Large low-energy oscillator strength for Eu 4f electrons of a rare-earth Zintl compound: EuIn₂P₂

A newly found Zintl compound EuIn₂P₂ has been investigated within the density-functional theory, and results of a detailed investigation of its electronic structure and related properties are reported. We employed the so-called LSDA+U method (LSDA: local-spin-density approximation) with varying U from 0 eV to 7 eV to take into account the strong on-site Coulomb repulsion between the Eu 4f-electrons and to reproduce the semiconducting energy gap of 3.2 meV experimentally observed by J. Jiang and S. M. Kauzlarich [Chem. Mater. 18, 435 (2006)]. The calculations, however, could not reproduce the energy gap even for the largest U value of 7.0 eV because the experimental gap was not an optical gap, but it was deduced from the temperature dependence of resistivity at 26-60 K; thus, the experimental gap was not a real 'semiconducting' gap. The calculated optical-conductivity spectrum with U = 7.0 eV could not reproduce the experimental spectrum, while with simple LSDA, the calculated spectrum resembled the experimental one rather successfully. Therefore, it seems that the LSDA+U method is not suitable to correctly simulate the optical properties of this compound. Contrary to the expectation, we found that EuIn₂P₂ had a large oscillator strengths for f→d transitions in the low-energy range (below 1.5 eV); i.e., the peaks located at ~0.6 eV and ~1.1 eV in the experimental spectrum are mainly due to transitions from f-bands to d-bands, in which joint density-of-states effects play a key role. We also found that the magneto-optical Kerr effect was quite large in the EuIn₂P₂ compound.