Measurements of OH- absorption and proton activation in Pb1-xBaxNb2O6 crystals with applications to holographic storage

Pb1-xBaxNb2O6 is a material under consideration for electro-optic and photorefractive applications. Single crystals of Pb1-xBa2Nb2O6 have been grown using the vertical Bridgman method in the composition range 0.5 < 1-x < 0.6. The formation and thermal fixing of holographic gratings in these crystals were recently demonstrated. In this study the OH- absorption spectra were measured by fourier-transform infrared spectrometry. As-grown crystals showed a strong OH- absorption near v = 3485 cm(-1), revealing the existence of protons (H+) which are essential for the thermal fixing of holograms. The dependence of the integrated OH- optical-absorption intensity on polarization showed that all of the O-H bonds were polarized either perpendicular or parallel to the c axis, with a greater proportion perpendicular. The proton concentration could be increased by heat treatment in a humid atmosphere and decreased in an ambient-or oxygen atmosphere. The diffusivity of protons perpendicular to the c axis was measured in Pb0.5Ba0.5Nb2O6 by proton in-diffusion experiments and was found to be D-0 approximate to 0.219 X 10(-6) cm(2)/s at 750 degrees C, which is comparable to the proton diffusivity in TiO2 (D-0 approximate to 0.190 X 10(-6) cm(2)/s) and LiNbO3 (D-0 = 0.193 similar to 2.507 X 10(-6) cm(2)/s) at the same temperature. Measurements of the thermal decay of holograms showed that the proton activation energy, Delta E, in Pb0.5Ba0.5Nb2O6 was 0.95 +/- 0.05 eV, which is close to the proton activation energies reported for LiNbO3 and KNbO3, 1.10-1.40 eV and 0.81-1.04 eV, respectively.