Counterintuitive effects of isotopic doping on the phase diagram of H2–HD–D2molecular 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 betweenH2,D2, and HD molecules come from mass difference and the different quantum exchange effects: fermionicH2molecules have antisymmetric wavefunctions, while bosonicD2molecules 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 ofH2–HD–D2with various isotope concentrations over a wide pressure–temperature (P-T) range. We find that mixtures ofH2, HD, andD2behave 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 pureH2orD2. 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.