We study the fixing of holographic gratings recorded with the K-vector perpendicular to the c-axis in an iron doped lithium niobate crystal. In this geometry the developed diffraction efficiency is very weak. We investigate the role of the photovoltaic and diffusion transport processes in the recording and developing of these gratings. We measured the diffraction efficiency during thermal decay of these gratings as a function of the temperature. This decay is related to thermal diffusion of protons in a direction perpendicular to the c-axis of the crystal. The analysis of the results provides information about the hydrogen diffusion coefficient in this direction. The values obtained are compared to the previous data of decays of fixed holograms with K-vector parallel to the c-axis.
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