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

Abstract

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.

Understanding the Doping Chemistry of High Oxidation States in Scheelite CaWO4 by Hydrothermal Conditions

“…SS-10 has a shorter W–O bond length, which explains its larger wavelength of the LMCT band compared to that of the nominal 10% Eu 3+ /K + -codoped CaWO 4 nanocrystals. The fine structures in the region 350–500 nm are the characteristic transitions of the ground state ( 7 F 0 ) to the excited state ( 5 D 4 , 5 G 2 , 5 L 6 , 5 D 3 , 5 D 2 ) of the 4f shell of Eu 3+ ions . All samples show sharp emission peaks with similar shapes, which could be assigned to the 5 D 0 → 7 F J ( J = 0–4) transitions (Figure b).…”

“…The fine structures in the region 350−500 nm are the characteristic transitions of the ground state ( 7 F 0 ) to the excited state ( 5 D 4 , 5 G 2 , 5 L 6 , 5 D 3 , 5 D 2 ) of the 4f shell of Eu 3+ ions. 42 All samples show sharp emission peaks with similar shapes, which could be assigned to the 5 D 0 → 7 F J (J = 0−4) transitions (Figure 4b). Theoretically, the electronic dipole transition of 5 D 0 → 7 F 2 is strictly forbidden, unless the Eu 3+ ions occupy the site without inversion symmetry.…”

Morphology Tailoring of ZnWO4 Crystallites/Architectures and Photoluminescence of the Doped RE3+ Ions (RE = Sm, Eu, Tb, and Dy)

“…This is primarily caused by an overlapping of WO 6 excitation (O 2− → W 6+ CT) and O 2− → RE 3+ CT (RE = Eu, Sm, and Dy) or 4 f 8 → 4 f 7 d 1 interconfigurational transition of Tb 3+ , since the latter bands usually occur at wavelengths shorter than 280 nm. 39 In addition, the intra-4f 6 excitation of Eu 3+ were clearly resolved at the longer wavelengths of ∼395 nm ( 7 F 0,1 → 5 L 6 ), 416 nm ( 7 F 0,1 → 5 D 3 ) and 465 nm ( 7 F 0,1 → 5 D 2 ), though the f−f excitation transitions of the other RE 3+ are negligibly weak. This is due to the fact that the 615 nm emission of Eu 3+ is less overlapped with WO 6 emission; thus, the PLE spectrum taken by monitoring the 615 nm emission shows much stronger Eu 3+ excitations (Figure 11c).…”

“…That is, the effect of Tar 2– can be enhanced by the lower pH and, therefore, much larger platelike crystallites were resulted. The high pH solutions of pH 9 and particularly pH 10 may enhance Zn 2+ hydrolysis, which expels out Tar 2– from the complex ion (smaller y ) and lower the reactivity of tungstate species. , As a consequence, a mixture of ZnWO 4 and ZnO, dehydrated from Zn­(OH) 2 , was formed (Figure c,d).…”

“…Once again, crystallite development via colattice of non-(00 l ) planes ((010) herein) was observed. EG molecules, though less chelating than Tar 2– , can selectively attach onto specific facets of the ZnWO 4 nuclei/crystallites, particularly those not sufficiently occupied by Tar 2– , to modify the pattern of crystallite growth and assembly. , The observed sublattice structure is usually caused by deviation of atoms from their ideal positions, which could be due to the synergistic constraining effects of adsorbed Tar 2– and EG.…”