Quaternary Selenides EuLnCuSe: Synthesis, Structures, Properties and In Silico Studies.

In this work, we report on the synthesis, in-depth crystal structure studies as well as optical and magnetic properties of newly synthesized heterometallic quaternary selenides of the EuLnCuSe composition. Crystal structures of the obtained compounds were refined by the derivative difference minimization (DDM) method from the powder X-ray diffraction data. The structures are found to belong to orthorhombic space groups (structure type BaMnS for EuLaCuSe and structure type EuCuS for EuLnCuSe, where Ln = Sm, Gd, Tb, Dy, Ho and Y) and (structure type KZrCuS for EuLnCuSe, where Ln = Tm, Yb and Lu). Space groups and were delimited based on the tolerance factor t', and vibrational spectroscopy additionally confirmed the formation of three structural types. With a decrease in the ionic radius of Ln in the reported structures, the distortion of the (LnCuSe) layers decreases, and a gradual formation of the more symmetric structure occurs in the sequence BaMnS → EuCuS → KZrCuS. According to magnetic studies, compounds EuLnCuSe (Ln = Tb, Dy, Ho and Tm) each exhibit ferrimagnetic properties with transition temperatures ranging from 4.7 to 6.3 K. A negative magnetization effect is observed for compound EuHoCuSe at temperatures below 4.8 K. The magnetic properties of the discussed selenides and isostructural sulfides were compared. The direct optical band gaps for EuLnCuSe, subtracted from the corresponding diffuse reflectance spectra, were found to be 1.87-2.09 eV. Deviation between experimental and calculated band gaps is ascribed to lower states of Eu in the crystal field of EuLnCuSe, while anomalous narrowing of the band gap of EuYbCuSe is explained by the low-lying charge-transfer state. Ab initio calculations of the crystal structures, elastic properties and phonon spectra of the reported compounds were performed.