Reactivity and volatility of astatine in a quartz column

Abstract

Astatine (At, Z = 85) is the rarest naturally occurring element and exhibits unique chemical properties influenced by relativistic effects. The short half-lives of its isotopes and its scarcity limits chemical experiments and methods to study and work with astatine. While some insight has been gained into its behavior in the liquid phase, substantial experimental challenges persist, and studies of its gas-phase chemistry remain scarce. Understanding its reactivity and volatility is important not only for optimizing the use of At in targeted alpha therapy but also a crucial step towards future investigations of its superheavy homolog, tennessine (Ts, Z = 117). Adsorption and interaction of At with a quartz surface were studied aiming at a conclusive understanding of the interaction strength between At and fused silica surfaces of different reactivity. In our work, the isotopes 207,208At (T1/2 = 1.63 h and T1/2 = 1.81 h, respectively) were produced via fusion-evaporation reactions by irradiating Bi2O3-targets with 3He beams. We used gas–solid thermochromatography in various gas atmospheres and applied several temperature gradients ranging from Tmax = 1000 °C to Tmin = − 170 °C. Silica surfaces with different degrees of hydroxylation were used. These experiments reveal the concentration of the hydroxyl groups on the surface, i.e. its reactivity, to play an important role in the chemical interaction of At with hot quartz surfaces. Advanced Monte Carlo simulations allowed determining the adsorption enthalpies of the At species, and thus, to elucidate the chemical interactions of At with quartz surfaces. The use of different carrier gases as well as surfaces of different reactivity allowed the production and observation of multiple chemical species. We assigned the most volatile species to elemental At, which was found to be chemically bound to the hydroxylated silica surface at temperatures between 300 and 500 °C.