Quantum path-integral study of the phase diagram and isotope effects of neon

TL;DR

This study uses quantum path-integral simulations to analyze neon's phase diagram and isotope effects, revealing quantum shifts in phase boundaries and isotope-dependent temperature variations consistent with experimental data.

Contribution

It provides the first quantum simulation-based phase diagram of neon, quantifies isotope effects, and compares quantum and classical results for phase coexistence.

Findings

Quantum effects shift coexistence lines to lower temperatures.

Triple-point isotope effect of about 0.15 K between 20Ne and 22Ne.

Quantum simulations agree with experimental isotope effect data.

Abstract

The phase diagram of natural neon has been calculated for temperatures in the range 17-50 K and pressures between 0.01 and 2000 bar. The phase coexistence between solid, liquid, and gas phases has been determined by the calculation of the separate free energy of each phase as a function of temperature. Thus, for a given pressure, the coexistence temperature was obtained by the condition of equal free energy of coexisting phases. The free energy was calculated by using non-equilibrium techniques such as adiabatic switching and reversible scaling. The phase diagram obtained by classical Monte Carlo simulations has been compared to that obtained by quantum path-integral simulations. Quantum effects related to the finite mass of neon cause that coexistence lines are shifted towards lower temperatures when compared to the classical limit. The shift found in the triple point amounts to 1.5 K, i.e., about 6 % of the triple-point temperature. The triple-point isotope effect has been determined for 20Ne, 21Ne, 22Ne, and natural neon. The simulation data show satisfactory agreement to previous experimental results, that report a shift of about 0.15 K between triple-point temperatures of 20Ne and 22Ne. The vapor-pressure isotope effect has been calculated for both solid and liquid phases at triple-point conditions. The quantum simulations predict that this isotope effect is larger in the solid than in the liquid phase, and the calculated values show nearly quantitative agreement to available experimental data.