Role of Stoichiometry and Atomic Order

Implicit in the above discussion is that alloying can either raise or decrease 7 either through affecting the density of states or electron-phonon interaction. These two parameters are related in a 

For example, NbgGe, when prepared by normal techniques, such as arc-melting, exhibits 7 ss 6.9° K and is deficient in Ge. When prepared by splat-cooling to obtain a metastable phase, superconductivity begins at 17°The transition is fairly broad. Geller was the first to recognize that Nb3Ge (7 = 6.9°K) exhibited a lattice parameter different from that predicted on the basis of stoichiometric material. Nearly stoichiometric material was obtained only by splat 

Similarly, 7 of normal NbjGa (7 = 14.5°K) can be raised to 20.3°K, presently the highest 7 for a binary phase, when prepared so that it is nearly stoichiometric. High-7 NbjGa is formed by quenching alloys from above the solidus followed by a low-temperature anneal. The lattice constant of near-stoichiometric 

Both NbjAu and VjGa exist at elevated temperatures in the disordered bcc state and can be retained by quenching to room temperature. These disordered phases are not high-7 superconductors. By an ordering anneal these phases transform to the CrjSi structure with 7 values of 11.5°K and 16°K, respectively. 

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