RULES FOR THE OCCURRENCE OF SUPERCONDUCTIVITY AMONG THE ELEMENTS, ALLOYS, AND COMPOUNDS
ARMY ELECTRONICS RESEARCH AND DEVELOPMENT COMMAND FORT MONMOUTH NJ
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An empirical rule predicting the occurrence of superconductivity among the elements was given by Matthias Matthias Rule, MR. The MR states that nonmetallic, ferromagnetic, and antifer romagnetic elements do not become supercon ductors, and that the empirical laws applying to the transition metals TM are quite differ ent from those applying to nontransition metals. Within these restrictions, the MR gives an esti mate of the expected superconducting transition temperature, Tc, in terms of three parameters 1 the atomic mass, M 2 the atomic volume Va and 3 the number of valence electrons per atom, n. The crystal symmetry also has some influence on the Tc value. The MR for the elements is with success applied also to alloys and compounds, when proper averages over the M and n values of the constituent atoms are used. Three supple mentary rules pertaining to the influence of compound formation on Tc are given by the author. The theoretical justification for the established empirical rules is outlined. Certain characteristic deviations from the MR are then discussed. A new rule is proposed for systems which contain TM atoms in the form of chains, or prisms, such as the TM borides. In other systems not obeying the MR, unexpectedly large changes of Tc are caused by small concentrations of vacancies, or of impurity atoms. Systems not obeying the MR should be of special importance for the development of new superconducting materials.
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