Determining the Kinetics of As₄O₆·2H₂ Formation at High Pressures

The first quantitative determinations of heterogeneous two-component reaction rates in single crystals under high pressure are reported. The rate of arsenolite inclusion compound with hydrogen, As₄O₆·2H₂, formation on the surface of an arsenolite single crystal has been determined at 1.47 (5) and 1.37 (5) GPa via synchrotron Fourier transform infrared spectroscopy. A thin plate of arsenolite has been completely transformed into the inclusion compound within 1 and 11 h, respectively. The inclusion compound growth rate was found to be linear with a slope of 7.7 (4) μm/h at 1.47 (5) GPa, indicating that reaction at the As₄O₆/As₄O₆·2H₂ phase boundary is rate-limiting. The kinetics of As₄O₆·2H₂ growth at 1.37 (5) GPa exhibits two domains of linear growth with slopes of 0.25 (1) and 1.2 (1) μm/h. Incidentally, a method of growing thin plates of As₄O₆ single crystals via spatial confinement is presented. This work shows that the single-crystal study of two-component reaction kinetics in diamond anvil cells is possible and presents one of the feasible ways to do it in a quantitative manner.