A theoretical model of the formation and behavior of holographic gratings in photorefractive crystals, based on the assumption of two types of active centers being involved and simultaneous electron-hole transport, is derived. A diffusion mechanism of recording is discussed. It is demonstrated that a photorefractive grating can change its phase by 180° during recording and erasure. At large angles between the recording beams, after a long recording or fixing the grating in darkness at room temperature, the grating decay is much slower than in the case when it is formed by diffusion and trapping of one type of carrier.

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