Comment on the structural phase transition in A15 compounds

The martensitic transformation in A15 compounds in the presence of internal strains and of external mechanical stress is examined on the basis of the Landau theory of phase transitions. The free energy of the model coincides with the free energies of the three-state Potts model and provides an explanation for the different martensitic transition temperatures for Nb 3 Sn and V 3 Si crystals. It is argued that in these compounds the linear chains of Nb and V atoms may be broken in the tetragonal phase. The critical behaviour is discussed.