The influence of external mechanical stress on the lattice transformations in V,Si a n d Nb,Sn is investigated on the basis of Landau's theory of phase transitions. The calculated phase diagrams for uniaxial

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The influence of the anomalous elastic behaviour of A15 compounds (e.g. Nb3Sn and V3Si) on its superconducting transition is investigated theoretically. The sensitive reaction of the superconducting properties to uniaxial stress is explained as a consequence of the elastic softening. In single crystals the change of the transition temperature depends strongly on the direction of the applied uniaxial stress. In polycrystals the width of the transition curve increases quadratically with the uniaxial stress. The shape of the broadened transition curve is calculated approximately. In a composite conductor the A15 layer is exerted to a non‐hydrostatic stress as a result of different thermal expansion coefficients. The present model explains the observed parabolic strain dependence of superconducting properties in Nb3Sn composite conductors and allows to estimate the strain at which damage of the A15 layer occurs.

The shift of the transition temperature with pressure for KH2AsO4 (dTc/dp = (−3.3 ± ± 0.2) deg/kbar) and for RbH2PO4 (dTc/dp = (−8.2 ± 0.3) deg/kbar) was determined by dielectric‐constant measurements under hydrostatic pressure up to 1.2 kbar. The experimental data are analysed using a formula for the shift of Tc with pressure derived from Kobayashi's theory.