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

A phase of silver iodate, $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{AgIO}}_{3}$, has been obtained at ambient temperature by compressing $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{AgIO}}_{3}$ to 1.60(5) GPa. The $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{AgIO}}_{3}$ crystal structure was identified via Rietveld refinement of high-pressure powder synchrotron x-ray diffraction. The $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{AgIO}}_{3}$ reflections were indexed to an orthorhombic lattice (Pbca) with unit-cell dimensions of $a\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}7.2945(12), b\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}15.0013(24), c\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}5.3904(9)\phantom{\rule{0.16em}{0ex}}\AA{}$, and $V\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}589.85(29)\phantom{\rule{0.16em}{0ex}}{\AA{}}^{3}$ at 2.20(5) GPa. Density-functional theory calculations predict that $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{AgIO}}_{3}$ is more stable than $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{AgIO}}_{3}$ above 0.15 GPa. The $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\gamma}$-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 \ensuremath{\alpha} and \ensuremath{\gamma} phases, however the $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\gamma}$-phase transition (indirect $\ensuremath{\rightarrow}$ indirect) is characterized by an abrupt band-gap energy increase of approximately $+0.18\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. This pressure induced band-gap evolution is rationalized based on the I-O bond lengths. The \ensuremath{\gamma} phase may correspond to an intermediate step between the previously known \ensuremath{\alpha} and \ensuremath{\beta} phases.