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

Significance When hydrogen and deuterium are mixed, they form H2 +HD + D2 mixtures at very low pressures and room temperature. We show that at high pressures and low temperatures these mixtures behave as an isotopic molecular alloy (ideal solution); exhibiting symmetry-breaking phase transitions between phases I, II, and III; and shifting the transformations to phases II and III to higher pressures as compared to pure isotopes. 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. 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 H2, D2, and HD molecules come from mass difference and the different quantum exchange effects: fermionic H2 molecules have antisymmetric wavefunctions, while bosonic D2 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 H2–HD–D2 with various isotope concentrations over a wide pressure–temperature (P-T) range. We find that mixtures of H2, HD, and D2 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 H2 or D2. 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.