Entropic phase transitions and accompanying anomalous thermodynamics of matter

Abstract

Thermodynamic consequences of subdivision for all first-order phase transitions (PT) into enthalpy- and entropy-driven ones (H-PT and S-PT), proposed previously [arXiv:1403.8053], are under discussions. Key-value for proposed discrimination is main benefit (decreasing) in enthalpy or (nega)entropy in spontaneous phase decomposition, i.e. the sign of latent heat of PT and consequent slope of its P(T)-dependence. The main driving mechanism for many isostructural S-PT is the decay (delocalization) of some kind of bound complexes, e.g. atoms, molecules etc. Thermodynamic features of H-PT and S-PT differ significantly. Entropic PT are always the part of more general thermodynamic anomaly—domain where great number of usually positive second cross derivatives (e.g. Grüneisen parameter, thermal pressure coefficient etc.) became negative simultaneously. This domain is restricted (in the case of isostructural S-PT) by remarkable bound, where all mentioned second derivatives are equal to zero. Isostructural S-PT has more complicated topology of stable, metastable and unstable states and their boundaries—binodals and spinodals in comparison with ordinary enthalpic PTs. Two new thermodynamic objects accompany isostructural S-PT: (i) appearance of third (additional) region of metastable states with positive compressibility, isolated from the regions of stable states; (ii) appearance of new additional spinodal, topped with a new singular point, separating metastable and unstable states. These additional spinodal and singular point obey to relation (∂P/∂V)T = ∞. All thermodynamic anomalies of entropic PTs correspond to conclusive and transparent geometrical feature of such PTs: multilayered structure of thermodynamic surfaces for temperature, entropy and internal energy as pressure- density functions U(P, V), S(P, V) and T(P, V).