NaLaW2O7(OH)2(H2O): Crystal Structure and RE3+ Luminescence in the Pristine and Annealed Double Tungstates (RE = Eu, Tb, Sm, and Dy).

Hydrothermal reaction of La(NO3)3 and Na2WO4·2H2O at 100 °C and pH 8 resulted in the formation of new compound NaLaW2O7(OH)2(H2O), as confirmed by the X-ray diffraction results, chemical composition, Fourier transform infrared, thermogravimetric/differential thermal analysis, and transmission electron microscopy analyses. The crystal structure was determined in the triclinic system (space group P1̅), with lattice constants a = 5.8671(2) Å, b = 8.2440(2) Å, and c = 9.0108(3) Å, axis angles α = 93.121(2)°, β = 75.280(2)°, and γ = 94.379(2)°, and cell volume V = 420.03(2) Å3. The structure contains two-dimensional layers of -(W1O6)-(W1O6)-(W2O6)-(W2O6)-(W1O6)-(W1O6)- and -LaO9-LaO9- chains alternating in the a-b plane, which are linked together through NaO6 octahedral trigonal prisms by edges to form a three-dimensional net. Dehydration of the compound proceeds up to a low temperature of ∼350 °C and results in the formation of technologically important NaLa(WO4)2 double tungstate, which is thus a unique precursor for the latter. Na(La,RE)W2O7(OH)2(H2O) and Na(La,RE)(WO4)2 solid solutions separately doped with the practically important activators for which RE = Eu, Tb, Sm, and Dy were also successfully synthesized and investigated for their structural features and photoluminescence properties, including excitation, emission, quantum yield, emission color, and fluorescence decay kinetics. The compounds were shown to exhibit dominantly strong red (∼616 nm for Eu3+; λex = 395 or 464 nm), green (∼545 nm for Tb3+; λex = 278 or 258 nm), deep red (∼645 nm for Sm3+; λex = 251 nm), and yellow (∼573 nm for Dy3+; λex = 254 nm) emission upon being irradiated with the peak wavelengths of their strongest excitation bands.