Our new paper, “Electronic and Nuclear Quantum Effects on the Ice XI/Ice Ih Phase Transition” has been accepted in Phys. Rev. B.
Experimentally, the temperature of the order/disorder phase transition of hexagonal ice is 72 K for light ice (H2O) and 76 K for heavy ice (D2O). Using ab initio DFT with van der Waals corrections and the quasi harmonic approximation to the free energy, we have calculated a 6K isotope effect on the phase transition temperature as compared to the experimental value of 4K.
By decomposing the free energy, we have explained that the source of this isotope effect is purely the zero-point energy difference between H2O and D2O. This is directly linked to the anomalous isotope effect on the volume of the hexagonal ice, which was discussed in our previous work. These results indicate that the same physics might be behind the isotope effects in transition temperatures between other ice phases.