Differences and Similarities between Lanthanum and Rare-Earth Iodate Anhydrous Polymorphs: Structures, Thermal Behaviors, and Luminescent Properties

The Ln­(IO₃)₃(HIO₃)_y (y = 1 or 1.33) compounds are isostructural with the La­(IO₃)₃(HIO₃)_y phases, but thermal studies reveal different behaviors. On the one hand, the partial thermal decompositions of these lanthanide compounds lead to the Ln­(IO₃)₃ formulation, with a room temperature structure different from the β-La­(IO₃)₃ obtained from La­(IO₃)₃(HIO₃)_y. On the other hand, the partial thermal decompositions of the La_1–xLn_x(IO₃)₃(HIO₃)_y compounds prepared with lanthanides ions (Ce, Pr, Nd, Sm, Eu, Gd, and Yb) lead to acentric β-La_1–xLn_x(IO₃)₃. As for β-La­(IO₃)₃, reversible structural transitions from β-La_1–xLn_x(IO₃)₃ to centrosymmetric γ-La_1–xLn_x(IO₃)₃ are observed. Differential scanning calorimetry analyses of La_1–xLn_x(IO₃)₃ solid solutions show that the transition temperatures vary with the lanthanide concentration in the solid solution. A transition is observed only up to a certain fraction of lanthanide-ion substitution; this substitution limit decreases with the cationic radius of the lanthanide ion. Finally, the β-La_1–xNd_x(IO₃)₃ and β-La_1–xYb_x(IO₃)₃ phases are investigated by luminescence spectroscopy.