Pentacoordinated and hexacoordinated silicon cations in a potassium silicate glass: Effects of pressure and temperature

Silicon cations with five or six oxygen first neighbors (SiO5 and SiO6, pentacoordinated and hexacoordinated) are known primarily from alkali and alkaline earth silicate glasses melted at pressures up to about 12 GPa, where their formation is thought to be an important part of the densification of the liquid. These species, especially SiO5 groups, have also been proposed as high-energy intermediate structures in the bond-swapping that controls network cation and anion diffusion, and hence viscous transport, in high temperature, silica-rich melts, as well as in low-temperature liquid-state silicate reactions, and have thus been the subject of numerous recent theoretical studies. In spite of this interest, the amount of experimental data on controls of pressure, temperature, and composition on concentrations of penta- and hexacoordinated silicon is very limited. Here we report new results from 29Si MAS NMR on high pressure glasses with the model composition K2Si4O9, which show that because of transient pressure drop during quench, previous studies probably significantly underestimated SiO5 and SiO6 concentrations. In addition, we refine the earlier results on SiO5 even in 1 bar glasses, and demonstrate a strong increase in its concentration at higher fictive temperature. A positive enthalpy for the reaction forming this species is consistent with its role as an energetically important transition complex in melt dynamics.