New framework hydrous silicate K_3Sc[Si_3O_9] · H_2O related to the high-temperature anhydrous silicate K_3Ho[Si_3O_9] and symmetry analysis of a phase transition with prediction of structures

摘要

Crystals of a new framework silicate K_3Sc[Si_3O _9] · H_2O, space group Pm2_1 n (nonstandard setting of space group Pmn2_1 = C _(2v) ~7), are obtained under hydrothermal conditions. The structure is determined without preliminary knowledge of the chemical formula. The absolute configuration is determined. The structure is close to that of the high-temperature K_3Ho[Si_3O_9] phase, which was obtained upon the heating of K_3HoSi_3O_8(OH) _2. This structural similarity is due to the specific conditions of synthesis and an analogous formula, where holmium is replaced by scandium. A symmetry analysis shows that the high local symmetry of a block (rod) is responsible for the first-order phase transition of both the order-disorder (OD) and displacement type. The number of structures in which the simplest and high-symmetry layers are multiplied by different symmetry elements are predicted.