Elucidating elusive quaternary selenide EuCeCuSe3: Synthesis, crystal structure, properties and theoretical studies

https://doi.org/10.1016/j.jre.2022.11.004

We report on the novel heterometallic quaternary selenide EuCeCuSe₃, the fabrication of which has been a challenge until this work. The structure of the reported selenide was elucidated from the powder X-ray diffraction data, which revealed the formation of EuCeCuSe₃ with excellent yield (96.7%) accompanied with a minor fraction of CeSe₂ (3.3%), and was best solved in orthorhombic space group Pnma with the BaLaCuS₃ structural type. Thus, the crystal structure of the title compound completes the row of the heterometallic quaternary selenides EuRECuSe₃ (RE = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y), of which the cerium-based derivative exclusively belongs to the BaLaCuS₃ structural type. The distortion of the CuSe₄ polyhedron was compared for the whole series of EuRECuSe₃ compounds using the τ₄-descriptor for four coordinated ions, which revealed the highest degree of distortion for the Ce³⁺-containing selenide, followed by the La³⁺-based derivative. Furthermore, the crystallographic and geometrical parameters of the reported selenide were discussed in comparison to the Ce³⁺-based sulfides SrCeCuS₃ and EuCeCuS₃. Ab initio calculations of the crystal structure, a phonon spectrum and elastic constants for the crystal of EuСeCuSe₃ were also performed. The types and wavenumbers of fundamental modes were determined and the involvement of ions participating in the phonon modes was assessed. The experimental IR spectrum of the reported selenide was interpreted and found to be in agreement with the calculated spectrum. The experimental direct band gap of EuCeCuSe₃ was measured to be 1.36 eV that is consistent with the concept of its origin due to interband transitions between orbitals emerging mainly from 4f (valence band) and 5d (conduction band) levels of the Eu²⁺ cation. The dependence of the Young's modulus on the direction demonstrates the anisotropy of the elastic properties, while the Vickers hardness for EuCeCuSe₃ was calculated to be 5.2 GPa. Finally, the title compound is paramagnetic above 4 K.