubChem = 6857674
| ChemSpiderID = 5257009
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 29380
| Gmelin = 326578
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/O7Si2/c1-8(2,3)7-9(4,5)6/q-6
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = KUDCBYUNCUYIDU-UHFFFAOYSA-N
| SMILES = [O-][Si]([O-])([O-])O[Si]([O-])([O-])[O-]
}}
|Section2={{Chembox Properties
| MeltingPtC =
| Formula_Charge = −6
| O=7 | Si=2
| ConjugateAcid = Pyrosilicic acid
}}
|Section3=
}}
A pyrosilicate is a type of chemical compound; either an ionic compound that contains the pyrosilicate anion {{chem|Si|2|O|7|6-}}, or an organic compound with the hexavalent ≡{{chem|O|3|Si}}-O-{{chem|Si|O|3}}≡ group. The anion is also called disilicate[http://www.chemspider.com/Chemical-Structure.5257009.html "Disilicate"]. Chemspider website, CSID:5257009, Accessed 2018-05-26 or diorthosilicate.
Ionic pyrosilicates can be considered salts of the unstable pyrosilicic acid, {{chem|H|6|Si|2|O|7}}. 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 {{chem2|AlSiO7(7-)}} and {{chem2|BSiO7(7-)}}.
Some notable synthetic pyrosilicates include
- sodium pyrosilicate {{chem|Na|6|Si|2|O|7}}, a possible component of water glass.
- sodium iron(II) pyrosilicate {{chem|Na|2|Fe|2|Si|2|O|7}}, a potential cathode material for batteries.Abhishek Panigrahi, Shin-ichi Nishimura, Tatau Shimada, Eriko Watanabe, Wenwen Zhao, Gosuke Oyama, and Atsuo Yamada (2017): "Sodium Iron(II) Pyrosilicate {{chem|Na|2|Fe|2|Si|2|O|7}}: A Potential Cathode Material in the {{chem|Na|2|O}}-{{chem|FeO}}-{{chem|Si|O|2}} System". Chemistry of Materials, volume 29, issue 10, pages 4361–4366. {{doi|10.1021/acs.chemmater.7b00764}}
- sodium manganese(II) pyrosilicate {{chem|Na|2|Mn|2|Si|2|O|7}}, another potential cathode material.Viktor Renman, Mario Valvo,
Cheuk-Wai Tai, and Cesar Pay Gómez (2017): "Manganese Pyrosilicates as Novel Positive Electrode Materials for Na-Ion Batteries". Uppsala University Publications, DIVA [http://uu.diva-portal.org/smash/record.jsf?pid=diva2%3A1158595&dswid=4799 urn:nbn:se:uu:diva-334063]Viktor Renman (2017): "Structural and Electrochemical Relations in Electrode Materials for Rechargeable Batteries", Doctoral Thesis, Uppsala University, Department of Chemistry. ORCID: 0000-0001-8739-4054
Structure
The pyrosilicate anion can be described as two {{chem|Si|O|4}} 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.Volker Kahlenberg, Thomas Langreiter, and Erik Arroyabe (2010): "{{chem|Na|6|Si|2|O|7}} – The Missing Structural Link among Alkali Pyrosilicates". Zeitschrift für anorganishe und allgemeine Chemie (Journal for Inorganic and General Chemistry), volume 636, issue 11, pages 1974-1979. {{doi|10.1002/zaac.201000120}}
Yuri Smolin and Yuri Shepelev determined in 1970 the crystal structures of pyrosilicates of rare earth elements with generic formula {{chem|Ln|2|Si|2|O|7}}, 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.Yu. I. Smolin and Yu. F. Shepelev (1970): "The crystal structures of the rare earth pyrosilicates". Acta Crystallographica Section B, volume B26, pages 484-492. {{doi|10.1107/S0567740870002698}} Other researchers also studied yttrium pyrosilicate {{chem|Y|2|Si|2|O|7}}.,Dias, H.W.; Glasser, F.P.; Gunwardane, R.P.; and Howie, R.A. (1990): "The crystal structure of δ-yttrium pyrosilicate, δ-{{chem|Y|2|Si|2|O|7}}". Zeitschrift für Kristallographie, volume 191, issues 1/2, pages 117-123; ISSN 0044-2968 gadolinium pyrosilicate {{chem|Gd|2|Si|2|O|7}}., and scandium pyrosilicate {{chem|Sc|2|Si|2|O|7}}.Anan'eva, G.V.; Karapetyan, V.E.; Korovkin, A.M.; Merkulyaeva, T.I.; Peschanskaya, I.A.; Savinova, I.P.; and Feofilov, P.P. (1982): "Strukturnye kharakteristiki i fizicheskie svojstva kristallov diorto(piro)silikatov lantanoidov, ittriya i skandiya, vyrashchennykh metodom Chokhral'skogo" ("Structural characteristics and physical properties of diortho(pyro)silicate crystals of lanthanides yttrium and scandium grown by the Czochralski technique"). Izv. Akad. Nauk SSSR, Neorg. Mater (translated in Inorganic Materials, USA) volume 18, issue 3, pages 442-445. ISSN 0002-337X.
Preparation
Rare earth pyrosilicates {{chem|Ln|2|Si|2|O|7}} can be obtained by fusing the corresponding oxide {{chem|Ln|2|O|3}} 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 {{chem2|Si\sO\sSi}} bond.{{cite patent|country=US|number=1953840|inventor=Myron C. Waddell|assign1=Grasselli Chemical Company|title=Process for making sodium pyrosilicate hydrate|status=patent|gdate=1934-04-03}} This proceeds according to the idealised equation
:{{chem2|2 SiO3(2-) + 2 OH(-) -> Si2O7(6-) + H2O}}
Additional alkali will degrade the bonds further, instead yielding orthosilicate.{{cite patent|country=US|number=2351385A|status=patent|inventor=Eduard Zintl|gdate=1944-06-13|pridate=1939-11-11|title=Process of preparing anhydrous sodium-orthosilicate}}