A family of phosphates of langbeinite structure. Crystal-chemical aspect of radioactive waste immobilization

Abstract

A systematic analysis of orthophosphates of framework (tetrahedral-octahedral) structure, belonging to langbeinite structural type (cubic cell, space group P213), is performed. The langbeinite structure is considered as favorable for environmentally safe crystalline forms of radioactive waste solidification. Experimental data on the compositions of known phosphates of such structure, on isomorphism of cations in various crystallographic positions, and on specific features of distribution of various cations in them are summarized. Possible formula compositions of isostructural phosphates are calculated. Some of them have already been synthesized (data given in this paper), whereas other phosphates are yet unknown. The structural-chemical aspect was also applied to langbeinite-like phosphates of f elements, some of which were prepared in accordance with the “crystal-chemical prediction.” Phosphates of the rhombohedral [NaZr2(PO4)3 type, NZP] and cubic (langbeinite type) tetrahedral-octahedral framework modifications, their cationic compositions, and structural features are compared. Properties of phosphates of langbeinite structure, useful in the development of new materials for radiochemical applications, and examples of possible use of crystal-chemical principles in solution of these problems are discussed.