Anomalies of Thermal Dilatation and Domain Structure in the Multiferroic Material PbK 2 LiNb 5 O 15
Résumé
PbK2LiNb5O15 belongs to the family of tungsten bronze compounds. It attracts attention because it exhibits a ferroelectric phase at room temperature, and undergoes in the neighbourhood of 650 K a complex phase transition between a tetragonal phase at high temperature (with P4/mbm space group) and an orthorhombic one (with the space group Pba2) at low temperature. This symmetry lowering, of paraelectric-paraelastic to ferroelectric-ferroelastic nature, allows the appearance of a domain structure which is discussed. Results of thermal dilatation in a direction perpendicular to the ferroelectric axis c (tetragonal axis at high temperature) exhibit an anomalous behaviour related both to the ferroelastic domains and to the complex phase transition. In agreement with other data, an intermediate state is shown to exist in a range of about 80 K in the region of the phase transition.