Mixing behavior and structural formation of quench-condensed binary mixtures of solid noble gases

Mixtures of two noble gases (neon, argon, krypton, and xenon) with similar molar fractions were quench condensed on a substrate held at constant temperature. Subsequently, the solidified mixtures were annealed and their desorption properties were studied by means of high-frequency surface acoustic waves acting as a microbalance. Depending on the preparation conditions, the binary alloys exhibit significant differences in their desorption behavior, namely continuous desorption over a wide temperature range, or steplike desorption at discrete substrate temperatures. Valuable information on the atomic structure of noble gas alloys can be obtained by systematically varying the condensation temperature and monitoring the desorption. If the atomic sizes of the participating atoms is comparable, e.g., in the case of $\mathrm{Ar}∕\mathrm{Kr}$ mixtures, substitutionally disordered solids are formed. With increasing size differences, e.g., in $\mathrm{Ne}∕\mathrm{Ar}$ mixtures, the properties of the alloys strongly depend on the conditions of film preparation. At low condensation temperatures, mixed phases with a fixed stoichiometry of the components ($A{B}_2$ and $A_{2}B$) are formed, whereas at higher condensation temperatures all neon mixtures show a clear tendency to separate into single-component phases.

Figure 1

Relative change of frequency as a function of substrate temperature for a pure neon film (lower part) and a pure argon film (upper part) during annealing. The heating rate was $2\mathrm{K}∕\mathrm{h}$ in both cases.

Figure 2

Desorption of a mixture of neon and argon with similar molar fraction, quench condensed at $T_{\mathrm{con}}=2.3\mathrm{K}$.

Figure 3

Desorption of $\mathrm{Ne}∕\mathrm{Ar}$ mixtures. The numerical values represent the condensation temperatures $T_{\mathrm{con}}$ at which the films were prepared. Only the onset of the argon desorption at about $30\mathrm{K}$ is shown.

Figure 4

Desorption of $\mathrm{Ne}∕\mathrm{Kr}$ mixtures. The numerical values represent the condensation temperatures. Only the onset of the krypton desorption is shown.

Figure 5

Desorption of $\mathrm{Ne}∕\mathrm{Xe}$ mixtures. The numerical values represent the different condensation temperatures. Only the beginning of the xenon desorption is plotted.

Figure 6

Desorption of quench-condensed $\mathrm{Ar}∕\mathrm{Xe}$ film prepared at three different condensation temperatures $T_{\mathrm{con}}$.

Figure 7

Desorption of mixtures of argon and krypton prepared at three condensation temperatures. The dashed line represents the expected frequency shift due to the release of the complete argon fraction.

Figure 8

Desorption of mixtures with similar fractions of krypton and xenon prepared at three condensation temperatures.

Figure 9

Summary of the experimental results. The effective mobility $(T_{\mathrm{con}}∕T_i)$ during condensation is plotted as a function of the atomic size ratio $\alpha$ for the different mixtures. The dotted lines are guides for the eyes. A detailed description is given in the text.