  We show that a first-order phase transition can take place in a metal in a
strong magnetic field if an electron Landau level approaches the Fermi energy
of the metal. This transition is due to the electron-phonon interaction and is
characterized by a jump in magnetostriction of the metal. If there are several
equivalent groups of charge carriers in the metal, a spontaneous symmetry
breaking of the magnetostriction can occur when the Landau level crosses the
Fermi energy, and this breaking manifests itself as a series of the structural
phase transitions that change a crystal symmetry of the metal. With these
results, we discuss unusual findings recently discovered in bismuth.[ In the
new version more explanations and examples added.]
