Murataite-Pyrochlore Ceramics as Complex Matrices for Radioactive Waste Immobilization: Structural and Microstructural Mechanisms of Crystallization

Murataite-pyrochlore titanate ceramics are attractive waste forms capable to immobilize radioactive waste streams of complex compositions, thus eliminating the need for further chemical separation. We have investigated structures of three types of murataite: 3C, -5C, and -8C phases and demonstrate their polysomatic nature and structural complexity. Structurally simple pyrochlore crystallizes first, followed by crystallization of murataite-5C containing pyrochlore cells surrounded by fragments of Keggin clusters. This phase is overgrown by murataite-8C containing both murataite and pyrochlore cells. The crystallization finishes with the formation of murataite-3C, which is the most stable and less actinide-rich. The microstructure formed via this mechanism reminds a Russian doll, which creates additional barrier for the actinide leaching from the pyrochlore core. The high chemical and structural complexity of the pyrochlore-murataite series is unparalleled in the world of crystalline materials proposed for the HLRW immobilization, which makes it unique and promising for further exploration.


Introduction
One of the most important tasks for the advanced nuclear cycle is the elaboration of waste forms capable to immobilize waste streams of complex compositions, thus eliminating the need for further chemical separation. In this regard, the murataite-pyrochlore titanate ceramics attract considerable attention due to their ability to immobilize radioactive wastes with different and complex chemical compositions, including actinides such as Pu-238. Over last few years, there has been a renewed interest in their synthesis and investigations (Maki et al. 2017;Lizin et al. 2018, etc.).
Murataite-(Y) is a complex titanate mineral first discovered in alkali pegmatites in St. Peters Dome area in Colorado, United States and later found in pegmatites in the Baikal region in Russia. Its crystal structure (cubic, space group F-43 m, a = 14.886 Å) was determined by Ercit and Hawthorne (1995) as based upon a framework of cornerlinked a-Keggin clusters hosting a complex metal-oxide substructure. The simplified formula of natural murataite-(Y) can be written as [8] TX 43 , where R = Y, HREE, Na, Ca, Mn, M1 = Ti, Nb, Na, M2 = Zn, Fe, Ti, Na, T = Zn, Si and X = O, F, OH. The interest in murataite-(Y) was renewed in 1982, when its synthetic analogue was identified in Synroc-type titanate ceramics with imitators of high-level radioactive waste at the Savannah River nuclear power plant (Morgan and Ryerson 1982). Laverov et al. (1998) reported the formation of murataite-type titanate phase in the uraniumbearing Synroc matrix from the Mayak factory, a radiochemical facility for the reprocessing of nuclear fuel located in Southern Ural, Russian Federation. It was found that five volume percent of synthetic murataite accumulate about 40% of the total amount of uranium present in the sample, which led to follow-up detailed studies of chemistry and properties of this material. Transmission electron studies allowed identification of synthetic varieties of murataite with 3 Â 3 Â 3, 5 Â 5 Â 5, 7 Â 7 Â 7 and 8 Â 8 Â 8 fluorite-like cubic supercells, referred in the following as murataite-3C, -5C, -7C and -8C phases (Laverov et al. 2011).

Methods and Approaches
We have studied crystal structures of murataite-3C, -5C and -8C using single-crystal X-ray diffraction analysis on the samples obtained by melting the mixture of oxides in an electric furnace at 1500°C with subsequent cooling to the room temperature as described by Laverov et al. (1998). The details of the experimental procedures used to obtain structure models have been described in detail in (Krivovichev et al. 2010;Pakhomova et al. 2013Pakhomova et al. , 2016. Urusov et al. (2005) proposed that synthetic murataites can be considered as members of murataite-pyrochlore polysomatic series consisting of different combinations of 2D modules. The structural determination of murataite-5C reported by Krivovichev et al. (2010) confirmed the assumption about the modular nature of the polysomatic series and demonstrated that the murataite-and pyrochlore-type modules are not layers but zero-dimensional blocks (nanoscale clusters), combination of which in a 3-dimensional space generates at least two different derivative structures, which combine structural features of both murataite and pyrochlore. In particular, the crystal structure of murataite-5C can be described as an ordered arrangement of pyrochlore unit cells immersed into the recombined murataite matrix, i.e. a substructure consisting of murataite structure elements.

Results and Discussion
The crystal structure of murataite-3C was reported by Pakhomova et al. (2013), who demonstrated its general identity to natural murataite, with some important chemical and structural modifications. The modular nature of the murataite-pyrochlore polysomatic series was discussed by Laverov et al. (2011).
The crystal structure of murataite-8C was reported by Pakhomova et al. (2016) as based upon a three-dimensional octahedral framework that can be described as an alternation of murataite and pyrochlore modules immersed into transitional substructure that combine elements of the crystal structures of murataite-3C and pyrochlore. The obtained structural model confirmed the polysomatic nature of the pyrochloremurataite series and illuminated the chemical and structural peculiarities of crystallization of the murataite-type titanate ceramic matrices.

Conclusions
The information-based complexity parameters for the members of the pyrochloremurataite are listed in Table 1. Both kinds of information-based complexity parameters (per atom and per cell) behave in a similar fashion: they are relatively small for the initial pyrochlore phase, then increase for murataite-5C, reach their maxima for murataite-8C and decrease for the final murataite-3C phase. This trend is also followed in the crystallization of murataite-pyrochlore ceramics: structurally simple and actiniderich pyrochlore crystallizes first, creating conditions for the saturation of melt with Keggin clusters, which triggers crystallization of murataite-5C containing pyrochlore unit cells surrounded by fragments of Keggin clusters. This pyrochlore-rich phase is overgrown by murataite-8C containing both murataite and pyrochlore unit cells. The crystallization finishes with the formation of pure Keggin phase murataite-3C, which is the most stable and less actinide-rich. The microstructure formed during such a crystallization reminds a Russian doll ('matryoshka'), which creates additional barrier for the actinide leaching from the pyrochlore (or crichtonite) core. The high chemical and structural complexity of the members of the pyrochlore-murataite series is unparalleled in the world of crystalline materials proposed for the high-level radioactive waste immobilization, which makes it unique and promising for further technological and scientific exploration.