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 ß-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 ß-La1-xLnx(IO3)3. As for ß-La(IO3)3, reversible structural transitions from ß-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 ß-La1-xNdx(IO3)3 and ß-La1-xYbx(IO3)3 phases are investigated by luminescence spectroscopy.