Cage Structure and Long-Range Order in Solid Rare Gas Matrixes: A Combined FTIR and XRD Study

Abstract

Infrared spectroscopy (FTIR) and X-ray diffractrometry (XRD) were applied in order to characterize solid rare gas matrixes containing SF6 or CH4 species as impurities. The two techniques may be considered as complementary insofar as FTIR probes the dynamics of the dopant species, which is more or less strongly influenced by the matrix environment, whereas XRD aims at the long-range order of the rare gas matrix, which is likely to depend on the type and amount of the dopant. In fact, the distortions induced in Ar matrixes by embedding perfectly isolated CH4 and SF6 monomer molecules are not seen by XRD since the respective contribution to the broadening of Bragg reflections is buried under the dominating contribution related to the crystallite size effect (Scherrer formula). The nonequilibrium conditions of the deposition process give rise to a mean crystallite diameter of less than 50 nm. From the trends observed for both position and width of the respective IR test bands in Ne, Ar, and Kr, it is concluded that the matrix cages for the CH4 monomer are single substitutional sites, whereas for the SF6 monomer the space required comprises that of six rare gas atoms in an octahedral arrangement. However, a tetrahedral cage consisting of four rare gas atoms may not be excluded definitely. The crystalline order of Ar is retained even at CH4 concentrations that are undoubtedly related to the previously reported miscibility gap and to significant dimer formation detected by FTIR. On the other hand SF6 dimer formation, again identified by FTIR, gives rise to a complete loss of coherent X-ray scattering of Ar and to the appearance of diffuse intensity in the XRD pattern attributed to an essentially amorphous Ar phase. The local Ar environment of an isolated monomer and of the monomer unit of an isolated dimer appear to be fundamentally different in the case of SF6 and resemble each other for CH4 as impurity. Obviously, the SF6 dimer formation in Ar immediately heralds in an efficient phase separation process already at 1 mol % SF6 even under the nonequilibrium conditions of matrix deposition. Under these conditions the phase separation in CH4/Ar mixtures appears to be strongly kinetically hindered.