K_0.72Na_1.71Ca_5.79Si₆O₁₉ – the first oligosilicate based on [Si₆O₁₉]-hexamers and its stability compared to cyclosilicates

Synthesis experiments were conducted in the quaternary system K₂O-Na₂O-CaO-SiO₂, resulting in the formation of a previously unknown compound with the composition K_0.72Na_1.71Ca_5.79Si₆O₁₉. Single crystals of sufficient size and quality were recovered from a starting mixture with a K₂O:Na₂O:CaO:SiO₂ molar ratio of 1.5:0.5:2:3. The mixture was confined in a closed platinum tube and slowly cooled from 1150°C at a rate of 0.1°C min^-1 to 700°C before being finally quenched in air. The structure has tetragonal symmetry and belongs to space group P4₁22 (No. 91), with a = 7.3659 (2), c = 32.2318 (18) Å, V = 1748.78 (12) ų, and Z = 4. The silicate anion consists of highly puckered, unbranched six-membered oligomers with the composition [Si₆O₁₉] and point group symmetry 2 (C₂). Although several thousands of natural and synthetic oxosilicates have been structurally characterized, this compound is the first representative of a catena-hexasilicate anion, to the best of our knowledge. Structural investigations were completed using Raman spectroscopy. The spectroscopic data was interpreted and the bands were assigned to certain vibrational species with the support of density functional theory at the HSEsol level of theory. To determine the stability properties of the novel oligosilicate compared to those of the chemically and structurally similar cyclosilicate combeite, we calculated the electronegativity of the respective structures using the electronegativity equalization method. The results showed that the molecular electronegativity of the cyclosilicate was significantly higher than that of the oligostructure due to the different connectivities of the oxygen atoms within the molecular units.