Alkali metal substitution effects in Mg1-xAxB2 (A = Li and Na)

We report the alkali metal substitution effects on the structure and superconductivity of Mg(1-x)A(x)B(2) (A = Li and Na) compounds. X-ray diffraction results show that nearly single phase Mg(1-x)A(x)B(2) can be obtained for x less than or equal to 0.15 Li and x less than or equal to 0.2 Na substitution. The lattice parameters a and c decrease with Li substitution. As substituted by Na, both a and c show an increase at low Na concentration, then decrease with substitution at higher Na. concentrations. The change of a with Li and Na substitution is explained by two overlapping effects: ion size and an increase of in-plane coupling upon hole doping. The tendency of c-axis contraction in Mg1-xLixB2 is consistent with theoretical prediction. The T-c decreases when Li concentration is increased, while the change of T-c upon Na substitution is not apparent. (C) 2001 Elsevier Science B.V. All rights reserved.