Structural Study of Europium Doped Gadolinium Polyphosphates LiGd(PO₃)₄ and Its Effect on Their Spectroscopic, Thermal, Magnetic, and Optical Properties

Abstract

Alkali metal-rare earth polyphosphates LiGd_(1-x)Eu_x(PO₃)₄ (LGP:Eu³⁺) (where x= 0, 0.02 and 0.04) were synthesized by solid-state reaction. The Rietveld refinement showed the following cell parameters: I 2/a space group, a=9.635(3) Å, b=7.035(3) Å, c=13.191(3) Å, β=90.082°, V= 894.214Å3, and Z=4. The similarity between R_F=4.21% and R_B=4.31% indicated that the realized refinement is reliable. The crystal structure consists of infinite zig-zag chains of (PO₄)^3- tetrahedra, linked by bridging oxygen. The acyclic structure of polyphosphates is confirmed by infrared and Raman (IR) spectroscopies. A good thermal stability up to 940°C and paramagnetic behavior of these compounds were also proved by thermal analyses and magnetic susceptibility measurements, respectively. Excitation spectra revealed the charge transfer phenomenon between O^2- and Eu³⁺ (CTB), the energy transfer from Gd³⁺ to Eu³⁺, and the intrinsic 4f-4f transitions of Eu³⁺ where the electronic transitions were also identified. Moreover, LGP:Eu³⁺ can emit intense reddish orange light under excitation at 394 nm. The strongest tow at 578 and 601 nm can be attributed to the transitions from excited state ⁵D₀ to ground states ⁷F₁ and ⁷F₂, respectively.

1. Introduction

Condensed alkali metal-rare earth polyphosphates, with the general formula M^IRE^III(PO₃)₄ (where M^I= are alkali metal and RE^III= are rare earth ions), have been extensively investigated thanks to their structural diversity [1–4] and their interesting magnetic [5], optic [6], and electric [7] proprieties. These polyphosphates are generally stable in normal conditions of temperature and humidity [8], which makes them useful for industrial applications. For example, Yamada et al. have used the LiNd(PO₃)₄ polyphosphate as a solid-state laser material [9, 10]. However, Z. Mua et al. have used LiEu(PO₃)₄ compound for white light-emitting diodes [11]. They are also used as promising scintillation material such as the Ce³⁺ doped MGd(PO₃)₄ compound [12].

In order to enhance the optical properties of polyphosphates, researches were oriented to doping them with metal-rare earths. In recent years, a great interest was accorded to europium earth-rare due to its outstanding photoluminescence feature [13–19]. In fact, the presence of well-defined energy levels in europium allows the emission of monochromatic and coherent radiations in solid laser.

The Gd–Eu couple is well known for its efficient conversion of the absorbed high-energy photons into two visible ones [20–24]. This phenomenon may be followed by a sequence of two steps of energies transfer. The first step is the transition ⁶G_J→⁶P_J of Gd³⁺, involving the ⁵D₀→⁷F₁ transition of the Eu³⁺ ion. The one red photon related to the Eu^3+  5D₀ → ⁷F_J transition is created. In the second step, the energy of Gd³⁺ is related to ⁶P_7/2→ ⁸S transition which is transported over the Gd³⁺ sublattice and then transferred to another Eu³⁺ ion. This phenomenon leads to ⁵D_J emission (J = 0, 1, 2, or 3) [25].

This work describes the synthesis of LiGd_(1-x)Eu_x(PO₃)₄ polyphosphate, doped with different low percentages of europium (2 and 4%). The structural study of all obtained compounds is carried out with XRD diffraction. The infrared and Raman spectroscopies and magnetic and thermal analyses were recorded at room temperature. Moreover, the optical study through excitation and emission of Eu³⁺ ions spectra was also undertaken.

2. Experimental

The condensed phosphates LiGd_(1-x)Eu_x(PO₃)₄ (where x= 0, 0.02 and 0.04) were synthesized by solid-state reaction (methods of Hammami et al. 2017) [26]. A mixture of the reagents, Li₂CO₃, Gd₂O₃, Eu₂O₃, and NH₄H₂PO₄, was prepared with the molar ratio (2.1:1:8) of Li:Gd:P, respectively. First, the raw materials were grounding in an agate mortar for one hour at least to homogenize the solid phase and improve the interatomic diffusion. Second, the mixture was introduced into the oven and submitted to the following thermal program. The first level was at 430°C to eliminate H₂O, NH₃, and CO₂, the second one was at 730°C to get LiGd_(1-x)Eu_x(PO₃)₄ pure phase. Then, the obtained products were cooled with the rate of 2°C/min to ensure a better crystallinity. Finally, the synthesized polyphosphates were washed with boiling water and nitric acid solution (1mol/L) to eliminate the residual raw materials from the final product.

The proposed chemical reaction for polyphosphate synthesis is

$$\begin{array}{c}\mathrm{8}{\mathrm{N}\mathrm{H}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{2}}{\mathrm{P}\mathrm{O}}_{\mathrm{4}}+\left(\mathrm{1}\text{-}\mathrm{x}\right){\mathrm{G}\mathrm{d}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}+{\mathrm{x}\mathrm{E}\mathrm{u}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}+{\mathrm{L}\mathrm{i}}_{\mathrm{2}}{\mathrm{C}\mathrm{O}}_{\mathrm{3}}\\ ⟶2\hspace{0.17em}{\mathrm{L}\mathrm{i}\mathrm{G}\mathrm{d}}_{\left(\mathrm{1}\text{-}\mathrm{x}\right)}{\mathrm{E}\mathrm{u}}_{\mathrm{x}}{\left({\mathrm{P}\mathrm{O}}_{\mathrm{3}}\right)}_{\mathrm{4}}+\begin{array}{c}\mathrm{8}{\mathrm{N}\mathrm{H}}_{\mathrm{3}}\uparrow \end{array}+\mathrm{12}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\\ +\begin{array}{c}{\mathrm{C}\mathrm{O}}_{\mathrm{2}}\uparrow \end{array}\end{array}$$ (1)

Samples were characterized using an INEL XRG 3000 (D5000T) diffractometer with monochromatic Cu K_α radiation. The diffraction pattern was recorded under 300K over the angular range 10-90° (2θ). The luminescence spectra were performed under ambient atmosphere via Xenius (the fluorescence Genius) spectrophotometer, at 591nm and 394nm for excitation and emission, respectively. The infrared spectra were recorded in the range of 250–1500 cm⁻¹  with a Thermo Scientific Nicolet N10 MX using sample dispersed in KBr pellets. Raman analysis was carried out at room temperature, with 514.5 nm radiation from an argon ion laser as the excitation beam. A microscope allowed a selection of high optical quality regions in the crystalline sample. Thermal stability of Eu³⁺ doped LGP was measured with differential thermal analysis SETARAM TAG 16 operating from room temperature up to 1000°C with heating rate of 5°C min⁻¹. Magnetic measurements were carried out using Quantum Design MPMSXL magnetometer with detection SQUID (at institute NEEL France).

3. Results and Discussion

3.1. Rietveld Refinement Data Analyses

The Rietveld refinement of X-ray diffraction patterns of synthesized LiGd(PO₃)₄ samples is shown in Figure 1. The graph presents the experimental and the calculated data as well as the difference between them. As it is shown, the presence of only single phase was checked by Rietveld fitting quality through the reliability R factors: R_exp, R_brag, profile R_p, and weighted profile R_wp, which should be less than 10%. The final R factors, atomic coordinates, site occupancy, thermal displacement parameters, and their estimated standard deviations in parentheses for LGP are shown in Table 1. Interatomic bond distances and angles are given in Table 2. The new lattice parameters, derived from the Rietveld refinement, are a=9.635(3) Å, b= 7.035(3) Å, c= 13.191(3) Å, and β= 90.082° and with monoclinic space group I 2/a.

Figure 1.

The Rietveld analysis of X-ray diffraction patterns for LiGd(PO₃)₄.

Table 1.

Refined structure parameters from powder X-ray Rietveld analysis for LiGd(PO₃)₄ in space group I 2/a.

Atom	Wyck	x	y	z	B	Occ
Gd	4e	0.75000	0.2982(3)	0.00000	1.35(8)	0.50
Li	4e	0.75000	0.79603(4)	0.00000	1.88(16)	0.50
P1	8f	0.48189(3)	0.06204(4)	0.143135(20)	1.88(16)	1.00
P2	8f	0.54601(3)	0.66399(4)	0.15354(2)	1.88(16)	1.00
OL12	8f	0.5665(6)	0.87995(6)	0.1598(14)	2.7(2)	1.00
OL21	8f	0.4057(4)	0.0815(16)	0.2447(3)	2.7(2)	1.00
OE21	8f	0.6520(11)	0.59342(6)	0.0754(8)	2.7(2)	1.00
OE11	8f	0.5743(13)	0.2282(14)	0.1098(11)	2.7(2)	1.00
OE22	8f	0.3953(6)	0.626(2)	0.1220(12)	2.7(2)	1.00
OE12	8f	0.3696(12)	0.0106(3)	0.0652(10)	2.7(2)	1.00
	R p = 1.237	R wp= 1.598	R exp = 1.134	R Bragg= 4.301	χ 2= 2

Table 2.

Atomic distances(Å) and angles in LiGd(PO₃)₄ with standard deviations in parentheses.

Tetrahedra	aroundP1
P1-OL12	1.534(4)	OL12 -OL21	2.381(12)	OL12 i —P1—OE11	111.61(38)
P1-OL21	1.535(4)	OL12 -OE21	2.446(11)	OL12 i —P1—OL21	101.76(23)
P1-OE11	1.534(11)	OL12 -OL21	2.463(14)	OL12 i —P1—OE12	105.86(29)
P1-OE12	1.535(12)	OL12 -OE11	2.538(11)	OL21—P1—OE11	117.46(53)
		OL12 -OE22	2.482(12)	OL21—P1—OE12	105.55(40)
		OL12 -OE12	2.448(15)	OE12—P1—OE11	113.37(47)
Tetrahedra	around P2
P2 -OL12	1.5343(13)	OE22 -OL12	2.482(12)	OE22—P2—OE21	113.09(39)
P2 -OL21	1.534(6)	OE22 -OL21	2.619(12)	OE22—P2—OL12	108.04(41)
P2 -OE21	1.535(10)	OE22 -OE21	2.560	OE22—P2—OL21 vi	117.27(36)
P2 -OE22	1.533(7)	OL21 -OL12	2.381(12)	OE21—P2—OL12	105.66(4)
		OL21 -OL12	2.440(11)	OE21—P2—OL21 vi	105.25(16)
		OE21 -OE22	2.560(13)	OL12—P2—OL21 vi	106.79(38)
Polyhedra	around Gd	Tetrahedra   around Li
Gd -OE21	2.489(6)	Li -OE21		Gd -Gd iv	5.592(2)
Gd -OE21	2.489(6)	Li -OE21		Li -Gd ii	3.533(2)
Gd -OE11	2.283(13)	Li -OE12		Li -Li v	5.068(0)
Gd -OE11	2.283(13)	Li -OE12		P1 - P2	2.8710(4)
Gd-OE22	2.197(13)			P2 -P1	2.7897(4)
Gd-OE22	2.197(13)			Gd -P1 iii	6.256(2)
Gd-OE12	2.605(7)
Gd -OE12	2.605(7)
Symmetry   code	i: x,1+y,z	ii:x,1+y,z   iii:1.5-x,1+y,-z		iv:2-x,1-y,-z   v:2-x,2-y,-z	vi:1-x,0.5+y,0.5-z

The LGP structure can be simply described as three-dimensional framework of GdO₈ polyhedra linked to (PO₄)^3– rings by Gd-O-P bridges. This framework delimits interesting tunnels with Li⁺ ions which are bonded to four oxygen atoms (LiO₄). Each LiO₄ tetrahedron shares all four O atoms with two LaO₈ polyhedra and four different PO₄ tetrahedra. A view of this structure projected along the b axis is shown in Figure 2.

Figure 2.

The structural arrangement of the LiGd(PO₃)₄ viewed in the (0 1 0) plane.

3.2. X-Ray Powder Diffraction

X-ray diffraction patterns of LiGd_(1-x)Eu_x(PO₃)₄ (where x= 0, 0.02 and 0.04) are shown in Figure 3. The obtained crystalline phases are isotypes of the mother-phase LiGd(PO₃)₄ [27]. Mainly, it is shown that XR diffraction peaks of studied solids, with different percentages of europium, are described in a cell with a super space group I 2/a instead of C 2/c usually used in crystallographic data of the old LGP. The same XRD pattern is obtained for almost of synthesized compounds, even at high Eu³⁺ concentrations. However, a shift of the diffraction peaks to the lower 2θ is observed. This shift can be explained by Bragg's theory “nλ= 2d_hklsinθ” (where λ is the X-ray wavelength (Cu K_α =1.5406Å), θ is diffraction angle, and d is interplanar distance of corresponding diffraction peaks). Therefore, λ is constant; it can be concluded that this shift is due to the increase of interplanar distance “d”. Considering the characteristics of Gd and Eu (ionic radius of Gd⁺³: 1.05 Å, Eu⁺³: 1.07 Å and atomic volume Gd: 19.9 cm³/mol, Eu: 28.9 cm³/mol), this phenomenon can be attributed to the distortion of the tetrahedra of polyphosphates upon europium insertion [28].

Figure 3.

XRD patterns of LiGd_(1-x)Eu_x(PO₃)₄ ( x=0, 0.02 and 0.04).

The crystallite size of obtained polyphosphates is calculated using Sherrer's equation below [29] and values are summarized in Table 3. Results show that the range of calculated crystallite size is between 42.49 and 42.79 nm, which prove that the synthesized compounds are nanometric.

$$\begin{array}{c}\text{Sherre'r\hspace{0.17em}\hspace{0.17em}equation:}\hspace{0.17em}D=\frac{0.9\lambda }{\beta \cos \theta }\end{array}$$ (2)

where λ is the X-ray wavelength (Cu K_α =1.5406Å), θ is the Bragg diffraction angle, and βe is the full width at half -maximum (FWHM) in radian of the main peak of each XRD pattern.

Table 3.

The size of the crystallite according to percentage of europium.

Percentages (%)	FWHM	Position(2θ)	D(nm)
0	0.189	20.630	42.49
2	0.189	20.628	42.79
4	0.189	20.621	42.79

3.3. Infrared and Raman Spectroscopy Investigations

3.3.1. Infrared

Figure 4 shows the IR spectra of all studied compounds. The comparison of spectra (Figure 4) and those obtained in previous works in literature for condensed polyphosphates [30, 31] proves that positions of infrared absorption bands are characteristic of phosphates with chain structures.

Figure 4.

IR spectra of LiGd_(1-x)Eu_x(PO₃)₄ ( x=0, 0.02 and 0.04).

IR bands attribution is carried out based on (O-P-O)⁻ groups and P-O-P bridges vibrations [32, 33]. The IR absorption spectra show the presence of two bands around 1249 cm⁻¹  which are assigned to the asymmetric stretching vibration (υ_as) of O-P-O. The weak band observed between 1071 and 1136 cm⁻¹  is attributed to the symmetric stretching vibration υ_s of O-P-O. The large and intense band around 944 cm⁻¹  is assigned to the asymmetric vibration υ_as of P-O-P. We can also attribute the few bands at 689-818 cm⁻¹  to the symmetric vibration υ_s (P-O-P). At low frequencies region, below 600 cm⁻¹ , it is very difficult to distinguish the symmetric and antisymmetric bending modes of the (O-P-O) and (P-O-P) groups. The frequencies of the corresponding bands are given in Table 4.

Table 4.

Attributions of main IR bands (cm⁻¹) of LiGd_(1-x)Eu_x(PO₃)₄ samples.

Assignment	x=0	x=0.02	x= 0.04
υ as OPO	1257	1241	1250
	1137	1127	1149
υ s OPO	1089	1089	109
υ as POP	1014	1014	1014
	944	944	947
υ s POP	689	689	689
	736	736	736
	761	758	761
	818	818	818
δPOP	437-	437	437
δPOP	456	456	457
	519	519	521
	586	585	585

The major difference between the IR spectra of cyclic polyphosphate and polyphosphate is the absence of vibration bands between 750 and 1000 cm⁻¹ . In this range, IR spectroscopy confirms the structure as long as polyphosphates chains.

3.3.2. Raman

The Raman spectra of LiGd_(1-x)Eu_x(PO₃)₄ (where x= 0, 0.02 and 0.04) at room temperature are shown in Figure 5. These spectra show the presence of many bands; the first intense band at 1178 cm⁻¹  and the second at 700 cm⁻¹  are assigned to antisymmetric stretching vibration mode υ_as (O-P-O) and symmetric stretching vibrations mode υ_s (P-O-P), respectively. The υ_as (P-O-P) asymmetric and υ_s (O-P-O) symmetric stretching vibration modes, respectively, appear in the 1000-1100 cm⁻¹  and 1212-1296 cm⁻¹  ranges. The bands under 599 cm⁻¹  are attributed to the symmetric and the asymmetric bending vibrations (δ_as and δ_s) of (O-P-O)⁻ and (P-O-P). The intense symmetric stretching vibrations bands around 700 and 1178 cm⁻¹  are characteristic of phosphoric anions (PO₃)₄⁴⁻ [34].

Figure 5.

Raman spectra of LiGd(PO₃)₄ (x=0, 0.02 and 0.04) at 300 K.

Distinguishing characteristic cyclotetraphosphates and polyphosphates compounds exit also in the Raman spectrum. The symmetric stretching vibration of the P-O-P (υ_s (P-O-P)) has a single band at 700 cm⁻¹ . This is the strongest of all the Raman vibration bands. That is because of the monoclinic symmetry of LGP doped Eu and the different positions of the lanthanide and alkali ions. The results of Raman spectroscopy can identify the structure of alkali metal lanthanide polyphosphates.

3.4. DTA (Differential Thermal Analysis)

The thermal stability of lithium polyphosphate is investigated using DTA. The curves of the Eu: LiGd(PO₃)₄ crystal are given in Figure 6. It is clearly observed that the curves present the same shape (evolution). Indeed, a single sharp endothermic peak is observed between 900 and 1000°C for all samples, which exhibits the characteristics of a first-order phase transition. This signal can be attributed to the decomposition of polyphosphates to GdPO₄. The stability of lithium gadolinium polyphosphates can be explained by heavily distorted of PO₄ tetrahedra as are the GdO₈ polyhedra. We thus conclude that all compounds are stable at high temperatures and it is monophasic.

Figure 6.

DTA of LiGd_(1-x)Eu_x(PO₃)₄ (x=0, 0.02 and 0.04).

3.5. Magnetic Study

The magnetic susceptibility and inverse magnetic susceptibility versus temperature of LiGd(PO₃)₄, LiGd_0.98Eu_0.02(PO₃)₄, and LiGd_0.96Eu_0.04(PO₃)₄ are shown in Figures 7, 8, and 9, respectively. The only other reported type of rare earth polyphosphate structures are those of LGP: Eu³⁺; and these were chosen because Gd³⁺ has an effective magnetic moment and the 4fⁿ electrons.

Figure 7.

The magnetic susceptibility (χ) and inverse magnetic susceptibility (1/χ) measurements as a function of temperature of LiGd(PO₃)₄.

Figure 8.

The magnetic susceptibility (χ) and inverse magnetic susceptibility (1/χ) measurements as a function temperature of LiGd_0.98Eu_0.02(PO₃)₄.

Figure 9.

The magnetic susceptibility (χ) and inverse magnetic susceptibility (1/χ) measurements as a function temperature of LiGd_0.96Eu_0.04(PO₃)₄.

These curves prove that all three rare earth polyphosphate compounds exhibit a paramagnetic response. The nondoped LGP is the most paramagnetic one; this is explained by their structural stability. Indeed, the addition of europium in the host disturbs samples in crystallinity. The response for LGP obeys Curie's Law very well; this is consistent with the (⁸S_7/2) ground state of Gd³⁺, which has no orbital angular momentum and so is unaffected by crystal field effects. Fitting, the temperature dependence of the inverse of susceptibility χ⁻¹ in high temperatures is given by the formula [35]

$$\begin{array}{c}\frac{\mathbf{1}}{\mathbf{\chi }}\mathbf{=}\frac{\left(\mathbf{T}\mathbf{-}{\theta }_{\mathbf{p}}\right)}{\mathbf{C}}\end{array}$$ (3)

where θ_P is the Weiss temperature and C is the Curie constant given by

$$\begin{array}{c}\mathbf{C}\mathbf{\approx }\frac{{\mathit{\mu }}_{\mathbf{0}}\mathbf{N}{\mathit{\mu }}_{\mathbf{e}\mathbf{f}\mathbf{f}}^{\mathbf{2}}}{\mathbf{3}{\mathbf{K}}_{\mathbf{B}}}\end{array}$$ (4)

where N is the number of carriers of magnetic moment, μ₀ is the vacuum permeability, K_B is the Boltzmann constant, μ_B is the Bohr magnetron, and μ_eff is effective moment of carriers. Samples' structure consists of one magnetic species (i), possessing each a magnetic moment μ_eff (i); the magnetic susceptibility is given by the relation:

$$\begin{array}{c}\mathbf{\chi }\mathbf{=}{\mathit{\mu }}_{\mathbf{0}}\frac{{\mathbf{n}}_{\mathbf{1}}{\mathit{\mu }}_{\mathbf{e}\mathbf{f}\mathbf{f}}^{\mathbf{2}}\left(\mathbf{1}\right)\mathbf{+}{\mathbf{n}}_{\mathbf{2}}{\mathit{\mu }}_{\mathbf{e}\mathbf{f}\mathbf{f}}^{\mathbf{2}}\left(\mathbf{2}\right)\mathbf{+}\mathbf{\cdots }\mathbf{+}{\mathbf{n}}_{\mathbf{i}}{\mathit{\mu }}_{\mathbf{e}\mathbf{f}\mathbf{f}}^{\mathbf{2}}\left(\mathbf{i}\right)}{\mathbf{3}{\mathbf{K}}_{\mathbf{B}}\mathbf{T}}\end{array}$$ (5)

Generally, the magnetic moment is determined by

$$\begin{array}{c}{\mathit{\mu }}_{\mathbf{e}\mathbf{f}\mathbf{f}}\mathbf{=}{\mathbf{g}}_{\mathbf{J}}\sqrt{\mathbf{J}\left(\mathbf{J}\mathbf{+}\mathbf{1}\right)}\end{array}$$ (6)

where g_J is the Lande factor and J is the total angular moment. The theoretical effective paramagnetic moment μ_eff^the for the samples can be calculated by

$$\begin{array}{c}{{\mathbf{\mu }}_{\mathbf{e}\mathbf{f}\mathbf{f}}^{\mathbf{2}}}^{\mathrm{t}\mathrm{h}\mathrm{e}}\hspace{0.17em}\mathbf{=}\left\{\mathbf{x}{\mathbf{g}}_{\mathbf{G}{\mathbf{d}}^{\mathbf{3}\mathbf{+}}}^{\mathbf{2}}{\mathbf{J}}_{\mathbf{G}{\mathbf{d}}^{\mathbf{3}\mathbf{+}}}\left({\mathbf{J}}_{\mathbf{G}{\mathbf{d}}^{\mathbf{3}\mathbf{+}}}+\mathbf{1}\right)\right\}{\mathbf{\mu }}_{\mathbf{B}}^{\mathbf{2}}\end{array}$$ (7)

Curves of χ⁻¹ versus temperature allow deducing μ_eff^exp values, which are summarized in Table 5. We can notice that the values of μ_eff^the decrease with the decrease of Gd percentage in the system, due to the important magnetic moment of Gd³⁺ ions (7.94μ_B). The comparison between the theoretical and the experimental effective moment values shows that the former are higher than the latter. This result can be associated with the increase of disorder in the matrix (LGP). On the other side, when the temperature increases to more than 75K, it induces a thermal agitation and causes magnetic moments disorientation of atoms in Eu doped LGP polyphosphates. Consequently, a decrease of paramagnetism is clearly observed (Figure 10).

Table 5.

Values of C, μ_eff^the (μ_B), and μ_eff^exp (μ_B) for the LiGd_(1-x)Eu_x(PO₃)₄ (x=0, 0.02 and 0.04) compounds.

	x=0	x=0.02	x=0.04
C (μB.KT−1)	5.26	5	2.86
μ eff the (μB)	7.94	7.86	7.78
μ eff exp (μB)	6.50	6.32	4.65

Figure 10.

Magnetic measurements of LiGd_(1-x)Eu_x(PO₃)₄ (x=0, 0.02 and 0.04).

3.6. Luminescence Properties

3.6.1. Excitation

Excitation spectra of LiGd(PO₃)₄, doped with europium (2, 4%) (Figure 11), are measured at 300K under emission with λ_em= 591 nm. Figure 11 shows broad band from 254 to 271 nm. These bands are assigned to the charge transfer bands (CTB), resulting from the transfer of an electron from the orbital 2p⁶ of the ligand O^2- to the empty state of the configuration [Xe]4f⁶ of the Eu³⁺ ion (Eu³⁺-O^2- transition). The maximum of the CTB is located at 245 nm. The differences between broadening and positions of the maxima intensities of the CTB in polyphosphate indicate their dependence on the host lattices [36]. This is due to the strong binding of the oxygen ligands in the polyphosphate compound [37].

Figure 11.

Excitation spectra with λ_em=591 nm of LiGd_(1-x)Eu_x(PO₃)₄ (x=0, 0.02 and 0.04) at 300 K.

At low frequencies, several groups of narrow bands in the spectral region 271-310 nm are clearly observed and assigned to ⁸S_7/2→⁶H_J, ⁸S_7/2→⁶G_J, ⁸S_7/2→⁶D_J, ⁸S_7/2→⁶I_J, and ⁸S_7/2→⁶P_Jtransitions of the Gd³⁺ ion. LiYF₄:Gd³⁺ crystal is used as reference to identify excitation bands, which describe the basis of the detailed energy level scheme proposed for the trivalent gadolinium [38]. The presence of band in the range between 271 and 310 nm indicates the presence of energy transfer between the two rare earths, which occurs from Gd³⁺ to Eu³⁺ in the matrix. However, there is no CTB of Eu³⁺–O^2- or energy transfer band Gd³⁺-Eu³⁺ above 310 nm. Excitation spectra within the wavelength range of 310–550 nm, show only the intrinsic transitions 4f-4f from the ground state ⁷F₀ to different excited levels (⁵D, or ⁵L) of Eu³⁺ ion. These transitions are assigned as follows: ⁷F₀→⁵H_J at 316 nm, ⁷F₀→⁵D₄ at 362 nm, ⁷F₀→⁵G₂, ⁵L₇ at 382 nm, ⁷F₀→⁵L₆ at 393 nm, ⁷F₀→⁵D₃ at 417 nm, ⁷F₀→⁵D₂ at 464 nm,and ⁷F₀→⁵D₁ at 502 nm. All these assignments and wavelengths are given in Table 6. Most of the excitation bands are broadened and some of them overlap together to form a strong band, particularly the band between 369 and 409 nm with FWHM of about 18 nm.

Table 6.

Excitation lines attribution of Eu³⁺ doped LiGd(PO₃)₄.

Wavelength (nm)	Attribution
287	7F0→5I6
294	7F0→5F4
297	7F0→5F2
6318	7F0→5H6
321	7F0→5H4
328	7F0→5H7
363	7F0→5D4
376	7F1→5D4
373-390	7F0→5GJ(2,4)
394	7F0→5L6
405	7F1→5L6
416	7F0→5D3
464	7F0→5D2
526	7F0→5D1

The perfect match of this excitation band with the emission wavelength of NUV In GaN-based LED chips makes these phosphors conveniently useful in white LEDs [39]. Figure 11 shows that the band intensities increase with europium concentration. However, they maintain the same shape and position.

3.6.2. Emission

The emission spectra of condensed phosphates are recorded at ambient temperature (300K) and in the range of 500-750 nm after excitation with λ_ex= 394 nm (Figure 12). These spectra present the same shapes, with bands intensity proportional to Eu³⁺ active ion concentration. However, we notice that the undoped LiGd(PO₃)₄ polyphosphate does not emit light. The observed emission bands are attributed to the following transitions: ⁵D₀→⁷F_J (where J = 0,1,2,3 or 4) of Eu³⁺ ion in the matrix LiGd(PO₃)₄ [40, 41].

Figure 12.

Emission spectra with λ_ex=394 nm of LiGd_(1-x)Eu_x(PO₃)₄ (x=0, 0.02 and 0.04) at 300 K.

Figure 12 proves the presence of five bands in the emission spectra where the most intense ones are those situated at 578-600 nm (⁵D₀→⁷F₁) and 604-634 nm (⁵D₀→⁷F₂). The other emission bands are observed at 554 nm (⁵D₀→⁷F₀), 660 nm (⁵D₀→⁷F₃), and 686-706 nm (⁵D₀→⁷F₄). The corresponding assignments and wavelengths of these emissions are given in Table 7. The relative intensities of the most intense transitions ⁵D₀ →⁷F₀, ⁷F₁, ⁷F₂  ⁷F₃ and ⁷F₄ are strongly influenced by the nature of the host and the crystalline environment [42]. Therefore, the dominance of magnetic dipole (MD) transition ⁵D₀→⁷F₁ of Eu³⁺ means that Eu³⁺ occupies a site in the crystal lattice with inversion symmetry. However, in the case of absence of symmetry inversion in the site of Eu³⁺, the main emission would be the electric dipole (ED) transition ⁵D₀→⁷F₂ [43]. The synthesized polyphosphates showed that orange emission transition (⁵D₀→⁷F₁) is slightly dominated. This indicates that Eu³⁺ occupies a site in the crystal lattice with symmetry inversion.

Table 7.

Emission attribution of Eu³⁺ doped LiGd(PO₃)₄.

Transitions	Wavelengths (nm)
5D0→7F0	554
5D0→7F1	578-601
5D0→7F2	604-634
5D0→7F3	660
5D0→7F4	686-706

4. Conclusion

Polyphosphates of rare earth and alkali metal LGP:Eu³⁺ were successfully synthesized by solid-state reaction at 730°C. XRD patterns proved that the obtained samples crystallize in a monoclinic single phase with space group I 2/a and following cell parameters a= 9.635(3) Å, b= 7.035(3) Å, c= 13.191(3) Å, β= 90.082°, V= 894.214 ų, and Z= 4. The synthesized polyphosphates showed a good thermal stability until 940°C. Spectroscopic analyses by IR and Raman spectra confirmed the acyclic zig-zag chain of (PO₄)^3– in LGP structure, involving GdO₈ dodecahedra and LiO₄ polyhedra. The magnetic susceptibility carried out on single crystals revealed that the title compounds were paramagnetic between 5 and 300 K. An increase in excitation and emission bands intensities was observed with the increase of europium concentration. The presence of band in the range between 271 and 310 nm in excitation spectra proved the energy transfer process from Gd³⁺ to Eu³⁺. The dominance of ⁵D₀→⁷F₁ transition in the emission spectra confirms that Eu³⁺ occupies a site in the crystal lattice with symmetry inversion. The change in transition bands intensity proves that LGP phosphates affect europium environment.

Conflicts of Interest

There are no conflicts of interest regarding the publication of this paper.