Structural and electrical transport properties of Al1-xMxB2 (M = Li, vacancy)

AlB2 and MgB2 are neighboring compounds and have similar crystal structures. However, their superconducting properties are very different because the sigma holes which play a key role in the superconductivity of MgB2 do not exist in AlB2. In this paper we reported our attempts to dope holes into AlB2 by substituting Li and vacancy for Al, and these holes were expected to induce superconductivity in the system. However, experimental results showed that Li was hard to substitute Al in AlB2. The variation of resistivity with temperature and x for samples with the nominal composition of Al1-xLixB2 was analyzed by considering the phase variation of the samples with x. The temperature dependence of resistivity for Al1-xB2 was also studied and orders of magnitude change in resistivity was observed when x was up to 0.4 and 0.5, similar to that observed in Mg1-xB2 system by Sharma et al. (A. Sharma, N. Hur, Y. Horibe, C. H. Chen, B. G. Kim, S. Guha, Marta Z. Cieplak, and S.W. Cheong, Phys. Rev. Lett. 89, 167003 (2002)). It is proposed that phase separation may exist in Al1-xB2, leading to the dramatic change of resistivity because of percolation.