LiRE(SO4)2 (RE = Y, Gd, Eu): noncentrosymmetric chiral rare-earth sulfates with very large band gaps

Abstract

Three new ternary lithium rare earth metal sulfates, LiRE(SO₄)₂ (RE = Y, Gd, and Eu), have been synthesized via high temperature solid-state and solvothermal reactions. The reported materials crystallizing in the noncentrosymmetric (NCS) nonpolar tetragonal space group, Pn2 (no. 118) exhibit three-dimensional structures consisting of REO₈, LiO₄, and SO₄ groups. Ultraviolet-visible diffuse reflectance spectra and density functional theory calculations indicate that the title materials exhibit very large band gaps of ca. 6.7 eV. The frameworks of the sulfates reveal high thermal stabilities of up to 873–1073 K depending on the constituting rare earth cations. Powder second-harmonic generation (SHG) measurements reveal that the sulfates with NCS structures have mild SHG responses of ca. 13 × α-SiO₂ and are non-phase matchable. Photoluminescence experiments on LiEu(SO₄)₂ revealing a bright orange emission show multiple peaks originating from f–f transitions. Detailed synthetic procedures, crystal structures, and full characterization studies of the new sulfates are presented.