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

Abstract

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 (P2/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 (P2 1 /a, no. 14) for Ln = La, triclinic (P1̅ , no. 2) for Ln = Pr−Sm, triclinic (P1̅ , no. 2) for Ln = Eu and Gd, and monoclinic (P2 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.

Acquisition of Emissive Single-Crystalline AEu(MoO4)2 (A = Na, K, Rb, and Cs) Double Molybdates by One-Pot Hydrothermal Synthesis: Relationship between Particle Morphology and Photoluminescence Properties

“…Particularly, the FTIR spectra for KEu(MoO 4 ) 2 and RbEu(MoO 4 ) 2 exhibit absorptions derived from N–H bonding, suggesting that NH 4 Eu(MoO 4 ) 2 may adopt a space group of Pbcn like KEu(MoO 4 ) 2 and RbEu(MoO 4 ) 2 . This is supported by the fact that the ionic radius of NH 4 + (1.48 Å) is similar to that of K + (1.38 Å) and Rb + (1.52 Å), and the space group of NH 4 Gd(MoO 4 ) 2 occupied by Gd 3+ with an ionic radius close to that of Eu 3+ has been reported to belong to the space group of Pbcn . The introduction of ammonium ions in A Eu(MoO 4 ) 2 likely originates from using (NH 4 ) 6 Mo 7 O 24 ·4H 2 O as the Mo source.…”

“…This is supported by the fact that the ionic radius of NH 4 + (1.48 Å) 37 is similar to that of K + (1.38 Å) and Rb + (1.52 Å), 34 and the space group of NH 4 Gd(MoO 4 ) 2 occupied by Gd 3+ with an ionic radius close to that of Eu 3+ has been reported to belong to the space group of Pbcn. 30 The introduction of ammonium ions in AEu(MoO 4 ) 2 likely originates from using (NH 4 ) 6 2). This is because NaEu(MoO 4 ) 2 does not have a layered structure but a scheelite structure with a three-dimensional structure, and the space group of CsEu(MoO 4 ) 2 , which has a layered structure, is not Pbcn but P2/c.…”

“…The FTIR spectra for KEu­(MoO 4 ) 2 (Figure d) exhibited absorptions in the ∼1000 cm –1 region for the antisymmetric stretching of the Mo–O bond (ν 3 ) and at ∼1410/1430 cm –1 (doublet), 2831, 3037, and 3159 cm –1 for the antisymmetric bending (ν 4 ), an overtone band (2ν 4 ), symmetric stretching (ν 1 ), and antisymmetric stretching (ν 3 ) of the N–H bond, respectively, with the reduction in the amount of KOH added . This FTIR spectrum resembles that of the related compound ammonium lanthanide molybdate, NH 4 La­(MoO 4 ) 2 , indicating that K + in KEu­(MoO 4 ) 2 was substituted by NH 4 + with the reduction in the amount of KOH added. This tendency was observed not only in KEu­(MoO 4 ) 2 but also in RbEu­(MoO 4 ) 2 (refer to Figures S3 and S4 and Table ).…”

“…22−28 Due to its interesting structural and optical features, AEu(MoO4)2 has been extensively studied as a phosphor material. 1,[3][4][5][6]9,15,16,29,30 Among these methods, hydrothermal synthesis stands out as one of the most effective "soft chemistry" techniques because it can proceed under mild conditions to obtain crystals. In contrast, coprecipitation and sol−gel methods often require high-temperature treatments to crystallize the particles.…”

“…Precise control of the synthesis technique is crucial for improving the properties of these materials. Several liquid-phase syntheses, including coprecipitation, hydro/solvothermal, and sol–gel methods, have been developed to synthesize submicrocrystals and nanocrystals. − Due to its interesting structural and optical features, AEu­(MoO4)­2 has been extensively studied as a phosphor material. ,− ,,,,, Among these methods, hydrothermal synthesis stands out as one of the most effective “soft chemistry” techniques because it can proceed under mild conditions to obtain crystals. In contrast, coprecipitation and sol–gel methods often require high-temperature treatments to crystallize the particles.…”