Abstract
A wide band gap is one of the essential requirements for metal iodates to be used as nonlinear optical materials. Usually, the band gap of these materials decreases under the application of pressure. Herein, we introduce a case in which the band-gap energy of a hydrated metal iodate, namely $\mathrm{Ca}{({\mathrm{IO}}_{3})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}_{2}\mathrm{O}$, has been successfully increased, from 4.52 to 4.92 eV, by applying external pressure without showing signs of saturation upon increasing pressure. The pressure-induced nonlinear band-gap opening correlates with the pressure-induced shortening of the I-O bond distances, as obtained from x-ray diffraction measurements. In addition, two pressure-induced isostructural phase transitions are observed in the pressure regions of 6.6--8.0 and 13.0--15.5 GPa. These two isostructural phase transitions cause a nonlinear pressure-induced evolution of the band-gap energy and crystal lattice parameters, as well as the occurrence of several extra peaks and peak splitting in Raman spectra.
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