Quaternary Selenides EuLnCuSe 3 : 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 Eu +2 Ln +3 Cu +1 Se 3 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 Pnma (structure type Ba 2 MnS 3 for EuLaCuSe 3 and structure type Eu 2 CuS 3 for EuLnCuSe 3 , where Ln = Sm, Gd, Tb, Dy, Ho and Y) and Cmcm (structure type KZrCuS 3 for EuLnCuSe 3 , where Ln = Tm, Yb and Lu). Space groups Pnma and Cmcm 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 3+ in the reported structures, the distortion of the (LnCuSe 3 ) layers decreases, and a gradual formation of the more symmetric structure occurs in the sequence Ba 2 MnS 3 → Eu 2 CuS 3 → KZrCuS 3 . According to magnetic studies, compounds EuLnCuSe 3 (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 3 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 3 , 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 d states of Eu 2+ in the crystal field of EuLnCuSe 3 , while anomalous narrowing of the band gap of EuYbCuSe 3 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.