Negative Thermal Expansion in the Polymorphic Modification of Double Sulfate β-AEu(SO₄)₂ (A-Rb⁺, Cs⁺)

New polymorphic modifications of double sulfates β-AEu(SO₄)₂ (A-Rb⁺, Cs⁺) were obtained by the hydrothermal method, the structure of which differs significantly from the monoclinic modifications obtained earlier by solid-state methods. According to single-crystal diffraction data, it was found that the compounds crystallize in the orthorhombic system, space group Pnna, with parameters β-RbEu(SO₄)₂: a = 9.4667(4) Å, b = 13.0786(5) Å, c = 5.3760(2) Å, V = 665.61(5) ų; β-CsEu(SO4)2: a = 9.5278(5) Å, b = 13.8385(7) Å, c = 5.3783(3) Å, V = 709.13(7) ų. The asymmetric part of the unit cell contains one-half Rb⁺/Cs⁺ ion, one-half Eu³⁺ ion, both in special sites, and one SO₄^2- ion. Both compounds exhibit nonlinear negative thermal expansion. According to the X-ray structural analysis and theoretical calculations, the polarizing effect of the alkali metal ion has a decisive influence on the demonstration of this phenomenon. Experimental indirect band gaps of β-Rb and β-Cs are 4.05 and 4.11 eV, respectively, while the direct band gaps are 4.48 and 4.54 eV, respectively. The best agreement with theoretical calculations is obtained using the ABINIT package employing PAW pseudopotentials with hybrid PBE0 functional, while norm-conserving pseudopotentials used in the frame of CASTEP code and LCAO approach in the Crystal package gave worse agreement. The properties of alkali ions also significantly affect the luminescent properties of the compounds, which leads to a strong temperature dependence of the intensity of the ⁵D₀ → ⁷F₄ transition in β-CsEu(SO₄)₂ in contrast to much weaker dependence of this kind in β-RbEu(SO₄)₂.