High-pressure tuning of d–d crystal-field electronic transitions and electronic band gap in Co(IO3)2

High-pressure optical-absorption measurements performed on polycrystalline $\mathrm{Co}{(\mathrm{I}{\mathrm{O}}₃)}₂$ 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 $\mathrm{Co}{(\mathrm{I}{\mathrm{O}}₃)}₂$. The results shed light on the electron-lattice coupling and show that $\mathrm{Co}{(\mathrm{I}{\mathrm{O}}₃)}₂$ 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 $(⁴T₁)$ is very stable in $\mathrm{Co}{(\mathrm{I}{\mathrm{O}}₃)}₂$. We have also determined the band-gap energy of $\mathrm{Co}{(\mathrm{I}{\mathrm{O}}₃)}₂$ 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.