The progression of thermodynamic anomalies in MX₂ networks with local tetrahedral geometries

Abstract

Key thermodynamic anomalies in density and compressibility, as well as the related stability limits, are determined using an ionic model for BeF2 which includes many-body polarization terms. BeF2 is chosen as an example of an archetypal network-forming system whose structure can be rationalised in terms of connected local tetrahedral coordination polyhedra. The anion dipole polarizability (which effectively controls the bond angles linking neighbouring tetrahedra) is used as a single free parameter in order to help rationalise the changes in the anomaly locations in phase space, whilst all other potential parameters remain fixed. The anomalies and stability limits systematically shift to lower temperature and higher pressure as the anion polarizability is increased. At high dipole polarizabilities the temperature of maximum density anomaly locus becomes suppressed into the supercooled regime of the phase space. The movements of the anomaly loci are analysed in terms of the network structure and the correlation with the inter-tetrahedral bond angles is considered. The high sensitivity of the anomalies to the details of the potential models applied is discussed with reference to previous works on related systems. The relationship to analogous studies on Stillinger–Weber liquids is discussed.

Thermodynamic anomalies in silicon and the relationship to the phase diagram

“…At lower values of λ = 18, the TMC and TminC loci may merge at high pressure [29], while high values of λ = 27 the anomalies weaken and the 'S' shape of the TminC locus is replaced by a line with only positive gradient in the pT projection [30,51]. A similar progression has been found in models for BeF 2 in which the anion polariability is used as a free parameter (in the analogue of the use of the SW λ parameter) [52]. The 'S' shape in the TminC locus in itself is a highly unusual feature of these systems.…”

“…The 'S' shape in the TminC locus in itself is a highly unusual feature of these systems. Until recently [51,52] it has been largely overlooked due to the focus on lower pressure behaviour, in particular in the region of phase space occupied by the thermodynamic anomalies and their associated critical points. However, simple analytic models, such as the two-state model, fail to predict an 'S' shaped compressibility anomaly locus for any of the main 'scenarios' commonly used to interpret anomalies in water and water-like systems [24,[53][54][55].…”

The physics of empty liquids: from patchy particles to water

“…In summary, liquid anomalies are strongly connected [112,117,146,149,[152][153][154][155][156] and the presence of a second liquid critical point is a sufficient but not necessary condition for their appearance. The anomalies can persist also in the limit in which the liquid-liquid critical temperature goes to zero.…”

The physics of Empty Liquids: from Patchy particles to Water

“…In summary, liquid anomalies are strongly connected [102,107,135,138,[141][142][143][144][145] and the presence of a second liquid critical point is a sufficient but not necessary condition for their appearance. The anomalies can persist also in the limit in which the liquid-liquid critical temperature goes to zero.…”