Atomic and magnetic structure of the amorphous Y3Fe5O12 obtained with fast neutron irradiation is investigated with neutron diffraction techniques. It is found that coordination polyhedra typical of the garnet structure remain in the amorphous state. The nearest interionic distances oxygen‐iron and iron‐iron are, in general, close to those in crystal. The local magnetic structure of the amorphous oxide is, in general, similar to spin ordering of the yttrium fcrrogarnet. The main difference consists in the following: in the amorphous state magnetic moments of the nearest neighbours are greatly shifted from antiparallel orientation. On the basis of the results and the known data it is shown that the differences in magnetic properties of the amorphous Y3Fe5O12 obtained by different methods are, to a great extent, caused by the properties of the local structure.

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Polycrystalline MgCr2O4 and CuCr2O4 samples are investigated by X‐ray and neutron diffraction methods before and after irradiation by fast neutrons (En ≧ 1 MeV) of fluence 2 × 1024 m−2. It is found that under irradiation in cubic magnesium and tetragonal copper chromites with the corresponding space groups O h7 – Fd3m and D 4h19 – I4/amd different types of cations are redistributed both, over “allowed” and “forbidden” sites of the initial spinel lattice. As a result, the lattice periods and crystal symmetries are changed. The structures of the irradiated chromites may be described in terms of the space group O h5 – Fm3m. Under annealing the irradiated samples restore their initial crystal structures. Structural changes occurring on different stages of annealing are analysed by X‐ray.

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