The structure and thermal properties of novel mesoporous inert-gas solids were studied by synchrotron x-ray diffraction. The materials were prepared by injecting a jet of helium containing dilute amounts of inert atoms and molecules (Ne, Kr, ${\mathrm{N}}_2$) into superfluid helium. The characteristic size of the constituent building blocks is $\sim 60Å$, and the density is $\gtrsim {10}^{20}\mathrm{impurity}$ atoms per c${\mathrm{m}}^3$. The Kr and ${\mathrm{N}}_2$ solids are stable outside of liquid He, up to temperatures above 10 K. These materials may find various applications as a new type of porous medium for fundamental physics, as well as in cluster physics, matrix isolation spectroscopy, and catalysis of low temperature chemical reactions.

We have used x-ray diffraction and ultrasound techniques to study the structure of mesoporous impurity–helium solids created after the injection of impurity particles $({\mathrm{D}}_2,$ Ne, ${\mathrm{N}}_2,$ Kr) into a volume of superfluid ${}^4\mathrm{He}.$ Clusters of impurities with size of order $50\pm 20\hspace{0.5em}\AA$ and density $\sim {10}^{20}\hspace{0.5em}\mathrm{impurities}/{\mathrm{cm}}^3$ were observed by x-ray diffraction. The presence of a wide distribution of pore sizes in Im–He solids was revealed by ultrasound (80 to $8600\hspace{0.5em}\AA )$ and by small-angle x-ray scattering (80 to $>400\mathrm{}\hspace{0.5em}\AA ).\mathrm{}$ Both x-ray and ultrasound methods detected irreversible structural changes when samples were warmed above $T_{\lambda }=2.17\mathrm{}\hspace{0.5em}\mathrm{K}.$ This is ascribed to the aggregation of small clusters caused by thermally activated diffusion. In addition to being of fundamental interest, the properties of the unique porous media studied in this work may be relevant to investigations of low temperature chemical reactions, storage of free radicals, matrix isolation spectroscopy, and superfluid ${}^4\mathrm{He}$ contained in the pores of an extremely compliant medium.

Macroscopic samples ($\mathrm{\text{volume}}\sim {\mathrm{cm}}^3$, $\mathrm{\text{atomic density}}\sim {10}^{19}–{10}^{20}{\mathrm{cm}}^{-3}$) of noble-gas nanoclusters ($\mathrm{\text{size}}\sim 5–6\mathrm{nm}$) were produced in superfluid helium by an impurity-helium gas injection technique. X-ray diffraction measurements show that the samples consist of weakly interacting nanoclusters with fivefold symmetry, such as icosahedra and decahedra. These results open new opportunities for fundamental research of nanoclusters of noble gases and other materials in well-controlled environments using experimental techniques requiring bulk samples.

Macroscopic ensembles of weakly interacting argon nanoclusters are studied using x-ray diffraction in low vacuum. As the clusters grow by fusion with increasing temperature, their structure transforms from essentially face-centered cubic (fcc) to hexagonal close packed as the cluster size approaches $\sim {10}^5$ atoms. The transformation involves intermediate orthorhombic phases. These data confirm extant theoretical predictions. They also indicate that growth kinetics and spatial constraints might play an important role in the formation of the fcc structure of bulk rare-gas solids, which still remains puzzling.