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 Co(IO3)(2) samples were used to characterize the influence of pressure on the electronic d-d transitions associated with Co2+ and the fundamental band gap of Co(IO3)(2). The results shed light on the electron-lattice coupling and show that Co(IO3)(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 Co2+ have been explained based on changes in the constituent CoO6 octahedral units using the semiempirical Tanabe-Sugano diagram. Our findings support that the high-spin ground state (T-4(1)) is very stable in Co(IO3)(2). We have also determined the band-gap energy of Co(IO3)(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.