Structural Phase Transitions in Langbeinites

Using Landau theory of phase transitions it is shown that in the langbeinite structure (space group T 4 ) four spatially homogeneous structural phase transitions are possible. Theree of them are improper ferroelectric and result from a phonon instability at the M point of the Brillouin zone. The fourth one is very similar to structural phase transitions in hig temperature superconductors of the β‐tungsten structure. The free energies providing a basis for macroscopic description of these phase transitions are presented.