Systematic Crystallization of NH4Ln(MoO4)2 as a Family of Layered Compounds (Ln = La-Lu and Y), Derivation of Ln2Mo4O15, Crystal Structure, and Photoluminescence

NH4Ln(MoO4)2 (Ln = La???Lu lanthanide, Y) was crystallized via hydrothermal reaction as a new family of layered materials, from which phase-pure Ln2Mo4O15 was successfully derived via subsequent annealing at 700 ??C for the series of Ln elements excluding Ce and Lu. Detailed structure analysis revealed that the ionic size of Ln3+ decisively determined the crystal structure and Mo/Ln coordination for the two families of compounds. NH4Ln(MoO4)2 was analyzed to be orthorhombic (Pbcn space group, no. 60) and monoclinic (P2/c, no. 13) for the larger and smaller Ln3+ of Ln = La???Gd and Ln = Tb???Lu (including Y), respectively, where both the crystal structures have a layered topology featured by the alternative stacking of a [Ln(MoO4)2]??? three-tier infinite anionic layer and interlayer NH4+. Four types of crystal structures were found for the Ln2Mo4O15 series, which are monoclinic (P21/a, no. 14) for Ln = La, triclinic (P1??, no. 2) for Ln = Pr???Sm, triclinic (P1??, no. 2) for Ln = Eu and Gd, and monoclinic (P21/c, no. 14) for Ln = Tb???Yb (including Y). The photoluminescence of NH4Ln(MoO4)2 (Ln = Eu, Tb) and Ln2Mo4O15:Eu3+ (Ln = La, Gd, Y) was thoroughly investigated in terms of spectral features, quantum efficiency, fluorescence decay, and CIE chromaticity. The thermal stability of luminescence was also studied for Ln2Mo4O15:Eu3+, and the observed charge-transfer excitation components were successfully correlated with the features of the Mo???O polyhedron/unit. Superscript/Subscript Available

Automated summary

NH4Ln(MoO4)2 was crystallized via hydrothermal reaction and subsequently annealed at 700°C to obtain phase-pure Ln2Mo4O15. The crystal structure and coordination of Mo/Ln in these compounds were determined by the ionic size of Ln3+ ions. NH4Ln(MoO4)2 and Ln2Mo4O15 exhibit layered topology. The photoluminescence properties of these materials were thoroughly investigated.