Role of charge doping and lattice distortions in codoped   Mg_{1-x}(AlLi)_{x}B_2 compounds
  We prepared a series of Mg_{1-x}(AlLi)_{x}B_2 samples with 0&#8804;x&#8804;0.45 in order to compensate with Li the electron doping induced by Al. Structural characterization by means of neutron and X-ray diffraction confirms that Li enters the MgB2 structure even though in an amount less than nominal one. We performed susceptibility, resistivity and specific heat measurements. Vibrational properties were also investigated by means of Raman spectroscopy. We compare these results with those obtained on a homologous series of Mg_{1-x}Al_{x}B_2 samples. The systematic success of scaling the relevant properties with the Al content rather than with the electron doping suggests that lattice deformation plays an important role in tuning the superconducting properties. 
M. Monni, C. Ferdeghini, P. Manfrinetti, A. Palenzona, M. Putti, M.   Affronte, P. Postorino, M. Lavagnini, A. Sacchetti, D....