Na2.4CeSi6O15 · 2.6H2O: Hydrothermal synthesis, characterization and properties of a new luminescent microporous cerium silicate

A new microporous sodium cerium silicate Na2.4CeSi6O15 · 2.6H2O containing mixed valence of CeIII/CeIV (denoted CeSiO-CJ2) was successfully synthesized under mild hydrothermal conditions. Its structure was determined by single-crystal X-ray diffraction (XRD) analysis, and characterized by SEM, powder XRD, ICP, TGA, and XPS analyses. CeSiO-CJ2 crystallizes in orthorhombic space group Cmm2 (No. 35) with a = 7.4165(15) Å, b = 30.966(6) Å, and c = 7.1539(14) Å, and Z = 4. It consists of flexuous [Si2O5]n2n- silicate layers that are connected by seven-coordinated Ce polyhedra to form a three dimensional (3D) open framework. There exist two different types of irregular 8-ring channels along [1 0 0] and 6-ring channels along [0 0 1] in which Na+ ions and water molecules reside. Magnetic measurement shows that CeSiO-CJ2 exhibits a Curie–Weiss behavior over the temperature range of 2–300 K with a Weiss constant θ of −5.34(1) K, and a Curie constant C of 0.12(7) K. The data of susceptibility show that there exists at least 31.2% trivalent Ce in CeSiO-CJ2 which is consistent with the XPS analysis. CeSiO-CJ2 exhibits a strong broad emission band containing two overlapped peaks centered at about 384 and 420 nm, respectively. Ion-exchange capacity of CeSiO-CJ2 was investigated by measuring the exchange amount of Na+ ions with various cations. Studies indicate that the Na+ ions can be nearly completely (97.2%) exchanged with NH4+ ions. The Na+ ionic conductivity measurement shows that its conductivity activation energy is 0.25 eV and that the conductivity at 573 K is 2.7 × 10−5 (Ω cm)−1.