Counterintuitive effects of isotopic doping on the phase diagram of H2-HD-D2 molecular alloy

Molecular hydrogen forms the archetypical quantum solid. Its quantum nature is revealed by behavior which is classically impossible and by very strong isotope effects. Isotope effects between H-2, D-2, and HD molecules come from mass difference and the different quantum exchange effects: fermionic H-2 molecules have antisymmetric wavefunctions, while bosonic D-2 molecules have symmetric wavefunctions, and HD molecules have no exchange symmetry. To investigate how the phase diagram depends on quantum-nuclear effects, we use high-pressure and low-temperature in situ Raman spectroscopy to map out the phase diagrams of H-2-HD-D-2 with various isotope concentrations over a wide pressure-temperature (P-T) range. We find that mixtures of H-2, HD, and D-2 behave as an isotopic molecular alloy (ideal solution) and exhibit symmetry-breaking phase transitions between phases I and II and phase III. Surprisingly, all transitions occur at higher pressures for the alloys than either pure H-2 or D-2. This runs counter to any quantum effects based on isotope mass but can be explained by quantum trapping of high-kinetic energy states by the exchange interaction.