An unusual member of the solid solution series between cristobalite and potassium ferrate(III) obtained from hydroflux

Dark octahedral crystals of K1-x[Fe1-xSixO2] with x approximate to 0.2 were synthesized under ultra-alkaline conditions in a KOH hydroflux at 200 degrees C. The compound is a member of the solid solution series between SiO2 and K[FeO2]. Due to its SiO2 content, K-0.8[Fe0.8Si0.2O2] is much less sensitive to moisture than K[FeO2]. The crystal structure is a stuffed cristobalite with a charged framework of vertex-sharing [MO4/2] tetrahedra (M = Fe3+, Si4+) and potassium counter ions in the large voids of the framework. It has the pseudo-symmetry and metrics of the cubic space group Fd (3) over barm, but adopts the tetragonal space group I4(1)md with four formula units in the cell. Unlike other tectosilicates of this type, the [MO4/2](-) tetrahedra are not regular but distorted to disphenoids. The O atom can be modeled by a huge disk-shaped ellipsoid or, better, by split atom positions forming a six-membered ring with a diameter of 1.1 angstrom. The M-O distances range from 1.66(2) to 1.96(2) angstrom, the M-O-M angles are 135(2)degrees, 137(1)degrees, and 151(1)degrees.

Automated summary

Dark octahedral crystals of K1-x[Fe1-xSixO2] were synthesized under ultra-alkaline conditions. The crystal structure is a stuffed cristobalite with a charged framework of vertex-sharing tetrahedra. The crystal adopts a tetragonal space group and the tetrahedra are distorted.