Recent syntheses of high-pressure alkali and alkaline earth silicates reveal a class of framework structures with corner-linked 4- and 6- coordinated silicon. These compounds possess the structural formula (A(4- 2x)/1+B(x)/2+)Si(m)/(VI)(Si(n)/(IV)O(2(m+n)+2)) where x, m, and n specify the amounts of alkaline earth, 6-coordinated silicon, and 4-coordinated silicon, respectively. Appropriate values of m and n yield a range of high- pressure structures, from fully 4-coordinated to fully 6-coordinated silicate frameworks. Recognition of this class of framework silicates leads to predictions of high-pressure structures as weft as room-pressure isomorphs of high-pressure silicates.
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