Moessbauer investigations of Zr-, Th-, Ce- and Gd-containing ferrite-garnets

Full text: Among the most promising matrixes for disposal of actinide-containing highly radioactive waste are phases of the composition A3B2T3O12 with garnet structure. Large cations are submerged into A-position with dodecahedral oxygen surrounding of a garnet frame of common-apex TO4 tetrahedrons and BO6 octahedrons. From crystal chemistry point of view intrusion of trivalent and tetravalent actinide elements into A-position of garnet structure should be supported by occupation of the positions B and T with relatively large and low-valent cations. The largest trivalent cation that can simultaneously join both octahedron B and tetrahedron T positions is Fe3+. That is why namely ferrites demonstrate the largest frame sizes consisting of octahedrons FeO6 and tetrahedrons FeO4 and, consequently, possessing maximal dimensions of dodecahedral positions AO8. In order to study the structure and properties of synthetic ferrite-containing garnets there were performed Moessbauer investigations of synthesized samples of the following composition: 1C - Ca2.5Ce0.5Zr2Fe3O12, 2C - Ca1.5GdCe0.5ZrFeFe3012, 1T-Ca2.5Th0.5Zr2Fe3012 and 2T-Ca1.5GdTh0.5ZrFeFe3012. The measurements were taken at room temperature involving Moessbauer spectrometer MS1101E. Analysis and processing of the spectra was performed using model decoding and reconstruction of distribution functions for partial spectra hyperfine parameters employing MSTools software. Obtained Moessbauer spectra revealed differences in structure state of iron atoms in two investigated groups of samples: 1T, 1C and 2T, 2C. In the first sample group there were registered two tetrahedral positions Fe3+ populated in proportion 2:1. At that in the sample 1T were not observed iron in octahedral position and in the sample 1C its amount comprises ∼ 8 %. In the second sample group was registered the only tetrahedral position Fe3+ in quite non-uniform local surrounding as well as considerable amount of Fe atoms in octahedral surrounding. Splitting up of the tetrahedral positions is the evidence of high short range ordering as a consequence of established long range Ordering in the structure. Therefore, samples 1T and 1C may have different ion crystalline structure compared to those of the samples 2T and 2C. Actually, performed by us roentgenographic investigations using Rietveld method of all four samples showed that samples 2C and 2T are represented by cubic (Ia3d) form of garnet with cell parameters a=12.622(5) and 12.660(5)A, correspondingly, and samples 1C and 1T - by tetrahedral (I41/acd) form of garnet. Besides, in the samples 1C and 1T with the help of X-ray phase analysis was verified presence of small perovskite fractions of the following composition CaZrO3: 8% (1C) and 3% (1T). As a result of performed investigations there were revealed crystal-chemical formulas of the synthesized samples: 1C - (Ca1.50Ce0.50)A1(Ca0..90Ce0.10)A2Zr2.00(Fe1.93)T1(Fe1.00)T2012 and 1T - (Ca1.50Th0.50)A1(Ca0.90Th0.10)A'2Zr2.00(Fe1.93)T'1(Fe100)T2O12.