Ferroelectrics in the lithium potassium niobate system

Abstract

Research into the Li2O-K2O-Nb2O5 system has revealed a large crystal field region exhibiting the tetragonal tungsten bronze structure which contains solid solution compounds with a wide range of lattice constants and chemical composition. However ferroelectric crystals are only obtained from melts containing large excesses of alkali and when the crystals' composition approach K6Li4Nb10O30; even so they are chemically inhomogeneous with Curie temperatures in the range of 450 to 580°C.An appreciation of crystal growth in the tetragonal tungsten-bronze region of the Li2O-K2O-Nb2O5 system is presented. Crystal quality, dielectric and electro-optic properties are compared for crystals grown by the Czochralski, Vertical Float Zone, and Flux Melt, methods. The extent of the tungsten-bronze field is outlined, in particular that region near the stoichiometric composition K6Li4Nb10O30, in which ferroelectric crystals can be found.An analysis of thermally stimulated currents in grossly alkali deficient samples gives deep traps (Ea = 2.05 eV). The problems of poling and switching are briefly described. Electro-optic measurements on stoichiometric melt, and nearly fully filled, crystals, show half wave voltages of 20 kV and 3 kV respectively. In the ferroelectric region (c0 > 4.02 Å) large variations in dielectric constant and Tc can occur in crystals from the same melt composition indicating chemical in-homogeneity. It is concluded that a maximum melting point composition does not exist within the tungsten-bronze field and that stoichiometry can be closely approached but not achieved.