Hydrothermal Syntheses and Crystal Structure of NH_4Ln_3F_(10) (Ln = Dy, Ho, Y, Er, Tm)

摘要

Ammonium rare earth fluorides NH_4Ln_3F_(10) (Ln = Dy, Ho, Y, Er, Tm) were synthesized by a hydrothermal method. Two polymorphs, of the hexagonal #beta#-KYb_3F_(10) and the cubic #gamma#-KYb_3F_(10) structure types, were formed under hydrothermal conditions for most of the rare earth fluorides except NH_4Dy_3F_(10), for which only the cubic #gamma#-phase was obtained. The crystal structures of MLn_3F_(10) (M = alkaline metal, NH_4~+ and Ln = rare earth) show a strong correlation to the ratio of ionic radii (R_M/R_(Ln)), which has been expressed in a structure phase diagram of the ionic radii of univalent and rare earth cations.