On the pressure evolution of the melting temperature and the glass transition temperature

The evolution of the melting temperature (Tm) and the glass temperature (Tg) from negative pressures up to very high pressures is discussed. It is based on the relation, Tg,m(P)=Tg,m0[1+ΔP/(πg,m+Pg,m0)1/b]exp(ΔP/c) where (Tg,m0,Pg,m0) are the reference temperature and pressure, ΔP=P-Pg,m0, c is the damping pressure coefficient and −πg,m estimates the negative pressure asymptote. Contrary to approximations used so far it is governed solely on pressure invariant coefficients πg,m, b and c. Their reliable estimation is possible basing on experimental data even limited to a moderate range of pressures. Both for Tm(P) and Tg(P) a possible maximum at extreme pressures and a negative pressure asymptote is suggested. The analysis was carried out for sodium, (Ca, Al)(Al, Si)O3 magmatic mixture, liquid crystalline 5CB, germanium, magmatic melt albite, selenium and epoxy resin EPON 828. For 5CB the isotropic–nematic orientational freezing was discussed, including the negative pressures domain. For EPON 828 the supplementary dielectric relaxation time (τ(P)) measurements were carried out. For the latter the analysis of τ(P  ) evolution is based on the modified Vogel–Fulcher–Tammann (VFT) equation, which makes an insight into the negative pressure domain possible: τ(P)=τ0Pexp[DP(P-PS)/(P-P0)], where P0 is the ideal glass VFT estimation, where DP is the fragility strength coefficient and PS is linked to the absolute stability limit. The obtained dependences enabled to address the question does fragility depends on pressure. For selenium both Tm(P) and Tg(P) behavior were possible to analyze, what yielded the experimental pressure dependence of the Turnbull’s Tg/Tm glass forming ability factor (GFA), linking the glass temperature and the melting temperature.