Growth of A15 Type Single Crystals and Polycrystals and Their Physical Properties

Abstract

Before the discovery of the high Tc superconductors, the highest values of Tc were observed in compounds crystallizing in the A15 type structure (Figure E2.5.1); in the meantime, more than 5000 works describing their properties have been published. At the present day, 64 A15 type compounds with the general formula A1–βBβ are known, where A stays for a transition metal: Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, while B represents either a transition element or an element at the right side of the periodic system. The compounds Nb3Ge, Nb3Ga, Nb3Al, Nb3Sn, V3Si and V3Ga exhibit the highest Tc values, in the range between 23 and 15.4 K. Stoichiometric A15 type compounds are characterized by the formula A3B (β = 0.25), but the A15 phase fields often extend over a certain range of composition, sometimes even at both sides of stoichiometry: in most cases, β in A1-βBβ is comprised within the range 0.18 ≤ β ≤ 0.30. For example, the phase field in the system V3Si ranges from β = 0.20 to 0.27, while for Nb3Sn, it extends from β = 0.18 to 0.255. There are compounds, e.g., the binaries Nb3Ge and Nb3Al, where the stoichiometric composition is stable at very high temperatures only (see the phase diagrams in Section E2.5.1.3, Figure E2.5.4). In these cases, the stoichiometric composition can only be stabilized either by ultrarapid quenching or by non-equilibrium formation processes, e.g., sputtering or chemical vapour deposition. The values for the superconducting transition temperature, Tc, the electronic specific heat coefficient, γ, and the upper critical field, Bc2(0), for a series of A15 type compounds are listed in Table E2.5.1.