High-pressure optical-absorption measurements performed on polycrystalline $\mathrm{Co}{\left(\mathrm{I}{\mathrm{O}}_3\right)}_2$ samples were used to characterize the influence of pressure on the electronic $d–d$ transitions associated with ${\mathrm{Co}}^{2+}$ and the fundamental band gap of $\mathrm{Co}{\left(\mathrm{I}{\mathrm{O}}_3\right)}_2$. The results shed light on the electron-lattice coupling and show that $\mathrm{Co}{\left(\mathrm{I}{\mathrm{O}}_3\right)}_2$ exhibits an unusual behavior because the compression of Co–O bond distances is not coupled to pressure-induced changes induced in the unit-cell volume. Experimental results on the internal $d–d$ transitions of ${\mathrm{Co}}^{2+}$ have been explained based on changes in the constituent $\mathrm{Co}{\mathrm{O}}_6$ octahedral units using the semiempirical Tanabe-Sugano diagram. Our findings support that the high-spin ground state $\left({}^{4}T_1\right)$ is very stable in $\mathrm{Co}{\left(\mathrm{I}{\mathrm{O}}_3\right)}_2$. We have also determined the band-gap energy of $\mathrm{Co}{\left(\mathrm{I}{\mathrm{O}}_3\right)}_2$ and its pressure dependence which is highly nonlinear. According to density-functional theory band-structure calculations, this nonlinearity occurs because the bottom of the conduction band is dominated by I-5p orbitals and the top of the valence band by Co-3d and O-2p orbitals, and because the Co–O and I–O bond lengths exhibit different pressure dependences.

• Received 21 December 2021
• Revised 11 February 2022
• Accepted 10 March 2022

DOI:https://doi.org/10.1103/PhysRevB.105.115204