High-pressure tuning of d-d crystal-field electronic transitions and electronic band gap in Co(I O3)2

Abstract

High-pressure optical-absorption measurements performed on polycrystalline Co ( I O 3 ) 2 samples were used to characterize the influence of pressure on the electronic d – d transitions associated with Co 2 + and the fundamental band gap of Co ( I O 3 ) 2 . The results shed light on the electron-lattice coupling and show that Co ( I 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 Co 2 + have been explained based on changes in the constituent Co O 6 octahedral units using the semiempirical Tanabe-Sugano diagram. Our findings support that the high-spin ground state ( 4 T 1 ) is very stable in Co ( I O 3 ) 2 . We have also determined the band-gap energy of Co ( I 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.