Entropy-Driven Reentrant Phase Transitions in Even-J/Odd-J Mixtures of Linear Rotors

Abstract

Depending on the parity of the rotational quantum number J, solid hydrogens exhibit either pressure-driven broken symmetry phase (BSP) transitions (even-J species – para-hydrogen (p-H2), and ortho-deuterium (o-D2)) or usual order-disorder phase transitions (odd-J species – o-H2 and p-D2) with the phase transition temperature increasing monotonically with pressure from the phase transition point at zero pressure. At the same time, for solid HD the BSP phase transition line displays a minimum, indicating that the disordered phase is reentrant. In this work a model of quantum linear rotors is used to study characteristic features of the P–T phase diagrams for ortho–para mixtures in solid H2 and D2. We developed a mean-field theory of even-J – odd-J mixtures of quantum linear rotors on a 3D lattice. Two limiting cases are considered: mixtures at thermodynamic equilibrium, where the conversion time is small or comparable with the thermalization time, and the opposite case, frozen mixtures, when the conversion time is large compared with other relevant times. We found that for all equilibrium linear rotor systems — the even-J – odd-J mixtures — the reentrant behavior of the phase transition lines is an entropy-driven phenomenon, as was previously found for the all-J system. Experimentally, conditions to find the reentrant BSP transition line are most favorable for H2 mixtures, whereas the frozen monotonic phase lines should be the case for D2 even-J – odd-J mixtures. These results may therefore be particularly useful for understanding the anomalous transition region of the BSP transition documented for D2 mixtures.