Structural environment of krypton dissolved in vitreous silica

X-ray absorption measurements on Kr dissolved homogeneously in vitreous sil ica (1.97 wt% Kr) have been carried out at the Kr K-edge at 4.5 K, represen ting the first direct determination of the sites occupied by a noble gas. T he presence of EXAFS oscillations shows that the Kr atoms are surrounded by a well-defined shell of nearest neighbors, identified as oxygen atoms. The mean Kr-O distances are 3.45 +/- 0.1 Angstrom, based on a simple model of Gaussian disorder, with a Debye-Waller factor of sigma(2) = 0.06 Angstrom(2 ). The large Kr-O distance, which is at the upper limit of the size of the holes existing in vitreous silica, together with the existence of well-defi ned sites suggests a forced, densely packed environment of oxygen around Kr atoms characteristic of clathrasil surroundings.