Positron annihilation studies of stage-1 donor graphite intercalation compounds

Abstract

Measurements of the 2γ-Angular Correlation Distribution (ACD) of the positron annihilation radiation in the stage-1 donor graphite intercalation compounds C 6Li, C 6Ba, C 8K, C 8Rb and C 8Cs are presented. The ACDs of C 6Li are found to be very similar to those of graphite, indicating that annihilations preferentially occur with graphitic σ-and π-electrons. By contrast, in the three heavy alkali metal compounds, an intense, strongly anisotropic, narrow component dominates the ACDs. This can be attributed to annihilations with electrons exhibiting a free-electron character parallel to the layers. The anisotropy decreases with increasing atomic number of the intercalant. This observation is related to the expansion of the distance between the carbon planes upon exchanging the intercalant. The ACDs of C 6Ba are found to be similar to those of the heavy alkali-graphite intercalation compounds; the anisotropy of the ACDs, however, is very small. Comparison with C 8K indicates that the narrow component of the ACDs in C 6Ba cannot be attributed solely to interlayer states exhibiting free-electron character parallel to the layers but rather to Ba metal states.