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

The first quantitative determinations of heterogeneoustwo-componentreaction rates in single crystals under high pressure are reported.The rate of arsenolite inclusion compound with hydrogen, As4O6 & BULL;2H(2), formation on the surface of anarsenolite single crystal has been determined at 1.47 (5) and 1.37(5) GPa via synchrotron Fourier transform infrared spectroscopy. Athin plate of arsenolite has been completely transformed into theinclusion compound within 1 and 11 h, respectively. The inclusioncompound growth rate was found to be linear with a slope of 7.7 (4)& mu;m/h at 1.47 (5) GPa, indicating that reaction at the As4O6/As4O6 & BULL;2H(2) phase boundary is rate-limiting. The kinetics of As4O6 & BULL;2H(2) growth at 1.37 (5) GPa exhibits two domainsof linear growth with slopes of 0.25 (1) and 1.2 (1) & mu;m/h. Incidentally,a method of growing thin plates of As4O6 singlecrystals via spatial confinement is presented. This work shows thatthe single-crystal study of two-component reaction kinetics in diamondanvil cells is possible and presents one of the feasible ways to doit in a quantitative manner.