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, -AgIO3, was obtained at 2.7 GPa by heating the -AgIO3 precursor above 240 degrees C. -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 -AgIO3, which crystallizes in the Pbc21 acentric orthorhombic space group with the lattice parameters a = 7.265(2) angstrom, b = 15.17(4) angstrom, c = 5.786(2) angstrom, and V = 638(2) angstrom 3, -AgIO3 crystallizes in the Pbca centric orthorhombic space group, with the following parameters: a = 6.137(2) angstrom, b = 16.980(4) angstrom, c = 5.827(1) angstrom, and V = 607.3(3) angstrom 3. Differential thermal analysis under high pressure (2.7 GPa) and structural studies showed an endothermic transformation at 260 degrees C corresponding to a first-order reconstructive phase transition from -AgIO3 to -AgIO3. The thermal behaviors of -AgIO3 and -AgIO3 were studied by differential scanning calorimetry at ambient pressure and in situ temperature-dependent X-ray powder diffraction.