Entropic crossovers in superionic fluorites from specific heat

Abstract

Neutron scattering/diffraction experiments and atomistic simulations reveal that anions in fluorite superionic conductors show rapid disordering at a characteristic temperature T α, which is distinct from the superionic or λ transition temperature T λ that is associated with a quasi-divergent behavior of the thermodynamic response functions. We demonstrate that both crossovers are unmistakably captured by the variation of the ratio of specific heat to the temperature (c/T)—a surprisingly seldom-used thermodynamic metric representing the rate of change of entropy with temperature (ds/dT). With increasing temperature, c/T decreases and portrays a minimum near T α; it then increases and depicts a maximum near T λ. The minimum in c/T corresponds to the onset of superionic state characterized by a rapid increase in the ionic conductivity while the maximum represents a quasi-second-order thermodynamic phase transition. The simple c/T metric appears to be a useful experimental indicator for entropic crossovers that arise from complex disordering in a wider class of superionic conductors.