Isomorphism in d-and f-element phosphates having framework crystal structure and crystallochemical conception of NZP matrix for radionuclide immobilisation

Abstract

The problem of isomorphism in phosphates that are structural analogues of mineral kosnarite (structural type of sodium zirconium orthophosphate NaZr2(PO4)3. NZP), monazite, and others, are discussed. Data on the crystal chemistry of orthophosphates of the framework structure are systematised according to the suggested classification of their formula types. The theoretically expected solid solutions and the realised ones are viewed. Especially isomorphism of f-elements in various structures of phosphates is presented. These data form the basis on monophase ceramics that can be easily adapted to many kinds of nuclear waste compositions.

Influence of rare earth elements (REEs) on the structure and optical properties of lithium zirconium phosphate (LZP)

“…Previously, PM approach has been reported to be suitable for realisation of the both substitution models 10,14,16 , while SSR has Table 1. REE-substitution models in LZP structure 9,10,13,14,[16][17][18] Please do not adjust margins Please do not adjust margins corresponded to single-phase low crystalline LZP phases. Thus, it can be concluded that the formation of the La-substituted LZP phase takes place directly from PM precursors without formation of intermediate products like in the case of SSR.…”

“…Crystallographycally, it results from stabilization of the rhombohedral LZP phase of NASICON type with high ionic conductivity. However, the formation of pure rhombohedral Li 1+x Zr 2-x REE x (PO 4 ) 3 phases was reported to take place only in a very limited substitution range (in most studies it is limited to x ≤ 0.4) 9,10,[16][17][18] . Realisation of this type of solid solutions takes place in LiZr 2 (PO 4 ) 3 -Li(REE) 2 (PO 4 ) 3 system where REE ions share crystallographic position with Zr 4+ described by the substitution model 'Zr/La': Zr 4+ → La 3+ + Li + (Table 1).…”

“…Please do not adjust margins Please do not adjust margins LiZr 2 (PO 4 ) 3 in such substitution model is Li(REE) 2 (PO 4 ) 3 , which adopts a totally unrelated structure type of monoclinic monazite, that excludes the possibility of the formation of the continuous solid solutions between LiZr 2 (PO 4 ) 3 and A(REE) 2 (PO 4 ) 3 18 . In this view, coexistence of two modifications of REE-substituted LZP ceramics in the low substitution range is an open scientific question that requires additional examination and proves.…”

Crystal chemistry of sodium zirconium phosphate based simulated ceramic waste forms of effluent cations (Ba2+, Sn4+, Fe3+, Cr3+, Ni2+ and Si4+) from light water reactor fuel reprocessing plants

“…Hawkins and Scheetz [11], Seida et al [12] and Troole et al [13] have reported that the immobilization of trivalent elements into Zr position was 0304-3894/$ -see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2007.08.057 questionable but in the light of various publications [8,[14][15][16][17][18][19][20][21][22][23][24][25] on immobilization of trivalent cations in NZP matrix the final word on the mechanism of solids state reactivity of trivalent elements requires more experimental evidence and better theoretical model. The NZP compounds are receiving attention for their potential applications in anti thermal shock devices, space technology, automobile industry, etc.…”

Sodium zirconium phosphate‐based glass‐ceramics as potential wasteforms for the immobilization of nuclear wastes

“…[45][46][47] The NZP structure has also been of particular interest in the area of nuclear waste immobilization, not only due to its low thermal expansion, but also its ability to accommodate a wide range of elements via cationic substitution within the crystal structure, especially fission products and actinides. 48,49 With a rhombohedral crystalline lattice (space group 𝑅 3𝑐), NZP consists of a skeletal structure of ZrO 6 octahedra joined to PO 4 3− tetrahedra by common vertices, forming a series of chains parallel to the c axis, which also join perpendicularly along the a axis via the PO 4 3− tetrahedra, giving rise to a porous three-dimensional network (Figure 1). 50 The practical significance of this network is the existence of two kinds of interstitial cavities (M1 and M2), with Na ions occupying the former and providing charge balancing.…”