Quaternary Selenides EuLnCuSe3: 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+2Ln+3Cu+1Se3 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 Ba2MnS3 for EuLaCuSe3 and structure type Eu2CuS3 for EuLnCuSe3, where Ln = Sm, Gd, Tb, Dy, Ho and Y) and Cmcm (structure type KZrCuS3 for EuLnCuSe3, 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 Ln3+ in the reported structures, the distortion of the (LnCuSe3) layers decreases, and a gradual formation of the more symmetric structure occurs in the sequence Ba2MnS3 → Eu2CuS3 → KZrCuS3. According to magnetic studies, compounds EuLnCuSe3 (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 EuHoCuSe3 at temperatures below 4.8 K. The magnetic properties of the discussed selenides and isostructural sulfides were compared. The direct optical band gaps for EuLnCuSe3, 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 Eu2+ in the crystal field of EuLnCuSe3, while anomalous narrowing of the band gap of EuYbCuSe3 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.

Structure, Thermal Stability, and Spectroscopic Properties of Triclinic Double Sulfate AgEu(SO4)2 with Isolated SO4 Groups.

Silver-europium double sulfate AgEu(SO4)2 was obtained by solid-phase reaction between Ag2SO4 and Eu2(SO4)3. The crystal structure of AgEu(SO4)2 was determined by Monte Carlo method with simulated annealing, and after that, it was refined by the Rietveld method from X-ray powder diffraction data. The compound crystallizes in the triclinic symmetry, space group P1̅ ( a = 0.632929(4), b = 0.690705(4), c = 0.705467(4) nm, α = 98.9614(4), ß = 84.5501(4), γ = 88.8201(4)°, V = 0.303069(3) nm3). Two types of sulfate tetrahedra were found in the structure, which significantly affects the spectroscopic properties in the IR-range. In the temperature range of 143-703 K, the average linear thermal expansion coefficients of cell parameters a, b, and c are very similar, (1.11-1.67) × 10-5 K-1 in magnitude, and therefore, AgEu(SO4)2 expands almost isotropically. Upon heating in argon flow, AgEu(SO4)2 is stable up to 1053 K. The luminescence spectra in the region of ultranarrow 5D0-7F0 transition contain a single narrow and symmetric line at 579.5 nm that is evidence of good crystalline quality of AgEu(SO4)2 and uniform local environment of Eu3+ ions in the structure. Distribution of luminescence bands is determined by the environment of Eu3+ ions in the structure. Influence of Ag+ ions on the electron density distribution at Eu sites is detected.