Hydrothermal Synthesis and Crystal Chemistry of Novel Fluorides with A7B6F31 (A = Na, K, NH4, Tl; B = Ce, Th) Compositions

Abstract

Two new monovalent metal thorium fluorides and three new monovalent metal cerium fluorides have been synthesized hydrothermally and structurally characterized. The structures of the five compounds are described in space group $${\text{R}}\bar{3}$$ : Tl7Th6F31 with a = 15.598(2) and c = 10.893(2), (NH4)7Th6F31 with a = 15.5979(2) and c = 10.952(2), K7Ce6F31 with a = 15.013(2) and c = 10.295(2), (NH4)7Ce6F31 with a = 15.232(2) and c = 10.749(2), and Na7Ce6F31 with a = 14.567(2) and c = 9.6921(19). Comparison of these materials was made based on their structures and synthesis conditions. The formation of these species in hydrothermal fluids appears to be largely dependent upon the concentration of the monovalent fluoride mineralizer solution and thus, the ratio of monovalent ions to thorium or cerium in the system. The crystal chemistry of the A7B6F31 (A = Na+, K+, Rb+, Tl+, NH4 +; B = Zr4+, Ce4+, Th4+) structure type is described by examining the title compounds alongside other isomorphs to gain a broader understanding of the crystallization of complex fluorides. Structural subtleties are also discussed, including substitutional disorder of Na+ and Ce4+ cations in Na7Ce6F31. The hydrothermal synthesis of fluorides having compositions A7B6F31 (A = Na, K, NH4, Tl; B = Ce, Th) is described. Structural comparisons are made to discuss the crystal chemistry within this family of compounds in terms of the combinations of A and B cations supported by the structure type. The study introduces Tl+ and NH4 + as new A-site building blocks, and Ce4+ as a new B-site building block for this structure type.