Facile hydrothermal crystallization of NaLn(WO₄)₂ (Ln=La-Lu, and Y), phase/morphology evolution, and photoluminescence

Abstract

With the successful synthesis of NaLn(WO 4 ) 2 for the full spectrum of Ln, the effects of lanthanide contraction on the structural features, crystal morphology, and IR responses of the compounds were clarified. The temperature-and timecourse phase/morphology evolutions and the phase conversion upon calcination were thoroughly studied for the Group I and Group II compounds with Ln=La and Lu for example, respectively. Unknown intermediates were characterized by elemental analysis, IR absorption, thermogravimetry, and differential scanning calorimetry to better understand their chemical composition and coordination. The photoluminescence properties of NaEu(WO 4 ) 2 and NaTb(WO 4 ) 2 , including excitation, emission, fluorescence decay, and quantum efficiency of luminescence, were also comparatively studied for the as-synthesized and calcination products.

Upconversion luminescence and favorable temperature sensing performance of eulytite-type Sr3Y(PO4)3:Yb3+/Ln3+ phosphors (Ln=Ho, Er, Tm)

“…Eulytite-type orthophosphate M 3 A(PO 4 ) 3 (M = Ca, Sr or Ba; A = La, Gd, Y or Lu) possesses high physical, chemical and structure stabilities [20,21], and may thus serve as an important family of phosphor hosts. It should be noted that many other types of inorganic compounds, such as GdPO 4 orthophosphate [22] and NaLn(WO 4 ) 2 tungstate (Ln = La-Lu, and Y) [23], also draw great interest for phosphor applications. For downconversion (DC) luminescence, You et al [24] prepared Eu 2+ /Mn 2+ co-doped Sr 3 Lu(PO 4 ) 3 by solidstate reaction at 1300°C for 3 h in a CO atmosphere and investigated its luminescence and Eu 2+ -Mn 2+ energy transfer; Liang et al [25] produced Ba 3 La(PO 4 ) 3 :Ln 3+ (Ln = Tb, Eu) phosphors via solid reaction at 1200-1250°C for 5-8 h in a thermal carbon atmosphere for color-tunable luminescence; Xia et al [26] synthesized a series of (Ba,Sr) 3 Lu(PO 4 ) 3 :Eu 2+ phosphors by solid reaction at 1300°C for 4 h under a 10%H 2 -90%N 2 gas mixture, and the blue shift of Eu 2+ emission with increasing Sr/Ba ratio was discussed in detail; Xia et al [27] also synthesized eulytite-type Ba 3 Eu(PO 4 ) 3 and Sr 3 Eu(PO 4 ) 3 compounds via solid reaction at 1250°C for 10 h in air, and systematically compared their crystal structures and photoluminescence; Guo et al [28] [11] and BiPO 4 [29].…”

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

“…The occurrence of the former suggested the presence of energy transfer from tungstate ligands to Eu 3+ activators. The shoulder peak at ∼317 nm overlaps with the 1 A 1 → 3 T 1 and 7 F 0 → 5 H 0 excitations of the [WO 4 ] 2– group and Eu 3+ , respectively . The sharp peaks in the longer wavelength region arise from electronic transitions within the 4f 6 shell of Eu 3+ , as labeled in the figure, with the 7 F 0,1 → 5 L 6 excitation at ∼395 nm dominating the whole excitation spectrum (Figure a).…”

“…The shoulder peak at ∼317 nm overlaps with the 1 A 1 → 3 T 1 and 7 F 0 → 5 H 0 excitations of the [WO 4 ] 2− group and Eu 3+ , respectively. 25 The sharp peaks in the longer wavelength region arise from electronic transitions within the 4f 6 shell of Eu 3+ , as labeled in the figure, with the 7 F 0,1 → 5 L 6 excitation at ∼395 nm dominating the whole excitation spectrum (Figure 8a). Tb 3+ similarly presents two groups of excitation bands (Figure 8c), but the broad band in the short-UV region (∼200−330 nm, centered at ∼278 nm) overwhelms any of the intra-4f 8 excitation of Tb 3+ in the ∼330−500 nm region and is also significantly stronger than that found for the Eu 3+ -doped sample.…”

“…As a result, subsequent annealing is needed to convert the precursor into the targeted ARE(WO 4 ) 2 :Ln compound (A = Na and K; RE = La, Y, Gd, and Lu; Ln = Eu 3+ , Tb 3+ , Sm 3+ , Dy 3+ , and Sm 3+ /Nd 3+ ; etc.). 5,9,16,25 This is primarily due to the rich solution chemistry of tungstate anions, which present themselves as monomeric [WO 4 ] 2− in a solution of sufficiently high alkalinity (pH >8), while the various protonated and polymerized forms of [H 18 (WO 4 ) 12 ] 6− , [H 10 (WO 4 ) 6 ] 2− , [H 7 (WO 4 ) 6 ] 5− , and [HWO 4 ] − appear in an acidic or neutral solution (pH 4−7). 26−35 Furthermore, the type and concentration of tungstate ions are affected by not only the solution pH but also the temperature, the initial concentration of the tungstate source (such as Na 2 WO 4 •2H 2 O in this work), and the attendance and concentration of alien anions.…”

“…As a result, subsequent annealing is needed to convert the precursor into the targeted ARE­(WO 4 ) 2 :Ln compound (A = Na and K; RE = La, Y, Gd, and Lu; Ln = Eu 3+ , Tb 3+ , Sm 3+ , Dy 3+ , and Sm 3+ /Nd 3+ ; etc. ). ,,, This is primarily due to the rich solution chemistry of tungstate anions, which present themselves as monomeric [WO 4 ] 2– in a solution of sufficiently high alkalinity (pH >8), while the various protonated and polymerized forms of [H 18 (WO 4 ) 12 ] 6– , [H 10 (WO 4 ) 6 ] 2– , [H 7 (WO 4 ) 6 ] 5– , and [HWO 4 ] − appear in an acidic or neutral solution (pH 4–7). − Furthermore, the type and concentration of tungstate ions are affected by not only the solution pH but also the temperature, the initial concentration of the tungstate source (such as Na 2 WO 4 ·2H 2 O in this work), and the attendance and concentration of alien anions. On the other hand, RE 3+ undergoes hydration and hydrolysis in an aqueous solution to form [RE­(OH) x (H 2 O) y ] 3– x complex ions, whose x value increases with an increase in the solution pH. , Numerous reaction pathways are, therefore, possible under hydrothermal/solvothermal conditions, which, in turn, make the product rather sensitive to the synthesis parameter. ,, It was recently shown that the reactivity of tungstate species toward [RE­(OH) x (H 2 O) y ] 3– x decreases with an increase in solution pH, and the hydrothermal product of a higher pH tends to have a lower W/RE molar ratio and vice versa. , The complicated solution chemistry and reaction pathway may account for why most of the studies to date left the hydrothermal/solvothermal precursors unanalyzed for chemical composition, phase purity, phase constituent, and phase structure.…”