A phase of silver iodate, $\gamma \text{−}{\mathrm{AgIO}}_3$, has been obtained at ambient temperature by compressing $\alpha \text{−}{\mathrm{AgIO}}_3$ to 1.60(5) GPa. The $\gamma \text{−}{\mathrm{AgIO}}_3$ crystal structure was identified via Rietveld refinement of high-pressure powder synchrotron x-ray diffraction. The $\gamma \text{−}{\mathrm{AgIO}}_3$ reflections were indexed to an orthorhombic lattice (Pbca) with unit-cell dimensions of $a=7.2945\left(12\right),b=15.0013\left(24\right),c=5.3904\left(9\right)\AA$, and $V=589.85\left(29\right){\AA }^3$ at 2.20(5) GPa. Density-functional theory calculations predict that $\gamma \text{−}{\mathrm{AgIO}}_3$ is more stable than $\alpha \text{−}{\mathrm{AgIO}}_3$ above 0.15 GPa. The $\alpha \to \gamma$-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 α and γ phases, however the $\alpha \to \gamma$-phase transition (indirect $\to$ indirect) is characterized by an abrupt band-gap energy increase of approximately $+0.18\mathrm{eV}$. This pressure induced band-gap evolution is rationalized based on the I-O bond lengths. The γ phase may correspond to an intermediate step between the previously known α and β phases.

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