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

The Ln­(IO<sub>3</sub>)<sub>3</sub>(HIO<sub>3</sub>)<sub><i>y</i></sub> (<i>y</i> = 1 or 1.33) compounds are isostructural with the La­(IO<sub>3</sub>)<sub>3</sub>(HIO<sub>3</sub>)<sub><i>y</i></sub> phases, but thermal studies reveal different behaviors. On the one hand, the partial thermal decompositions of these lanthanide compounds lead to the Ln­(IO<sub>3</sub>)<sub>3</sub> formulation, with a room temperature structure different from the β-La­(IO<sub>3</sub>)<sub>3</sub> obtained from La­(IO<sub>3</sub>)<sub>3</sub>(HIO<sub>3</sub>)<sub><i>y</i></sub>. On the other hand, the partial thermal decompositions of the La<sub>1–<i>x</i></sub>Ln<sub><i>x</i></sub>(IO<sub>3</sub>)<sub>3</sub>(HIO<sub>3</sub>)<sub><i>y</i></sub> compounds prepared with lanthanides ions (Ce, Pr, Nd, Sm, Eu, Gd, and Yb) lead to acentric β-La<sub>1–<i>x</i></sub>Ln<sub><i>x</i></sub>(IO<sub>3</sub>)<sub>3</sub>. As for β-La­(IO<sub>3</sub>)<sub>3</sub>, reversible structural transitions from β-La<sub>1–<i>x</i></sub>Ln<sub><i>x</i></sub>(IO<sub>3</sub>)<sub>3</sub> to centrosymmetric γ-La<sub>1–<i>x</i></sub>Ln<sub><i>x</i></sub>(IO<sub>3</sub>)<sub>3</sub> are observed. Differential scanning calorimetry analyses of La<sub>1–<i>x</i></sub>Ln<sub><i>x</i></sub>(IO<sub>3</sub>)<sub>3</sub> 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<sub>1–<i>x</i></sub>Nd<sub><i>x</i></sub>(IO<sub>3</sub>)<sub>3</sub> and β-La<sub>1–<i>x</i></sub>Yb<sub><i>x</i></sub>(IO<sub>3</sub>)<sub>3</sub> phases are investigated by luminescence spectroscopy.