Crystallography and structural phase transitions, an introduction

Structural phase transitions which involve the generation of lattice strain (i.e. in ferroelastic and co-elastic crystals) are reviewed. Landau-Ginzburg theory is found to lead to an appropriate description of order parameters following the theoretical predictions over large temperature intervals. A quantum extension of Landau-Ginzburg theory is presented which describes the saturation of order parameters at low temperatures. Coupling between different order parameters is relevant if more than one phase transition (or lattice instability) occurs in a crystal. Finally, the importance of microstructures and kinetic processes for the structural behaviour of crystals is pointed out.