Examination of Synthetic Murataite Structure Using Data of Mossbauer Spectroscopy
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([DPLQDWLRQRI6\QWKHWLF0XUDWDLWH6WUXFWXUH8VLQJ'DWDRI0|VVEDXHU6SHFWURVFRS\ Vadim S. Urusov1, Vyacheslav S. Rusakov1, Sergey V. Yudintsev2, Sergey V. Stefanovsky3 1 Moscow State University, Vorob’evy Gory, 119992 Moscow, Russia 2 Institute of Geology of Ore Deposits, Staromonetny 35, 119017 Moscow, Russia 3 SIA “RADON”, 7-th Rostovsky 2/14, 119121 Moscow, Russia $%675$&7 Murataite-based ceramics were recently suggested as promising matrices for immobilization of rare earths and actinides from high-level waste (HLW). Nevertheless, the crystal-chemical formula of the phase has not been accurately determined yet. We have examined structural features of murataite with Mössbauer spectroscopy. Initial batches were prepared from CaO, Al2O3, Fe2O3, MnO2, ZrO2, TiO2, ZrO2, and UO2. Mixtures were melted in platinum crucibles in a resistive furnace in air at 1450°C for 1 h followed by cooling to 1000°C at the rate of 10°C/min. and final cooling down with the furnace switched-off. Study with XRD, SEM, and TEM showed the samples are composed of two co-existing murataite-type phases with five- and eight-fold elementary fluorite cells. To investigate the valence and structural position of iron ions, Mössbauer spectroscopy on 57Fe nuclei in geometry “on absorption” was used. The data obtained let us conclude that in both murataite varieties trivalent iron is distributed almost statistically (3:1) between % octahedra and & five-vertex polyhedra, while tetrahedral sites 7 are probably not populated. Taking into account these suggestions, the idealized crystal chemical formula of synthetic murataites can be simplified to 3[8] 6[6] 2[5] 22-[2. Recalculation of chemical analyses to atomic numbers results in the conclusion that average cation valence for murataites is 3.33. Then, calculation for the generalized formula 11 22-[/2 (0= $%&) gives a value [/2 = 3.7, i.e., the number of O atoms in the formula is 22-3.7 = 18.3. Final conclusions on the murataite formulae have to be verified by direct structural research on a single crystal. ,1752'8&7,21 Recently a murataite-type phase - a complex oxide of rare earths, titanium, iron and some other elements - was proposed as a potential matrix for radioactive waste. Synthetic analogues of murataite were found in ceramics produced by solid phase synthesis at T = 1000-1200°C and P = 14-60 MPa in the USA [1] and by melting-crystallization in Russia [2]. The crystal structure of natural murataite with idealized formula 6 12 4 40- ( = Y, Na; = Ti; & = Fe; 7 = Zn; ; = O, F) has been solved in ref. [3] as cubic with space group )43P, D = 14.89 Å, = = 4. The structure contains four different cationic sites: the [8] – site is an eight-coordinated distorted cube, the [4] – site is tetrahedrally coordinated, the %>@– site is octahedrally coordinated, and the [5] – site is a triangular bipyramid. Three [6] octahedra share edges to form a compact %3;13 group. Four of these %3;13 groups link by sharing corners to form a tetrahedral cage in the center of which is the 7 site. The resulting
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