High-pressure phase transitions — examples of classical predictability

Abstract

We present 19 examples of materials whose high pressure phase transition points can be determined within a particular classical theory of dense matter. Theoretical predictions are compared with experimental results, and some possible causes of discrepancies are discussed.

Predicting phase transition possibility pressure in solids: A semi-classical possibility

“…For example,the precision of experimental data and of the input parameters in the calculations account for approximately ±10%.An important source of the discrepancies is the form of the interparticle potential.The SK takes into account only the pure Coulomb part of the interparticle potential,without the contributions of the charge distribution overlap and of the dispersive and repulsive forces.Now,the relative contribution of the dispersive and repulsive forces to the full inter-particle potential is minimal for C,H,N,O.Interestingly,the discrepancies between the SK and experimental values of the phase transition pressure is also minimal for the hydrocarbons [7].…”

“…Taking into account the existence of the charge distribution overlap gives rise to three additional terms in the expression for the accumulated energy.The sum of these terms is pressure dependent and it can be positive,negative or zero [7] .The existence of these terms induces an error in the calculated values of the phase transition pressure,and the magnitude of this error is also pressure dependent.Due to space limitations,we have here outlined the influence of just two factors which contribute to the the relative discrepancies between the SK values of the phase transition pressure in various materials and those obtained experimentally.A detailed analysis is avaliable in [7].…”

“…The algorithm proposed by SK for the calculation of the phase transition pressure was applied to 22 different materials [7], [8].Apart from 20 materials for which values of phase transition pressure were known experimentally or from various theoretical calculations (C 6 H 6 ; CH 4 ; CCl 4 ; C 60 ; CsI; CF 2 ≡ CF 2 ; CdS; Al; Ba; CaO; Kr 2 ; N e; LiH; T e; T eO 2 ; M g 2 SiO 4 ; P b; S 2 ; AlP O 4 ;…”

Selected results and open problems in a semiclassical theory of dense matter

“…(5) and (12) with a = 10 × r nm. Applying the procedure described above,an analysis of the applicability of the SK theory to real materials under high pressure was made [7]. A set of 19 materials for which experimental data on phase transitions under high pressure were easily avaliable was analyzed.The aim was to calculate within the SK theory values of pressure at which first order phase transitions could be expected,and then compare the results with the experimental data and analyze possible causes of the discrepancies.It was shown in [7] that the relative discrepancies between the measured values of phase transition pressure and those calculated within the SK theory are material and pressure dependent.Two basic causes of the discrepancies were identified: one is due to that fact that the SK theory takes into account only the simplest form of the electrostatic potential,while in reality in atoms and molecules one "deals" with charge distributions; the second "source of problems" is represented by the fact that the SK theory neglects the contribution of various non-electrostatic components to the overall intermolecular potential.This is expectable for a semiclassical theory,but it clearly increases the discrepancy between the measured and calculated values of the phase transition pressure.More details on these two problems are avaliable in [7].…”