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Today the mixed oxides with a fluorite structure of the composition (Zr, Y) O2-x are widely used to harden ferritic steels used as fuel element cladding materials for the new generation of fast neutron reactors operating under irradiation and high temperatures. This work is devoted to study of the state and local structure of samples of zirconium oxides modified by yttrium and magnesium, prepared by coprecipitation and calcined at a temperature of 1250 °C. XAFS (XANES/EXAFS) spectra (Y-K, Zr-K edges) of studied samples were recorded at SSTRC, Novosibirsk. It was shown that from the yttrium side, the XANES spectra of both samples were almost identical. On the zirconium side, the XANES spectra have minor differences. It can be assumed that the charge state and the nearest environment of yttrium also practically does not change depending on the composition of the samples. For zirconium, with the charge state unchanged, some changes in the nearest oxygen environment appear to occur due to the composition of the samples. It was found out that the curves of the radial distribution of atoms (RDFs) obtained from the EXAFS spectra of the studied samples have a number of characteristic features. Only the first coordination spheres of the Me-O and Me-Me types are observed, but long-range coordination spheres (more than ~0.45 nm) are practically absent, which may indicate distortions of the long-range order in the structures. The local yttrium arrangement is stable when the composition of the samples changes, which may indicate the formation of clusters that include only Y and O elements. The local zirconium arrangement depends on the composition of the samples, since there are some differences in the RDFs curves. Thus, the amplitudes of the peaks assigned to the coordination spheres Zr-O and Zr-Me are significantly reduced (by more than 25%) for the Y,Mg- modified sample in comparison with those for the Y- modified sample, which indicates distortions of the original fluorite structure. The observed changes in the Zr-O and Zr-Me distances (more than 0.05) are greater than the changes due to differences in the cell parameters for the compared samples. Using the model of statistical solid solution with a fluorite structure, it is suggested that the presence of Y-O clusters prevents refinement of structures.
The work was carried out in the framework of the RFBR project no. № 19-03-00530. The work was done at the shared research center SSTRC on the basis of the Novosibirsk FEL/VEPP-4-VEPP-2000 complex at BINP SB RAS, using equipment supported by project RFMEFI62119X0022.
