Density-driven structural transformations inB2O3glass

Abstract

The method of in situ high-pressure neutron diffraction is used to investigate the structure of B 2 O 3 glass on compression in the range from ambient to 17.5(5) GPa. The experimental results are supplemented by molecular dynamics simulations made using a newly developed aspherical ion model. The results tie together those obtained from other experimental techniques to reveal three densification regimes. In the first, BO 3 triangles are the predominant structural motifs as the pressure is increased from ambient to 6.3(5) GPa, but there is an alteration to the intermediate range order which is associated with the dissolution of boroxol rings. In the second, BO 4 motifs replace BO 3 triangles at pressures beyond 6.3 GPa and the dissolution of boroxol rings continues until it is completed at 11-14 GPa. In the third, the B-O coordination number continues to increase with pressure to give a predominantly tetrahedral glass, a process that is completed at a pressure in excess of 22.5 GPa. On recovery of the glass to ambient from a pressure of 8.2 GPa, triangular BO 3 motifs are recovered but, relative to the uncompressed material, there is a change to the intermediate range order. The comparison between experiment and simulation shows that the aspherical ion model is able to provide results of unprecedented accuracy at pressures up to at least 10 GPa.

Recent advances in identifying the structure of liquid and glassy oxide and chalcogenide materials under extreme conditions: a joint approach using diffraction and atomistic simulation

“…Edge-sharing conformations do, however, have a notable presence at the ~16% level, and most of the oxygen atoms (~81%) are shared among three or more polyhedra. This model for the liquid, in which AlO 4 tetrahedra are the predominant structural motifs, is consistent with the information available from high-temperature 27 Al nuclear magnetic resonance (NMR) experiments [14]. A comparison with Zachariasen's rules helps to explain why liquid Al 2 O 3 is not a glass-forming material.…”

“…Firstly, there is a reorganisation on quenching that leads to a reduction in the Al-O coordination number from 4.20(4) to 4.04(3), corresponding to a removal of the AlO 5 polyhedra and threefold-coordinated oxygen atoms that are present in the liquid, and the establishment of a network of corner-sharing AlO 4 tetrahedra in the glass. The liquid state coordination number of 4.20(4) compares to an estimate of 4.16 from high-temperature 27 Al NMR experiments, based on the temperature dependence of the chemical shift [19]. Secondly, edge-sharing Ca-centred polyhedra occur in the liquid, and there is an enhancement of these connections in the glass (Figure 3(b)).…”

“…The pressure dependence of the total structure factors measured for glassy B 2 O 3 by neutron diffraction and XRD are shown in Figure 7(a) and (b), respectively [27,33]. Here, the complementary nature of the information provided by these structural probes is indicated by the differences between the diffraction patterns at a given pressure, where neutron diffraction and XRD are more sensitive to the B and O atom pair-correlation functions, respectively.…”

“…The development of in situ high-pressure neutron diffraction to investigate the structure of glasses and liquids is reviewed elsewhere [25]. In order to illustrate the advances that have been made, we will focus on the structures of the prototypical glassy materials SiO 2 [26], B 2 O 3 [27] and GeSe 2 [28] under cold compression, i.e. pressurisation at ambient temperature.…”

Pressure-induced structural changes in the network-forming isostatic glassGeSe4: An investigation by neutron diffraction and first-principles molecular dynamics

“…The pressure versus load calibration curves were taken from Ref. [77], and the measurement and data analysis procedures are described in detail elsewhere [35,[76][77][78]. In the case of the PEARL experiments, the data analysis procedure employed a Lorentzian function to extrapolate a measured S N (k) function to k = 0 for use in Eq.…”