Systematic Crystallization of NH 4 Ln(MoO 4 ) 2 as a Family of Layered Compounds (Ln = La-Lu and Y), Derivation of Ln 2 Mo 4 O 15 , Crystal Structure, and Photoluminescence.

NH 4 Ln(MoO 4 ) 2 (Ln = La-Lu lanthanide, Y) was crystallized via hydrothermal reaction as a new family of layered materials, from which phase-pure Ln 2 Mo 4 O 15 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 Ln 3+ decisively determined the crystal structure and Mo/Ln coordination for the two families of compounds. NH 4 Ln(MoO 4 ) 2 was analyzed to be orthorhombic ( Pbcn space group, no. 60) and monoclinic ( P 2/ c , no. 13) for the larger and smaller Ln 3+ 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(MoO 4 ) 2 ] - three-tier infinite anionic layer and interlayer NH 4 + . Four types of crystal structures were found for the Ln 2 Mo 4 O 15 series, which are monoclinic ( P 2 1 / a , no. 14) for Ln = La, triclinic ( P 1̅, no. 2) for Ln = Pr-Sm, triclinic ( P 1̅, no. 2) for Ln = Eu and Gd, and monoclini c ( P 2 1 / c , no. 14) for Ln = Tb-Yb (including Y). The photoluminescence of NH 4 Ln(MoO 4 ) 2 (Ln = Eu, Tb) and Ln 2 Mo 4 O 15 :Eu 3+ (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 Ln 2 Mo 4 O 15 :Eu 3+ , and the observed charge-transfer excitation components were successfully correlated with the features of the Mo-O polyhedron/unit.