Molecular dynamics simulation of polymorphic and polyamorphic transitions in tetrahedral network glasses: BeF2 and GeO2

It is well know that tetrahedral network glasses have anomalous properties such as a density maximum and bulk modulus minimum. The magnitudes of such anomalous properties are different among tetrahedral network glasses. The origin of this anomaly has not been explained yet. We had already analyzed the local structural changes in SiO 2 in terms of transformation of 'structon'. The fragments of Si 2 O 7 were categorized into four types named alpha-, beta-, gamma-, and delta-'structons'. In this study we use molecular dynamics simulations to investigate the structural changes in crystalline and vitreous BeF 2 and GeO 2 . First the simulated BeF 2 cristobalite and quartz clearly reproduce their alpha-beta transitions and the density in the vitreous BeF 2 shows a maximum around 2300 K. On the contrary, GeO 2 shows weak alpha-beta like transitions in cristobalite and quartz, and a density maximum is not observed in the vitreous phase. Next we perform the 'structon' analyses on BeF 2 and GeO 2 . For the BeF 2 system the conversion between alpha- and beta-'structons' are almost complete, as previously observed in our study on the SiO 2 system. On the other hand, such transformation is incomplete in the GeO 2 system. We discuss similarities and dissimilarities between SiO 2 , BeF 2 , and GeO 2 .