MOSSBAUER DIFFRACTION IN CRYSTALS WITH INCOMMENSURATE MODULATION. E.N.Ovchinnikova, Moscow State University, Physical Department, Moscow 119899, Russia

A set of possible reflections was considered for a case of Mossbauer resonant diffraction in incommensurately modulated crystals with quadrupole interaction on nuclei. A structure, formed by electrical field gradients (EFG) on Mossbauer nuclei supposed to be modulated too. We believe, that atomic displacements in a modulation wave are enough small and they cause mainly the rotation of EFG main axes. Our propositions are based on the results of the published earlier NMR studies of EFG's in incommensurate ionic crystals.In this case a quasicontinuous distribution of quadrupole splittings exists, and EFG tensors on nuclei can be considered as a sum of EFG's corresponding to an average structure and additional modulation tensors. These approximations were used to study the features of Mossbauer diffraction pattern. A simple harmonic law of modulation was used. It was shown, that the diffraction pattern consists of main Bragg reflections and satellites. It differs from x-ray diffraction pattern in those cases, when purely nuclear Bragg reflections occur. Satellites of purely nuclear reflections are also purely nuclear. Their energy spectra are broaden dublets. Energy forms of all other reflections and their satellites are determined by interference between potential and resonant scattering. The satellite reflections energy spectra contains an information about additional modulation tensors. We have considered incommensurate phases of some crystals containing Mossbauer isotopes, in which the discussed phenomena may be observed.