The layered silicophosphate Si(HPO 4 ) 2 was prepared via a novel synthesis approach using silicon nanopowder and ortho-phosphoric acid at 150°C providing a polycrystalline product. Silicophosphate compounds with sixfold coordinated silicon atoms obtained from pyrophosphoric acid H 4 P 2 O 7 and tetraalkoxysilanes via a sol-gel route exhibit the same shortrange order. The solid products were analyzed with XRD, ele- IntroductionSilicophosphates, i.e. in narrower sense compounds in the element system Si/P/O/(H), can show in contrast to most naturally occurring silicates and related silicon-oxygen compounds a sixfold coordination of the silicon with oxygen. [1] Typically, this phenomenon is only known from several high pressure materials and minerals, e.g. stishovite. [2] Not only in phosphorus containing glasses [3][4][5][6] but also in a number of crystalline silicophosphates, e.g. SiP 2 O 7 , [7][8][9] Si 5 P 6 O 25 , [10] and Si(HPO 4 ) 2 ·xH 2 O (x = 0,1), [11,12] the silicon is present in [SiO 6 ] units at ambient pressure. Additionally, molecular compounds with [SiO 6 ] moieties are also known in literature. [13][14][15][16] Furthermore, a tris(imidodiphosphinato) silicon complex synthesized from bis(diphenyldiphosphinyl)imine and silicon(IV) chloride, offers [SiO 6 ] structures surrounded by phosphorus atoms. [17] The role of phosphorus for building up [SiO 6 ] octahedra in silicophosphates has been discussed in several theoretical investigations. [18,19] Furthermore, the optical [20] and material properties [21] of silicophosphates, which are partly different from silicate materials due to the phosphorus content, are of basic interest relating to applications in laser technique, [22,23] implants research, [24] or [a]

Reactions of SiCl 4 with R 2 PO(OH) (R=Me, Cl) yield compounds with six‐fold coordinated silicon atoms. Whereas R=Me afforded the hexacoordinated tetra‐cationic silicon complex [Si(Me 2 PO(OH)) 6 ] 4+ with chloride counter‐ions, R=Cl caused release of HCl with formation of a cyclic dimeric silicon complex [Si(Cl 2 PO(OH))(Cl 2 PO 2 ) 3 (μ‐Cl 2 PO 2 )] 2 with bridging bidentate dichlorophosphates.

Silicodiphosphonates synthesized by two different pathways show interesting chemical shifts of five- and sixfold coordinated silicon.

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