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Abstract: Anhydrous yttrium iodate presents polymorphism; two crystalline phases are obtained under hydrothermal conditions. α-Y(IO3)3 crystallises in the monoclinic space-group P21/c with a three-dimensional structure, whereas β-Y(IO3)3 crystallises in the monoclinic space-group P21/n with a two-dimensional structure. The lattice parameters are a =7.038(1)Å, b =8.466(1)Å, c =13.317(1)Å, β =99.65(1)°, V =782.3(2)Å3, Z =4 for α-Y(IO3)3 and a =8.685(1)Å, b =5.964(1)Å, c =14.958(1)Å, β =96.99(2)°, V =769.0(2)Å3, Z =4 for β-Y(IO3)3. The α-form is isostructural with α-Dy(IO3)3 studied in this work and α-Ln(IO3)3 (Ln=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb, and Lu) already studied. The β-form is isostructural with β-Ln(IO3)3 (Ln=Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho and Er) studied in this work. The structural evolutions as a function of the Ln3+ cationic radius are studied. α-Y(IO3)3 and β-Y(IO3)3 present good thermal stability since they decompose at 600°C. They are transparent up to 11.5μm and they have good optical damage thresholds on powder. [Copyright &y& Elsevier]