The structure of liquid silicates is commonly described as a statistical mixture of various atomic entities with relative abundances that can vary with pressure, temperature, and composition. Unfortunately, this view remains largely theoretical due to scarce experimental reports on the silicate melt structure, in particular under pressure. We performed X-ray diffraction of the SiO2 end member to probe the melting curve up to ~ 120 GPa and 7000 K, and the melt structure up to ~ 80 GPa. We confirm the steep increase of the melting curve above ~ 14 GPa when stishovite becomes stable over coesite in subsolidus conditions, with a slope of about 80 K/GPa. Then, around 45 GPa and 5400 K, the melting curve flattens significantly, an effect most likely reflecting the densification of the SiO2 melt structure. The signal of diffuse X-ray scattering is compatible with a change of the Si coordination number from 4 to 6 along the melting curve, in agreement with previous works reporting a similar evolution during the cold compression of SiO2 glass. Because of the limited pressure range (within 10 to 20 GPa) in which the melting curve changes its slope, we speculate a difficult coexistence of tetrahedral SiO4 and octahedral SiO6 units in SiO2 melt at high pressures.

中文翻译：

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SiO2 的熔化行为高达 120 GPa

液态硅酸盐的结构通常被描述为各种原子实体的统计混合物，这些原子实体的相对丰度会随压力、温度和成分而变化。不幸的是，由于很少有关于硅酸盐熔体结构的实验报告，特别是在压力下，这种观点在很大程度上仍然是理论上的。我们对 SiO2 末端构件进行了 X 射线衍射，以探测高达 ~ 120 GPa 和 7000 K 的熔化曲线，以及高达 ~ 80 GPa 的熔体结构。我们证实，当 stishovite 在亚固相线条件下在柯石英上变得稳定时，熔化曲线在~14 GPa 以上急剧增加，斜率约为 80 K/GPa。然后，在 45 GPa 和 5400 K 附近，熔化曲线显着变平，这种效果很可能反映了 SiO2 熔体结构的致密化。漫射 X 射线散射的信号与沿熔化曲线的 Si 配位数从 4 到 6 的变化相一致，这与先前报道的 SiO2 玻璃冷压缩过程中类似的演变一致。由于熔融曲线改变其斜率的压力范围有限（在 10 到 20 GPa 内），我们推测在高压下 SiO2 熔体中四面体 SiO4 和八面体 SiO6 单元难以共存。