Electron paramagnetic resonance study of sodium guests in silicon clathrates, a cage-like crystalline silicon allotrope*

Si clathrates are cage-like crystalline Si allotropes with potentially exciting properties. They are synthesized in the presence of alkali guest atoms, e.g. Na, which occupy interstitial sites in the cages. However, Na guests degenerately dope the crystal. Realizing the potential of these materials requires fundamental understanding of guest properties in the host crystal structure. We present an EPR study of Na guests in Type II Si clathrate powders with ≤1 at. % Na. At low temperature, the spectrum shows four lines from the hyperfine interaction of the Na valence electron with the ²³Na nucleus (I=3/2). We attribute a line to Si dangling bonds in a highly disordered phase surrounding each grain, and identify doublets around the four hyperfine lines as arising from interactions with a ²⁹Si nucleus (I=1/2) in the surrounding cage. We estimate about half of the electronic wavefunction on the Si sub-lattice extends past the confining cage. Additional structure is observed halfway between the strong hyperfine lines, which we attribute to interactions of two ²³Na nuclei (I=3) in adjacent cages. A relatively broad line is attributed to clustered sodium. The relative intensities show the Na distribution is inhomogeneous.