First Evidence of a Phase Transition in a High-Pressure Metal Iodate: Structural and Thermal Studies of AgIO sub(3) Polymorphs

A new phase of silver iodate, beta -AgIO sub(3), was obtained at 2.7 GPa by heating the alpha -AgIO sub(3) precursor above 240 degree C. beta -AgIO sub(3) 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 alpha -AgIO sub(3), which crystallizes in the Pbc2 sub(1) acentric orthorhombic space group with the lattice parameters a = 7.265(2) Aa, b = 15.17(4) Aa, c = 5.786(2) Aa, and V = 638(2) Aa super(3), beta -AgIO sub(3) crystallizes in the Pbca centric orthorhombic space group, with the following parameters: a = 6.137(2) Aa, b = 16.980(4) Aa, c = 5.827(1) Aa, and V = 607.3(3) Aa super(3). Differential thermal analysis under high pressure (2.7 GPa) and structural studies showed an endothermic transformation at 260 degree C corresponding to a first-order reconstructive phase transition from alpha -AgIO sub(3) to beta -AgIO sub(3). The thermal behaviors of alpha -AgIO sub(3) and beta -AgIO sub(3) were studied by differential scanning calorimetry at ambient pressure and in situ temperature-dependent X-ray powder diffraction. beta -AgIO sub(3) is the first iodate phase obtained under high pressure. The reconstructive phase transition from the alpha -AgIO sub(3) acentric phase to the dense beta -AgIO sub(3) centric phase is an endothermic transformation that occurs at 260 degree C at 2.7 GPa.