Properties and improvements of tungsten bronze ferroelectrics

Recent work has shown that some ferroelectrics of the tungsten-bronze-type are potentially useful because they exhibit large linear electrooptic and second harmonic generation coefficients, are easily growable, and do not suffer from laser damage. We review these crystal systems in terms of a biased quadratic electro-optic effect gijfrom which the linear electrooptic properties can be predicted or vice versa. These relations are experimentally valid for KSr2Nb5O15. The resulting values of gijare similar to those found for other NbO6-type crystals (e.g., KNbO3, etc.), as are the measured values of the coefficients in the Devonshire free energy expansion. Thus fundamental properties of these tungsten-bronze ferroelectrics appear to be determined by the properties of the lattice of NbO6octahedra which are linked together by sharing oxygen corners as in the case of the structurally related perovskites. In view of the importance of the NbO6octrahedra in determining the behavior of the niobate ferroelectrics, we consider next the problems of improving the properties of the tungsten-bronze type materials while maintaining a homogeneous sublattice of NbO6octahedra. This has been done by moving the transition temperature Tcclose to room temperature as in (Ba, Sr)Nb2O6and KSr2Nb5O15+xLaNb3O9. We have also examined the system KxNa1-xBa2Nh5O15and show that the advantageous properties of NaBa2Nb5O15can be effectively retained while eliminating the deleterious effect of microtwinning in these crystals. Finally we Show that at least the order of magnitude of the strain-optic effect which relates change of index to strain, can be understood in terms of the shifts of electronic bands with strain. This is analogous to the understanding of gijin terms of shifts of electronic bands with polarization, a subject that is reviewed in this paper.