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S. J. Wen, G. Campet, and J. P. Manaud,（1993)，proposed  “A valuation of the “electron-acceptor-strength” of the ionized doping elements, Mm+，can be carried out using the empirical relationship established by Y. Zhang(1982) :
L = Z/rk2 – 7.7 Xz + 8.0
Where L symbolizes the “lewis acid strength” of the element. Z/rk2 (where z is the charge number of the atomic core and rk2 is the ionic radius) is related to electrostatic force and Xz symbolizes the electronegativity of elements in valence states."
"We have indeed reported elsewhere (12) that the “electron-acceptor strength” of the ionized donor centers, Mm+, decreases as the “Lewis acid strength”, L, of the element increases. Consequently, high mobilities, and thereby high conductivities, are likely to occur for ITO (or IO) samples heavily doped with donor centers having high L values, particularly in their oxidized form."
 S. J. Wen, G. Campet, and J. P. Manaud,（1993)，Active and Passive Elec.Comp.,1993,Vol.15,pp.67-74
 Y. Zhang(1982)，Yonghe Zhang, Inorg. Chem. 21, 3386, 3389 (1982).