Structure and Lattice Expansion of Pure Sodium Confined inside Nanoporous Vycor Glass at Low Temperatures

Abstract

Behavior of sodium metal subject to geometrical constraints has been studied at variable temperatures. A porous medium, Vycor glass of an average pore size of 9.2 nm, was loaded with Na by vacuum vaporization. Synchrotron X-ray powder diffraction of the specimen revealed the existence of close-packed phases such as FCC, HCP, 9R and 4H already at room temperature whereas the bulk BCC phase of Na was completely absent. In contrast, close-packed phases in bulk sodium are only observed at temperatures below 35 K upon cooling. This result can be qualitatively understood in view of earlier zero Kelvin calculations from which it is well known that the differences of structural energies between BCC and close-packed phases are very small. As a consequence, also various close-packed structures can be formed at ambient temperature under certain conditions and environments. The lattice expansion in the restricted geometry has been determined in the range of 160 K to 300 K and compared with the bulk metal. Confined Na exhibits smaller lattice expansion as compared to that of the bulk which likewise can be explained as being due to confinement.