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 La1–xLnx(IO3)3(HIO3)y compounds prepared with lanthanides ions (Ce, Pr, Nd, Sm, Eu, Gd, and Yb) lead to acentric β-La1–xLnx(IO3)3. As for β-La­(IO3)3, reversible structural transitions from β-La1–xLnx(IO3)3 to centrosymmetric γ-La1–xLnx(IO3)3 are observed. Differential scanning calorimetry analyses of La1–xLnx(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-La1–xNdx(IO3)3 and β-La1–xYbx(IO3)3 phases are investigated by luminescence spectroscopy.