High-pressure evolution of silver iodate (AgIO3) and the gamma-AgIO3 phase

A phase of silver iodate, gamma-AgIO3, has been obtained at ambient temperature by compressing alpha-AgIO3 to 1.60(5) GPa. The gamma-AgIO3 crystal structure was identified via Rietveld refinement of high-pressure powder synchrotron x-ray diffraction. The gamma-AgIO3 reflections were indexed to an orthorhombic lattice (Pbca) with unit-cell dimensions of a = 7.2945(12), b = 15.0013(24), c = 5.3904(9) angstrom, and V = 589.85(29) angstrom(3) at 2.20(5) GPa. Density-functional theory calculations predict that gamma-AgIO3 is more stable than alpha-AgIO3 above 0.15 GPa. The alpha -> gamma -phase transition is characterized by a decrease in the volume per formula unit of approximately 2% and it is reversible on decompression. Single-crystal optical-absorption measurements and density-functional theory calculations reveal the electronic band gap to decrease monotonically with increasing pressure in both alpha and gamma phases, however the alpha -> gamma-phase transition (indirect -> indirect) is characterized by an abrupt band-gap energy increase of approximately +0.18 eV. This pressure induced band-gap evolution is rationalized based on the I-O bond lengths. The. phase may correspond to an intermediate step between the previously known alpha and beta phases.