Composites of two atomic lanthanide impurities (2Ln, where Ln = La, Ce, Sm, Gd) and a finite single-walled carbon nanotube (SWCNT) are investigated by use of a plane-wave density functional theory method. A symmetrically truncated SWCNT of the zigzag type is chosen as host species. Endohedral and exohedral configurations of the 2Ln(n,0), system with n = 9, 10 are explored. A molecular dimer solution for Ln = Sm, Gd, involving an Ln2 subunit at the tube center, is found to be substantially less stable than alternative geometries with separated Ln atoms, each localized at a tube edge. Addition of the Ln impurities strongly affects the magnetism of the pure SWCNT host. For the most stable isomers identified, the magnetic structure of 2Ln(n,0) is entirely determined by the magnetic moments of the Ln(4f) shell. Analyzing 2Ln insertion into the host along the SWCNT axis yields the prediction of vanishing energy barrier for the special case of 2Gd(10,0).
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