First Evidence of a Phase Transition in a High-Pressure Metal Iodate: Structural and Thermal Studies of AgIO3 Polymorphs



A new phase of silver iodate, β-AgIO3, was obtained at 2.7 GPa by heating the α-AgIO3 precursor above 240 °C. β-AgIO3 is the first iodate obtained at high pressure that stabilizes under normal conditions. The crystal structure was determined by X-ray powder diffraction and Rietveld refinement. In comparison to α-AgIO3, which crystallizes in the Pbc21 acentric orthorhombic space group with the lattice parameters a = 7.265(2) Å, b = 15.17(4) Å, c = 5.786(2) Å, and V = 638(2) Å3, β-AgIO3 crystallizes in the Pbca centric orthorhombic space group, with the following parameters: a = 6.137(2) Å, b = 16.980(4) Å, c = 5.827(1) Å, and V = 607.3(3) Å3. Differential thermal analysis under high pressure (2.7 GPa) and structural studies showed an endothermic transformation at 260 °C corresponding to a first-order reconstructive phase transition from α-AgIO3 to β-AgIO3. The thermal behaviors of α-AgIO3 and β-AgIO3 were studied by differential scanning calorimetry at ambient pressure and in situ temperature-dependent X-ray powder diffraction.