Type-I silicon clathrates containing lithium

Abstract

Abstract The intermetallic phase [Li x Ba8−x ][Li y Si46−y ] is the first example of a clathrate-I silicide containing lithium with the peculiarity that the Li atoms occupy both cage and framework positions. The phase was obtained by ambient-pressure solid-state reactions at 400°C between Ba4Li2Si6 and the binary clathrate phase Ba8−x Si46 applying different experimental approaches. In this reaction, Li atoms substitute Si atoms of the framework at site 6c (space group Pm3̅n) and fill up the Ba-deficient dodecahedral Si20 cages at site 2a. The presence of Li atoms in the clathrate phase was proven by combined X-ray powder diffraction, 7Li and 29Si solid-state NMR analyses. Incorporation of lithium markedly increased the lattice parameter of the clathrate phase, e.g. from a=10.3200(2) Å for Ba7.48(2)Si46 to a=10.3715(3) Å for [Li0.59(2)Ba7.41(2)][Li2.3(1)Si43.7(1)]. The critical temperature T c for the transition to the superconducting state decreased from 7.7 K to 3.2 K upon Li incorporation.