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

Abstract

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), β = 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.