Growth and characterization of tungsten-bronze ferroelectric lead barium niobate crystals for holographic data storage

This dissertation focuses on the development of a new photorefractive material, lead barium niobate (Pb_1-xBa_xNb₂O₆, PBN), which appears to be a promising candidate for use in holographic storage. One of the unique properties of PBN is that it simultaneously has a high ferroelectric transition temperature (300-500^oC depending on composition) and large electro-optic coefficients, which is possible due to the existence of a nearly vertical morphotropic phase boundary (MPB) separating two ferroelectric phases near 1 - x = 0.63. In spite of the many advantages of PBN, this material has not been widely studied for photorefractive storage applications because of the difficulty in growing large, optical-quality single crystals. Although single crystals a few millimeters in size have previously been grown by the Czochralski method, the rapid loss of PbO from the melt has led to poor control over stoichiometry and compositional homogeneity. We have developed the vertical Bridgman growth method involving the use of sealed Pt crucibles in which PBN charges are unidirectionally solidified. Large single crystals of centimeter size have been grown without PbO loss by this method. It was also found that the mass transport in PBN melt was dominated by diffusion, making it possible to obtain a crystal of uniform composition from an incongruent melt. Many important properties of PBN have been examined which include the ferroelectric, optical, electro-optic and photorefractive properties. The spontaneous polarization of tetragonal PBN (1-x <~0.63) was in the range of 0.40-0.70 μC/mm² depending on composition and increased as the composition approached the MPB. PBN crystals showed high diffraction efficiency, photorefractive gain and sensitivity. Another encouraging feature of PBN with respect to holographic storage applications is that it contained a high concentration of protons (H⁺) essential for the thermal fixing of stored holograms. Formation of the secondary ionic grating at elevated temperatures and the resulting thermal fixing of holograms have been demonstrated in these crystals, and the fixing efficiency increased with increasing proton concentration. These new findings serve to illustrate the importance of conducting materials development in parallel with device research.