Structures, Thermal Behaviors, and Luminescent Properties of Anhydrous Lanthanum Iodate Polymorphs

The
structural and thermal studies of six anhydrous lanthanum iodate polymorphs
are presented. The variation of the [IO3–]:[La3+] molar ratio in the starting solution and the
evaporation rate of the solution leads to either the centric La­(IO3)3(HIO3) or the acentric La­(IO3)3(HIO3)1.33 phases. The crystal
structure of La­(IO3)3(HIO3)1.33 was determined. The thermal treatments of these two phases up to
490 °C lead to β-La­(IO3)3, observed
at room temperature. To better understand the similar thermal behaviors
of La­(IO3)3(HIO3)1.33 and
La­(IO3)3(HIO3) compounds and their
structural evolution, thermogravimetry–differential thermal
analysis (TG-DTA), differential scanning calorimetry (DSC) analyses
and in situ temperature-dependent powder X-ray diffraction
(XRD) experiments were carried out. These experiments allowed us to
highlight the successive formation of δ-La­(IO3)3 and γ-La­(IO3)3. δ-La­(IO3)3 is observed from the beginning of thermal decomposition
of La­(IO3)3(HIO3)1.33 (at
340 °C) or La­(IO3)3(HIO3) (at
300 °C) up to 440 °C. A phase transition from δ-La­(IO3)3 to γ-La­(IO3)3 then
occurs at 440 °C. Finally, the phase transition from γ-La­(IO3)3 to β-La­(IO3)3 occurs
at 140 °C. A cycle of heating and cooling shows the reversible
phase transition at 185 and 140 °C, respectively. β-, γ-,
and δ-La­(IO3)3 are three polymorph phases
of the first α-La­(IO3)3 already characterized.
The structure of β-La­(IO3)3 and γ-La­(IO3)3 were determined on powder XRD analyses. The
iodate compounds present a very broad domain of transparency from
the visible range to the beginning of the far-infrared range. The
intensities of SHG light generated by α-La­(IO3)3, β-La­(IO3)3, La­(IO3)3(HIO3)1.33, and α-LiIO3 compounds with acentric structures were compared: β-La­(IO3)3 < La­(IO3)3(HIO3)1.33 < α-La­(IO3)3 ≈ α-LiIO3. Finally, the luminescence spectroscopy
of La­(IO3)3(HIO3)1.33:Nd3+, α-La­(IO3)3:Nd3+,
and α-La­(IO3)3:Yb3+ is studied.