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Semiconductor properties of Cu-based delafossites revealed by an electric field gradient study

In this paper we offer an interpretation of the previously observed trend of the electric field gradient (EFG) values measured in a group of semiconducting delafossites CuBO2 (B = Al, Fe, Cr, Nd) at Cd impurities which substitute either Cu or B atoms. Our theoretical study indicates that this EFG trend reveals one of the most subtle details in the electronic spectrum of the compounds, namely whether the impurity states are formed within or out of the band gap. When Cd substitutes the Cu, it exhibits a larger EFG value in CuAlO2 and CuFeO2 than in CuCrO2 and CuNdO2. This occurs because the Cd states form in the first two compounds a shallow band within the gap, but in the second two compounds they do not. When Cd substitutes the B atom it exhibits almost the same EFG in all delafossites. In this case, Cd states are not formed within the gap in any of the compounds. The same interpretation can be applied to the whole family of CuBO2 delafossites, whatever the B atom is. To arrive at these conclusions we analysed and calculated various systems (Cd-doped CuAlO2 and CuCrO2 compounds, and molecular clusters) using the full-potential linear augmented plane wave method.