On the nature of pressure‐induced coordination changes in silicate melts and glasses

Abstract

Abstract. Progressive decreases in the Si-O-Si angles between comer-shared silicate tetrahedra in glasses and melts with increasing pressure can lead to arrangements of oxygen atoms that can be described in terms of edge-or face-shared octahedra. This mechanism of compression can account for the gradual, continuous increases in melt and glass densities from values at low pressure that indicate dominantly tetrahedral coordination of Si to values at several tens of GPa that suggest higher coordination. It also can explain the unquenchable nature of octahedrally coordinated Si in glasses, the absence of spectroscopically detectable octahedrally coordinated Si in glasses until they are highly compressed, the gradual and reversible transformation from tetrahedral to octahedral coordination in glasses once the transformation is detectable spectroscopically, and the fact that this transformation takes place in glass at room temperature. It may also have relevance to pressure-induced transformations from crystalline to glassy phases, the difficulty in retrieving some metastable high pressure crystalline phases at low pressure, and the observed differences between the pressures required for phase transformations in shock wave experiments on glasses and crystals.

Morse Stretch Potential Charge Equilibrium Force Field for Ceramics: Application to the Quartz-Stishovite Phase Transition and to Silica Glass

“…2(b) shows that the fivefold to sixfold coor- 100 GPa, the percentage of sixfold coordinated atoms for silica glass is less than for the structures obtained from quartz. These results are consistent with the interpretation by Stolper and Ahrens [14] of the experiments [15].…”

“…Thus, relaxing the load from 100 GPa back to zero (at 21 GPa͞ps) results in a structure with a density of 3.7 g͞cm 3 at zero pressure. This is consistent with experimental observations of shock wave release isentropes [14,15].…”

“…In Fig. 1(b), we see that the phase transition changes from 19.5 GPa (300 K) to 16.5 GPa (500 K), 15.5 GPa (1000 K), and 14.5 GPa for 1500 K. Experiments indicate a transition starting at about 14 GPa for 500 K [14], in reasonable agreement with our result of 16.5 GPa at 500 K. We also found that, if the parameters were biased to get better cristobalite structures, the phase transition from a quartz to stishovite again occurred, but at a higher pressure of 26 GPa at 300 K. We also performed a simulation on a quartz using fixed charges; the transition pressure dropped by 1 GPa, however, the final structure has 2% lower density.…”

“…Thus, the charges readjust to the changes in atomic configuration. In addition, the charges depend intrinsically on the pressure and temperature, making MS-Q suitable for studying the phase transitions between the silica forms [quartz and coesite (both fourfold coordinated Si) have been observed to transform to stishovite at ϳ15 GPa in shock experiments [14,15] ].…”

Pressure-induced coordination change of Ti in silicate glass: a XANES study

“…The influence of cationic coordination on the properties of silicate melts has been widely discussed in the literature (Bottinga et al 1982;Bottinga 1985;Rigden et al 1984;Stolper and Ahrens 1987). Perhaps the most direct consequence imaginable is the effect of cationic coordination on melt density (Waft 1975).…”