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 g
ij
from which the linear electrooptic properties can be predicted or vice versa. These relations are experimentally valid for KSr
2
Nb
5
O
15
. The resulting values of g
ij
are similar to those found for other NbO
6
-type crystals (e.g., KNbO
3
, 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 NbO
6
octahedra which are linked together by sharing oxygen corners as in the case of the structurally related perovskites. In view of the importance of the NbO
6
octrahedra 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 NbO
6
octahedra. This has been done by moving the transition temperature T
c
close to room temperature as in (Ba, Sr)Nb
2
O
6
and KSr
2
Nb
5
O
15
+xLaNb
3
O
9
. We have also examined the system K
x
Na
1-x
Ba
2
Nh
5
O
15
and show that the advantageous properties of NaBa
2
Nb
5
O
15
can 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 g
ij
in terms of shifts of electronic bands with polarization, a subject that is reviewed in this paper.