Structural relationships,crystal chemistry and anion substitution processes for M(III)X3 systems of the lanthanides and actinides

An examination of data for lanthanide and actinide phases with UCl₃-type and PuBr₃-type M(III)X₃ structures has shown that these systems are conveniently described by alternating layers of MX₂]ⁿ⁺_n and X]^n?_n. The relationships between the UCl₃- and PuBr₃-type structures are described and expanded to include a variety of anion substitution systems, M(III)X_3?xY_x. The two different types of MX₂]ⁿ⁺_n layers observed in these systems are consistent with the existence of a novel structural unit, M₂X₄]²⁺. The effects of radius ratio constraints and layering mechanisms on the phase equilibria and anionic substitution processes, polymorphism and crystal growth in the MX_3?xY_x systems are discussed.