Pressure-Driven Symmetry-Preserving Phase Transitions 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 (4T1) 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 densityfunctional 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.