A combination of in situ high-pressure neutron diffraction at pressures up to 17.5(5) GPa and molecular dynamics simulations employing a many-body interatomic potential model is used to investigate the structure of cold-compressed silica glass. The simulations give a good account of the neutron diffraction results and of existing x-ray diffraction results at pressures up to $\sim 60\mathrm{GPa}$. On the basis of the molecular dynamics results, an atomistic model for densification is proposed in which rings are “zipped” by a pairing of five- and/or sixfold coordinated Si sites. The model gives an accurate description for the dependence of the mean primitive ring size $⟨n⟩$ on the mean Si-O coordination number, thereby linking a parameter that is sensitive to ordering on multiple length scales to a readily measurable parameter that describes the local coordination environment.

In the present work the structural properties of low silica content calcium aluminosilicate melts with concentration ratio CaO/Al2O3 = 1 are investigated in the liquid and undercooled states by neutron diffraction experiments and ab initio molecular dynamics simulations. The results show the presence of AlO5 units and triclusters as well as non-bridging oxygen in the fully charge balanced compositions. Moreover, our findings allow us to identify a possible interplay between these structural units. Finally, we discuss the influence of these defective structural units on the properties of tetrahedral network and more particularly their implication on the evolution of the viscosity and the fragility.

This study shows how a few nanometer-sized crystals markedly increase the viscosity of a magma leading to explosive eruptions.

The structure of GeO2 glass at pressures extending from ambient to 8.6(5) GPa was measured at $\simeq${}25\,\ifmmode^\circ\else\textdegree\fi{}C by using in situ neutron diffraction. The results show a gradual change in the intermediate range order with increasing density as manifested by an increase in position and reduction in height of the first sharp diffraction peak in the total structure factor. By contrast, the local ordering, as characterized by the Ge-O bond length and coordination number, remains constant for the pressure range from ambient to $\simeq${}5$\hskip0.3em${}GPa. As the pressure is increased further to 8.6(5) GPa, however, a steady increase from 4.0(1) to 4.9(1) is observed in the Ge-O coordination number as the corresponding distance increases from 1.73(2) to 1.77(2)$\hskip0.3em${}\AA{}. The results are therefore consistent with the operation of two densification mechanisms, the low-pressure one associated with squeezing the open network of corner-linked tetrahedral motifs and the high-pressure one associated with a transformation of those motifs. There is no evidence in support of an abrupt transformation of the network structure over the investigated pressure range. The structure of permanently densified GeO2 glass was also investigated by high-energy x-ray diffraction. The results show that there is a threshold pressure at $\simeq${}5$\hskip0.3em${}GPa below which the structure of a recovered glass is similar to that of the high-pressure material. Above the threshold pressure there is, however, a reorganization of both the local and intermediate range ordering once the pressure is released and the Ge-O coordination number returns to 4.