The Importance of the Transition Metal Volume in A15 Superconductors
It has been suggested1,2 that an essential feature of superconductors with a high transition temperature Tc is that the system be on the verge of a lattice instability. Testardi et al.3,4 have also argued that the high transition temperatures in some Al5 superconductors, notably V3Si, are related to their low-temperature lattice instabilities. We are currently investigating the superconducting properties under pressure of A15 superconductors of the type T1−y X y , where T is a transition metal atom and X is a simple metal exhibiting s—p bonding. In general, these systems form a single well-defined Al5 phase over a range of composition 0.2 0≲ y ≦ 0.25. For y > 0.25 the V—Si system has a two-phase region of (V3Si + V5Si3). The change in T c produced by the application of hydrostatic pressure can be of either sign in these systems. Likewise, the lattice parameter increases in some systems and decreases in others as one alloys away from the stoichiometric compound at y = 0.25 in the single-phase A 15 region. In investigating these systems, we have noted a strong correlation between the sign of dT c / dP and the sign of the lattice parameter change as one approaches the stoichiometric compound. For example, in V3Si the volume decreases with a decrease in the amount of excess V,5 and dT c / dP is positive.6 This behavior is contrary to that predicted by Testardi,4 who calculates a large quadratic strain dependence for T c This would give a rapid depression of T c with pressure. We do not believe that the correlation is coincidental and are currently investigating the changes in T c produced by changing the transition metal atom spacing. Smith6 has observed a particularly large increase in T c with pressure (1°K per 29 kbar) for the A15 compound V3Si. Since this compound is thought to be on the verge of a lattice instability, it is useful to determine if the occurrence of some phase transition in V3Si might limit the increase in T c attainable by the application of pressure. Hence we are studying the compressibility and phase stability of V3Si at pressures up to 300 kbar. Preliminary results of this study are reported here.
KeywordsBulk Modulus Lattice Instability Stoichiometric Compound High Transition Temperature National Bureau ofStandards
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