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

New polymorphic modifications ofdouble sulfates & beta;-AEu(SO4)(2) (A-Rb+, Cs+) wereobtained by the hydrothermal method, the structure of which differssignificantly from the monoclinic modifications obtained earlier bysolid-state methods. According to single-crystal diffraction data,it was found that the compounds crystallize in the orthorhombic system,space group Pnna, with parameters & beta;-RbEu(SO4)(2): a = 9.4667(4) & ANGS;, b = 13.0786(5) & ANGS;, c = 5.3760(2) & ANGS;, V = 665.61(5) & ANGS;(3); & beta;-CsEu(SO4)2: a = 9.5278(5) & ANGS;, b = 13.8385(7) & ANGS;, c = 5.3783(3) & ANGS;, V = 709.13(7) & ANGS;(3). The asymmetric part of the unit cell contains one-half Rb+/Cs+ ion, one-half Eu3+ ion, both inspecial sites, and one SO4 (2-) ion. Bothcompounds exhibit nonlinear negative thermal expansion. Accordingto the X-ray structural analysis and theoretical calculations, thepolarizing effect of the alkali metal ion has a decisive influenceon the demonstration of this phenomenon. Experimental indirect bandgaps of & beta;-Rb and & beta;-Cs are 4.05 and 4.11 eV, respectively,while the direct band gaps are 4.48 and 4.54 eV, respectively. Thebest agreement with theoretical calculations is obtained using theABINIT package employing PAW pseudopotentials with hybrid PBE0 functional,while norm-conserving pseudopotentials used in the frame of CASTEPcode and LCAO approach in the Crystal package gave worse agreement.The properties of alkali ions also significantly affect the luminescentproperties of the compounds, which leads to a strong temperature dependenceof the intensity of the D-5(0) & RARR; F-7(4) transition in & beta;-CsEu(SO4)(2) in contrast to much weaker dependence of this kind in & beta;-RbEu(SO4)(2). Newpolymorphic modifications of double europium sulfateswith alkali metals were obtained by the method of hydrothemal crystallization.The crystal structure of the compounds was established and the coordinationenvironment of its main elements was analyzed in detail.