Phase transitions in H2−HD−D2 mixtures up to 350 GPa

Abstract

Utilizing the high-pressure low-temperature Raman and optical transmission/absorption spectroscopies, we have mapped out the phase diagram of H2-HD-D2 alloy with the initial H:D concentration 50:50 up to 350 GPa between 10 and 300 K. We followed the phase lines between all known solid phases [I, II, III, IV, and IV′ (IV+V)] of pure H2 and D2 constraining their locations in the wide space. We trace the phase boundary separating phases III and IV(IV′)/V demonstrating that this phase line has a steep negative slope (−0.95 K/GPa), which becomes shallower at above 250 GPa (−0.55 K/GPa). The phase line follows the trend observed in the pure species. Additionally, we find that the band gap of the mixture between 300 and 360 GPa is close to those of the pure species suggesting that alloying one isotope by another would not significantly shift the metallization pressure. These observations imply that hydrogen deuteride (HD) has very similar properties to those exhibited by the pure species. Published by the American Physical Society 2025