English

• 0.   Introduction

  Rare earth phosphate has excellent chemical and thermal stability and unique structural characteristics, which make it have excellent optical and magnetic properties. Therefore, it is widely used in optical materials, magneto-optical/resistive materials, dielectric materials, laser materials, and chemical sensors^[1]. GdPO₄ has been extensively studied for its unique properties in various rare earth-doped phosphates. Under the effect of seven non-paired electrons, Gd³⁺ions have strong paramagnetism. It can be used to design new magnetic resonance functional materials with electron relaxation time in the order of micro nanoseconds^[2-3]. The study of Sm³⁺ - doped phosphate luminescent materials is helpful to explain the luminescent mechanism of rare earth ions in phosphate matrix and has certain basic theoretical significance for the development and utilization of rare earth in lumines cence and magnetic resonance imaging^[4-5].

  Ouertani et al.^[6] studied the impact of Sm³⁺ doping concentration and excitation wavelength on the spectral characteristics of GdPO₄∶Sm³⁺ phosphors. Li et al. ^[7] prepared a series of red-orange phosphors Ba₃Gd_1-x (PO₄)₃∶xSm³⁺ through the high- temperature solid -phase reaction. It had very strong absorption and bright red-orange emission in the near - ultraviolet region. Liu et al. ^[8] synthesized a series of Sr₃GdNa(PO ₄)₃F∶Sm³⁺ through a high - temperature solid- phase reaction, explored that the Sm³⁺ concentration quenching was attributable to the electric dipole-electric dipole interaction, and calculated that the activation energy of thermal quenching was 0.242 eV. Hu et al. ^[9] proposed introducing Tb³⁺ ions into YPO₄∶Sm³⁺ crystals to improve their luminescent properties, which increased the luminescence intensity and duration of Sm³⁺ at room temperature by about 14 times. Nair et al. ^[10] synthesized Ca₃Mg ₃(PO₄)₄∶Sm³⁺ phosphors through the combustion method and the prepared phosphors were potential candidates for orange light - emitting diode (LED). Zou et al.^[11] synthesized LaPO₄∶Sm³⁺ nanocrystalline luminescent material with lactic acid (LA) as the cosolvent using the solvothermal method. LA not only significantly restricts the inherent one -dimensional (1D) growth of hexagonal LaPO₄ crystals along the [001] direction, resulting in the shape change from microfilaments to nanoparticles, but also promotes the growth of crystals. Our research group^[12-13] synthesized a series of nanophosphors using the hydrothermal method and studied the concentration quenching and energy conversion mechanism of Sm³⁺ and Eu³⁺.

  The luminescence mechanism of rare earth samarium ions has been studied more, but the high temperature luminescence mechanism and magnetic properties of phosphors have not been studied much^[14]. Therefore, based on previous research, the impact of the optimal calcination temperature on the luminescent properties of GdPO₄∶Sm³⁺ phosphors was investigated first, then the Sm³⁺ doping concentration and concentration quenching mechanism were optimized in this paper. The high temperature luminescence characteris- tics and magnetic properties of the optimal products were mainly studied. The research results have impor- tant guiding significance in the development of rare earth resources in the field of magneto - optical materials, and the potential applications in bioimaging and magnetic resonance imaging.

  1.   Experimental

  1.1   Reagents and instruments

  Gd₂O₃, concentrated phosphoric acid, concentrated nitric acid, and NaOH (AR) were purchased from Sinopharm Group Chemical Reagent Co. Sm₂O₃ (99.999%) was purchased from the Baotou Rare Earth Research Institute. The experimental water was all deionized water.

  1.2   Preparation of GdPO₄•H₂O∶Sm³⁺

  The hydrothermal method was used to prepare GdPO₄•H ₂O∶Sm³⁺. The molar fraction (x) of Sm³⁺ dop- ing were 0%, 1%, 2%, 3%, 4%, 6%, and 10%, respectively. First, Sm₂O₃ and Gd₂O₃ were dissolved in 6 mol• L^-1 HNO3 solution to prepare 0.5 mol•L^-1 Gd(NO₃)₃ and 0.05 mol•L-1 Sm(NO ₃)₃ solutions, respectively. In a 100 mL beaker, a certain amount of Gd(NO₃)₃ and Sm(NO₃)₃ solution was mixed. H ₃PO₄ (1.5 mol•L^-1) solution was added dropwise to the mixture. After enough reaction, 6 mol•L^-1 NaOH was used to bring the pH of the reaction system down to 1 - 2, and the solution was stirred for 10 min and then dispersed by ultrasonic for 30 min to produce about 60 mL of suspension liquid. This suspension liquid was transferred into the Teflon liner of a 100 mL high - pressure reactor with a filling degree of 80%. Then, the reactor was placed in an air - dry oven with a reaction temperature of 200 ℃ and maintained for 8 h until the reaction was completed. The cation and anion′s molar ratio was 1∶3 at this point. After the reaction, the product was cooled to room temperature, performed centrifugal separation, washed, and dried at 80 ℃ for grinding, and then the phosphor precursor GdPO₄•H₂O, GdPO₄•H₂O∶1%Sm³⁺, GdPO₄•H₂O∶2%Sm³⁺, GdPO₄•H₂O∶3%Sm³⁺, GdPO₄•H₂O∶4%Sm³⁺, GdPO₄• H₂O∶6%Sm³⁺, GdPO₄•H₂O∶10%Sm³⁺ was obtained, which is corresponded to x=0%, 1%, 2%, 3%, 4%, 6%, and 10%.

  1.3   Phosphor precursor calcination

  The prepared precursor GdPO ₄•H₂O∶2%Sm³⁺ was put into a corundum crucible and a muffle furnace to calcine for 3 h at 800, 900, 1 000, 1 100, or 1 200 ℃, respectively. The finished product was then ground with an agate mortar, cooled to room temperature, packaged for measurement, and then GdPO₄∶2%Sm³⁺ calcined at different temperatures were obtained.

  1.4   Characterization

  The structural characterization was carried out using Bruker D8 Advance X-ray diffractometer (Cu Kα, λ =0.154 06 nm, I=200 mA, and U=40 kV, scanning range of 10°-70°, scanning speed of 4 (°)•min^-1). The crystal size, morphology, particle size, distribution, and constituent element of the samples were measured by a Sigma 500 AMCs scanning electron microscope, and the working voltage was 3 kV. The hysteresis curves of the samples were detected by SQUID-VSM magnetometer. The excitation and emission spectra of the samples were obtained using a Hitachi F - 4600 fluorescence spectrophotometer (Japan). The phosphorescence life time was measured by FL920 series steady/transient state luminescence spectrometer. All the samples were tested at room temperature. In addition, the temperature - dependent emission spectra of the samples were measured using a Hitachi F-4600 fluorescence spectro- photometer (Japan), and the temperature was adjusted by the Janis VPF-800 temperature-controlling system.

  2.   Results and discussion

  2.1   Study on the calcination temperature of GdPO₄∶2%Sm³⁺

  2.1.1   Analysis of phase structure and morphology

  From Fig. 1a, the positions of the diffraction peaks of the precursor GdPO₄•H₂O∶2%Sm³⁺ are the same as the hexagonal crystal system GdPO₄•H₂O (P3₁21 (152), PDF No. 39 - 0232). It indicates that the precursor is a pure - state hexagonal structure. It can be known from the standard card that the space group of GdPO₄•H₂O is P3₁21 (152). The crystal cell parameters are a=b= 0.690 5 nm, c=0.632 6 nm, α=β=90°, γ=120°, Z=3, V=0.261 2 nm³. No other peak (such as SmPO₄) was observed in Fig. 1a, indicating that Sm³⁺ did not change the crystal structure of the GdPO₄ matrix, and better replaced the Gd³⁺ ion site in the crystal lattice, achieving effective doping^[15-17].

  Figure 1

  

  Figure 1.  XRD patterns of the precursor GdPO₄·H₂O∶2%Sm³⁺ (a) and GdPO₄∶2%Sm³⁺ obtained by calcination at different temperatures (b)

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  Fig. 1b shows the XRD patterns of the products GdPO₄∶2%Sm³⁺ obtained by calcination at 800, 900, 1 000, 1 100, and 1 200 ℃, respectively. The diffraction peak positions of all the samples were the same, but were different from the characteristic diffraction peak of the precursor, indicating that the calcined phosphors had the same lattice structure, which are different from that of the precursor^[18]. Through comparison of the diffraction peak of the calcined phosphor and the characteristic peak in the standard pattern (PDF No.32- 0386) of GdPO ₄ in the monoclinic crystal system, the positions of the peaks were completely consistent, and there was no extra peak, indicating that the final product is GdPO₄ in the monoclinic crystal system P2₁/n (14). The crystal cell parameters are a=0.665 3 nm, b= 0.684 5 nm, c=0.644 3 nm, Z=4, and V=0.279 9 nm³. The phosphors prepared with calcination temperatures of 800 to 1 200 ℃ had strong diffraction peaks at (200), (120), and (112) planes, and the strongest peak was the (120) plane. Through comparison of the intensity of the diffraction peaks in Fig. 1b, the crystallization properties of the phosphor prepared with a calcination temperature of 1 000 ℃ were the best.

  It indicates that the phosphor precursor changed from a hexagonal crystal system to a monoclinic crystal system after calcination. The hexagonal crystal system GdPO₄ •H₂ O∶2%Sm³⁺ lost crystal water through calcination so the final product was the monoclinic crystal system GdPO ₄∶2%Sm³⁺. The crystal lattice transformation process is shown in Fig. 2. The distributions of atoms in the two crystal structures are similar. It is related to the difference in the number of coordinating oxygens of gadolinium. In the hexagonal crystal system GdPO₄• H₂O, Gd³⁺ and 6 oxygen atoms form a quadrilateral bipyramid. After dehydration, Gd³⁺ and 9 oxygen atoms form a triangular prism, and one phosphorus atom and three oxygen atoms form a tetrahedron.

  Figure 2

  

  Figure 2.  Lattice transformation diagram of phosphors

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  Fig. 3 shows that the morphology of the precursor GdPO₄•H₂O∶2%Sm³⁺ was rod-like. The morphology of GdPO₄∶2%Sm³⁺ obtained by calcination at 800 ℃ was still nanorod, but the size of the nanorod was 2-3 times larger than that of the precursor, and the nanoparticles had better dispersibility. GdPO₄∶2%Sm³⁺ obtained by calcination at 900 ℃ maintained a nanorod morphology, but the nanoparticles showed obvious agglomeration and some edges were melted. GdPO₄∶2%Sm³⁺ obtainedby calcination at 1 000 ℃ exhibited an ellipsoid - shaped micro-block morphology, with uniform distribution and better dispersibility. GdPO₄∶2%Sm³⁺ obtained by calcination at 1 100 ℃ exhibited an ellipsoid - shaped micro - block morphology that aggregated into clusters, forming larger irregular particles with uneven shape distribution. GdPO₄∶2%Sm³⁺ obtained by calcination at 1 200 ℃ indicated nanoparticles were fusion, blurred boundaries, and forming clusters. From the above analysis, with the temperature increasing, the morphology changed from nanorods to micro - and nano- like spherical particles until the emergence of sintered masses. The mechanism of the phenomenon: in the hexagonal structure, the c - axis has a high symmetry, thus the samples tend to grow along the c -axis, whereas the growth of the other two axes may be compressed. However, with the increased temperature, the atoms slightly shift and break the symmetry of the crystal structure due to the removal of water from GdPO₄•H₂O. Therefore, the samples will no longer grow along with the c - axis. Meanwhile, it is suggested that they can grow along the a - axis and b - axis. Thus, it is reasonable to observe that the morphology of the sample transforms from nanorods to spheroidal particles^[19-20].

  Figure 3

  

  Figure 3.  SEM images of the precursor GdPO₄·H₂O∶2%Sm³⁺ and GdPO₄∶2%Sm³⁺ obtained by calcination at different temperatures

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  2.1.2   Excitation and emission spectra

  Fig. 4a shows that the samples had a series of excitation peaks in the range of 250-450 nm. The peak at 273 nm is due to the absorption of the Gd³⁺ charge transfer band from ⁸S_7/2 to ⁶I_11/2, while the peaks at 361, 373, and 401 nm correspond to the transitions of Sm³⁺ from ⁶H_5/2 to ⁴H_7/2, ⁶H^5/2 to ⁴P_7/2, and ⁶H^5/2 to ⁴F_7/2, respectively. The peak at 401 nm is the strongest excitation peak^[21-22]. Fig. 4b shows that when the product is excited by ultraviolet light at 401 nm, the luminescent spectra of the samples at 562, 599, and 646 nm exhibited three main characteristic emission peaks of Sm³⁺, which belonged to the ⁴G_5/2 → ⁶H_J (J=5/2, 7/2, 9/2) transition, respectively^[23-24]. The luminous intensity of the emission peak was the strongest at 600 nm. In Fig. 4c, under the excitation of ultraviolet light at 273 nm, the emission peak of the luminescent spectrum of the GdPO₄∶2%Sm³⁺ at 599 nm was the strongest characteristic peak of Sm³⁺. The emission peak at 626 nm is that of Gd³⁺, which belonged to the electronic transition of⁶G_3/2 → ⁶P_7/2^[25].

  Figure 4

  

  Figure 4.  Excitation (a) and emission (b, c) spectra of the precursor GdPO₄·H₂O∶2%Sm³⁺ and GdPO₄∶2%Sm³⁺ obtained by calcination at different temperatures

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  The results show that the shapes and positions of the excitation and emission peaks of the precursor and products were the same at different calcination temperatures, but the intensities of the excitation and emission peaks of the precursor were very weak. The intensities of the excitation and emission peaks of the GdPO ₄∶ 2%Sm³⁺ (800 and 900 ℃) were about 4 times that of the precursor. The intensities of the excitation and emission peaks of the GdPO₄∶2%Sm³⁺ obtained by calcination at 1 000 ℃ reached the maximum value, which was almost 20 times that of the precursor. It is suggested that the luminescence performance of phosphors with monoclinic crystals is better than that of phosphors with hexagonal crystal systems because the hydroxyl groups in the water molecules in hexagonal crystal systems have a quenching effect on the lumines- cence^[15]. The spheroidal sample is beneficial to decrease the non - radiative rate and thus increase the luminescent intensity. Because of the fewer defects of the spheroidal morphology, it is beneficial to decrease the non - radiative rate and thus there are more electrons to relax down to the ground state, leading to an increase in the luminescent intensity. It also indicates that the optimum calcination temperature of GdPO₄∶ 2%Sm³⁺ was 1 000 ℃.

  Under the excitation by 401 nm near - ultraviolet light, the color coordinates of GdPO₄∶2%Sm³⁺ obtained at different calcination temperatures are drawn in Fig. 5. The color coordinates of the precursor were X= 0.542 3, Y=0.456 5, and the emitting color was orange. The color coordinates of the GdPO₄∶2%Sm³⁺ phosphor calcined at 1 000 ℃ were X=0.583 4, Y =0.415 8. The colors of the phosphors were all orange - red. The color coordinate of the sample does not change much with the increase in the calcination temperature. It indicates that such phosphors changed from orange to orange-red.

  Figure 5

  

  Figure 5.  Color coordinates of precursor and GdPO₄∶2%Sm³⁺ obtained by calcination at different temperatures

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  2.1.3   Phosphorescence lifetime

  Fig. 6 shows the luminescence decayed curves of GdPO₄∶2%Sm³⁺ for the transition from ⁴ G_5/2 → ⁶H_7/2 (599 nm). These decay curves can be fitted with Formu-la1^[26]

  Figure 6

  

  Figure 6.  Phosphorescence lifetimes of the precursor and GdPO ₄∶2%Sm³⁺ obtained by calcination at different temperatures Fig. 7 XRD patterns of GdPO₄∶Sm³⁺ doped with 0%-10% S

  Ch is the fitting coefficient

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  $ I_t=I_0+A_1 \mathrm{e}^{\frac{t}{\tau_1}}+A_2 \mathrm{e}^{\frac{t}{\tau_2}} $

  (1)

  Where I_t is the emission intensity at time t, I₀ is the emission intensity at t=0, τ₁ and τ₂ are the long life- times and short lifetimes, and A₁ and A₂ are constants. The average lifetime (τ_av) can be calculated with Formu- la 2^[27-28]:

  $ \tau_{\mathrm{av}}=\frac{A_1 \tau_1^2+A_2 \tau_2^2}{A_1 \tau_1+A_2 \tau_2} $

  (2)

  The fitting results are shown in Fig. 6. The lifetime of the calcined phosphor obtained by calcination at 1 000 ℃ (4.57 ms) is more than twice that of the precursor (1.99 ms), which further proved that the phosphor in the monoclinic system had better luminescence properties. It might be due to the different crystal structures, crystallinity, and morphology of phosphors. The change laws for the intensity of the excitation and emission spectra of the phosphor in Fig. 4 at various calcination temperatures were the same as those for the phos- phorescence lifetime of GdPO₄∶2%Sm³⁺, indicating that the luminescence properties of micro - scale phos- phors were stronger than those of nano - scale phos- phors. From the above results, the luminescence perfor- mance of the monoclinic crystal structure and uniformly dispersed sphere-like morphology GdPO₄∶2%Sm³⁺ is better. It indicates that 1 000 ℃ is the ideal calcination temperature for the phosphors.

  2.2   Study on Sm³⁺ doping concentration

  At the calcination temperature of 1 000 ℃, the effect of Sm³⁺ doping concentration was studied.

  2.2.1   Crystal structure and morphology

  The diffraction peaks in Fig. 7 were consistent with those in Fig. 1b and no other heterogeneous peaks appeared as the doping concentration was increased, demonstrating that the prepared samples were a pure phase. The doping of Sm³⁺ did not change the crystal structure of the matrix GdPO₄, and the prepared samples are monoclinic crystal systems^[13]. The characteristic peak of SmPO₄ did not appear in the XRD pattern. Because Gd³⁺ (0.105 3 nm) and Sm³⁺ (0.107 9 nm) have similar radii and crystal structures, so Sm³⁺ easily replaces the lattice of Gd³⁺.

  Figure 7

  

  Figure 7.  XRD patterns of GdPO₄∶Sm³⁺ doped with 0%-10% Sm³⁺

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  The morphology of GdPO₄ is an irregular block, with uneven size distribution. The morphologies of GdPO ₄∶1%Sm³⁺ and GdPO₄∶2%Sm³⁺ were very similar. Some are like spherical particles, while others are irregular blocks with uneven size distribution but good dispersion. The morphology of GdPO₄: 6%Sm³⁺ and GdPO ₄∶10%Sm³⁺ was an irregular block, which is similar to that of GdPO₄ (Fig. 8a). It indicates that when the Sm³⁺ doping concentration was relatively low, the morphology of the product was refined. As the Sm³⁺ doping concentration increased to 6% - 10%, the particle size of the product remained unchanged. According to the EDS analysis of Fig. 8f, GdPO₄∶2%Sm³⁺ was only composed of O, Gd, Sm, and P. From the above results, the Sm³⁺ doping concentration has little effect on the morphology of the phosphor^[15].

  Figure 8

  

  Figure 8.  (a-e) SEM images of GdPO₄∶Sm³⁺ doped with 0%-10% Sm³⁺ and (f) EDS spectrum of GdPO₄∶2%Sm³⁺

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  2.2.2   Fluorescence properties

  Fig. 9 shows that the intensity of the excitation and emission spectra of GdPO₄∶Sm³⁺ increased first and then decreased gradually with the Sm³⁺ doping concentration. The intensities of the excitation spectrum of GdPO₄∶2%Sm³⁺ reached the maximum at 330-450 nm. The strongest peak was located at 401 nm, which was the characteristic excitation peak of Sm^{3+ [29]}. The intensity of the excitation spectrum of GdPO₄∶3%Sm³⁺ at 273 nm reached the maximum, which was the characteristic excitation peak of Gd³⁺. It indicates that when the Sm³⁺ doping concentration was 3%, the characteris- tic excitation peak of Gd³⁺ was the strongest, and the absorption capacity was the highest.

  Figure 9

  

  Figure 9.  Excitation (a) and emission (b, c) spectra of GdPO₄∶Sm³⁺

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  Fig. 9b shows that the luminescence intensity of GdPO₄∶2%Sm³⁺ reached the maximum. Fig. 9c shows that under the excitation of ultraviolet light at 273 nm, the characteristic emission peaks of Gd³⁺ were obvious in GdPO₄, which were located at 590 nm (_13/2^6G → ⁶P_7/2), 596 nm (⁶G_13/2 → ⁶P_7/2), 617 nm (⁶G_7/2 → ⁶P_7/2), and 626 nm (⁶G_3/2 → ⁶P_7/2), respectively^[30]. As the Sm³⁺ doping concentration gradually increased, the emission spectra were mainly at 599 and 646 nm, which are the char- acteristic emission peaks of Sm³⁺. The intensities of the characteristic emission peaks of Gd³⁺ and Sm³⁺ increased first and then decreased gradually. The intensity of the emission peak of Sm³⁺ in GdPO₄∶2%Sm³⁺ reached the maximum. The optimal doping concentration of Sm³⁺ was 2% - 3%. The concentration quenching phenomenon appeared when the doping concentration of Sm³⁺ was higher than 2%. The emission peak of Gd³⁺ almost disappeared when the doping concentration of Sm³⁺ was higher than 3%. It indicates that Gd³⁺ effectively transferred the absorbed energy to Sm³⁺ during the lumines- cence process of GdPO₄∶Sm^{3+ [31]}, as shown in Fig. 10.

  Figure 10

  

  Figure 10.  Gd³⁺ and Sm³⁺ energy transfer in GdPO₄∶Sm³⁺

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  The color coordinates of GdPO₄∶Sm³⁺ under the excitation of ultraviolet and near - ultraviolet light at 273 and 401 nm were calculated using the CIE 1931 software. The corresponding color coordinates diagrams (Fig. 11) were drawn. Under the excitation of 273 nm UV light, the luminescence color of the phosphor was red in Fig. 11a. Under the excitation of 401 nmnear-UV light, the luminescence color of the phosphor changed from red to orange - red with the increase of Sm³⁺ doping concentration in Fig. 11b.

  Figure 11

  

  Figure 11.  Color coordinate diagrams of GdPO₄∶Sm³⁺: (a) 273 nm; (b) 401 nm

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  2.2.3   Phosphorescence lifetime properties

  Fig. 12 shows the luminescence decay curves of the ⁴G_5/2 → ⁶ H_9/2 (599 nm) transition of Sm³⁺ for GdPO ₄∶ Sm³⁺ under the excitation by 401 nm. These decay curves can be fitted with Formula 1. The fitting results are shown in Fig. 12. The phosphorescence lifetimes increased from 4.04 to 4.25 ms as the Sm³⁺ doping concentration increased from 1% to 2%. The phosphores- cence lifetime tended to decrease significantly with the increase of doping concentration when the doping con- centration was higher than 2%. The change law is con- sistent with that of luminescence intensity with Sm³⁺ doping concentration in Fig. 9. It further proved that the optimal doping concentration of Sm³⁺ was 2%. This is because the concentration quenching phenomenon occurs when the Sm³⁺ doping concentration is greater than 2%, which increases the energy transfer rate between Sm³⁺ thereby reducing the phosphorescence lifetime^[32-33].

  Figure 12

  

  Figure 12.  Phosphorescence lifetime diagram of GdPO₄∶Sm³⁺

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  2.2.4   Concentration quenching mechanism and critical distance of energy transfer

  The critical distance (D_c) between neighboring Sm³⁺ can be used to determine the type of interaction mechanism. Blasse^[34] obtained through research that the D_c of the energy transfer of rare - earth ions can be expressed with Formula 3:

  $ D_{\mathrm{c}}=2 \times \sqrt[3]{\frac{3 V}{4 \pi c N}} $

  (3)

  Where V is the volume of the crystal cell; c is the critical quenching concentration; N is the number of cations that can be replaced in the unit crystal cell. For the GdPO₄ crystal in the monoclinic crystal system, V is 0.279 9 nm³, N=4, c=0.02-0.03. D_c=1.646-1.884 nm was obtained by Formula 3. This distance is much greater than the minimum required for energy transfer between activators (0.5 nm). Therefore, it can be indicated that the interaction between electric multipoles is the cause of the concentration quenching mechanism of GdPO₄∶Sm³⁺. The concentration quenching mechanism of GdPO₄∶Sm³⁺ was s tudied further according t o Dexter′s energy transfer theory^[35-36]:

  $ I/x \propto {\left( {\beta {x^{s/3}}} \right)^{ - 1}}{\rm{ or }}\lg (I/x) = C - ( - s/3)\lg x $

  (4)

  Where β and C are constants and s is the electrical multilevel interaction index. Taking lg x as abscissa and lg(I/x) as ordinate, the slope of a straight line is-s/3.

  Fig. 13 shows that the curve of lg(I/x) and lg x was linear, with a slope of -2.3 which is closest to 2. It can be obtained that the s was 6. Therefore, the concentration quenching mechanism between Sm³⁺ is the electric dipole-electric dipole interaction.

  Figure 13

  

  Figure 13.  Relationship between lg(I/x) and lg x for GdPO₄∶Sm³⁺

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  2.3   High - temperature luminescence characteris- tics

  Fig. 14a shows the high-temperature luminescence spectra of GdPO₄•H₂O: 2%Sm³⁺ from 305 to 580 K under the excitation by 401 nm. Fig. 14a shows that as the temperature increased, the shape and peak position of the emission spectrum did not change, but the intensity changed greatly. From Fig. 14b, the luminescence intensity of the emission peak at 599 nm did not change from 305 to 320 K. The luminescence intensity suddenly rose from 420 K. The luminescence intensity at 430 K was 114% of that at 305 K. The luminescence intensity at 450 K was 238% of that at 305 K. The intensity of the emission peak at 599 nm changed slightly from 450 to 580 K. It indicates that the thermal stability of the GdPO₄ •H₂O∶2%Sm³⁺ in the hexagonal crystal system is very poor. It is mainly because the GdPO₄•H₂O∶2%Sm³⁺ in the hexagonal crystal system is unstable at high temperatures and transformed to GdPO ₄∶2%Sm³⁺ in the monoclinic crystal system^[37-38], which is consistent with the analysis results in Fig. 1.

  Figure 14

  

  Figure 14.  (a) Emission spectra of GdPO₄·H₂O∶2%Sm³⁺ at different temperatures and (b) the relationshipbetween temperature and luminescence intensity at 599 nm

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  Fig. 15a shows the luminescence spectra of GdPO₄∶2%Sm³⁺ under the excited by 401 nm at high temperatures (from 305 to 530 K). The shape and peak position of the phosphor′s emission spectrum essentially remained unchanged as the temperature rose, but the intensity gradually decreased. From 305 to 380 K, there was essentially no change in the luminescence intensity in Fig. 15b. As the temperature rose from 380 K, the luminescence intensity gradually decreased, possibly due to the thermal quenching effect^[39-40]. The lumi nescence intensity of GdPO ₄∶2%Sm³⁺ at 455 K was 87% of that at 305 K. The luminescence intensity at 530 K was 75% of that at 305 K. GdPO₄∶2%Sm³⁺ in the monoclinic crystal system had good luminescence thermal stability. By fitting the Arrhenius equation, it is possible to determine the thermal quenching activa- tion energy (ΔE) of these phosphors^[41-42]:

  Figure 15

  

  Figure 15.  (a) Emission spectra and (b) luminescence intensities at 599 nm of GdPO₄∶2%Sm³⁺ at different temperatures; (c) Relationship between ln[(I₀/I)-1] and 1/(kT)

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  $ {I_T} = {I_0}{\left[ {1 + C{\rm{exp}}\left( { - \frac{{\Delta E}}{{kT}}} \right)} \right]^{ - 1}} $

  (5)

  Where I₀ is the initial luminescence intensity, I_T is the luminescence intensity at different temperatures, C is a constant, k is Boltzmann′s constant (8.62×10^-5 eV• K^-1), and T is the temperature. Fig. 15c shows that the slope of the straight line fitted linearly was -0.157, corresponding to a ΔE value of 0.157 eV for GdPO₄∶ 2%Sm³⁺.

  2.4   Analysis of magnetic properties

  In Fig. 16, the magnetization intensity of GdPO₄• H₂O∶2%Sm³⁺ was about 1.767 emu•g^-1, and the mass susceptibility was 1.18×10-4 emu•g-1•Oe-1. The magnetization intensity of GdPO₄∶2%Sm³⁺ was about 1.836 emu•g^-1, and the mass susceptibility was 1.22×10^-4 emu•g^-1•Oe^-1. when the field intensity was 15 kOe. From the above results, the magnetic property of GdPO₄∶2%Sm³⁺ was better than that of GdPO₄•H₂O∶ 2%Sm³⁺. The magnetization intensity of GdPO₄∶ 2%Sm³⁺ had a good linear relationship with the external magnetic field. The coercivity and residual magne- tism are almost zero, demonstrating the product′s strong paramagnetism^[43-44]. The magnetism came from Gd³⁺ in GdPO₄∶2%Sm³⁺. Due to Gd³⁺ having 7 unpaired inner 4f electrons, which are tightly combined with the nucleus, effectively shielded by the outer shell electron 5_s²5 p⁶, and not disturbed by other ions. Therefore, Gd³⁺ had a high theoretical magnetic moment of 7.94 μ_B, resulting in paramagnetism^[45]. The magnetic property of the monoclinic crystal structure and uniformly dispersed sphere-like morphology of GdPO₄∶2%Sm³⁺ is better.

  Figure 16

  

  Figure 16.  Magnetization plots of GdPO₄·H₂O∶2%Sm³⁺ (a) and GdPO₄∶2%Sm³⁺(b) at 298 K

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  3.   Conclusions

  GdPO₄•H₂O∶Sm³⁺ were prepared using the hydrothermal method. Firstly, the calcination temperature was optimized, and then the doping concentration of Sm³⁺ was investigated. The results indicate that the crystal structure of phosphors was changed from the precursor GdPO₄∶Sm³⁺•H 2 O in the hexagonal crystal system to GdPO₄∶Sm³⁺ in the monoclinic crystal system. The morphology was changed from nanorods to like -sphere particles. The optimal calcination temperature was 1 000 ℃. The intensity of excitation and emission spectra of GdPO₄∶Sm³⁺ was 20 times that of the precursor, and the phosphorescence lifetime was more than 2 times that of the precursor. The optimal doping concentration of Sm³⁺ was 2%-3%, and the phosphores- cence lifetime of the optimal product was 4.25 ms. The critical distance for energy transferred of GdPO₄∶ 2%Sm³⁺ was 1.884 nm, and the concentration quenching mechanism between Sm³⁺ is the electric dipole - electric dipole interaction. GdPO ₄∶2%Sm³⁺ had good thermal stability, with the thermal quenching activation energy as - 0.157 eV. Moreover, it had good paramag- netism, and its mass susceptibility was 1.22×10^-4 emu• g^-1•Oe^-1. The multifunctional GdPO ₄∶2%Sm³⁺ with luminescence and magnetic properties can be useful in bioimaging and magnetic resonance imaging.

  
  
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• 

  Figure 1  XRD patterns of the precursor GdPO₄·H₂O∶2%Sm³⁺ (a) and GdPO₄∶2%Sm³⁺ obtained by calcination at different temperatures (b)

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  Figure 2  Lattice transformation diagram of phosphors

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  Figure 3  SEM images of the precursor GdPO₄·H₂O∶2%Sm³⁺ and GdPO₄∶2%Sm³⁺ obtained by calcination at different temperatures

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  Figure 4  Excitation (a) and emission (b, c) spectra of the precursor GdPO₄·H₂O∶2%Sm³⁺ and GdPO₄∶2%Sm³⁺ obtained by calcination at different temperatures

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  Figure 5  Color coordinates of precursor and GdPO₄∶2%Sm³⁺ obtained by calcination at different temperatures

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  Figure 6  Phosphorescence lifetimes of the precursor and GdPO ₄∶2%Sm³⁺ obtained by calcination at different temperatures Fig. 7 XRD patterns of GdPO₄∶Sm³⁺ doped with 0%-10% S

  Ch is the fitting coefficient

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  Figure 7  XRD patterns of GdPO₄∶Sm³⁺ doped with 0%-10% Sm³⁺

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  Figure 8  (a-e) SEM images of GdPO₄∶Sm³⁺ doped with 0%-10% Sm³⁺ and (f) EDS spectrum of GdPO₄∶2%Sm³⁺

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  Figure 9  Excitation (a) and emission (b, c) spectra of GdPO₄∶Sm³⁺

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  Figure 10  Gd³⁺ and Sm³⁺ energy transfer in GdPO₄∶Sm³⁺

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  Figure 11  Color coordinate diagrams of GdPO₄∶Sm³⁺: (a) 273 nm; (b) 401 nm

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  Figure 12  Phosphorescence lifetime diagram of GdPO₄∶Sm³⁺

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  Figure 13  Relationship between lg(I/x) and lg x for GdPO₄∶Sm³⁺

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  Figure 14  (a) Emission spectra of GdPO₄·H₂O∶2%Sm³⁺ at different temperatures and (b) the relationshipbetween temperature and luminescence intensity at 599 nm

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  Figure 15  (a) Emission spectra and (b) luminescence intensities at 599 nm of GdPO₄∶2%Sm³⁺ at different temperatures; (c) Relationship between ln[(I₀/I)-1] and 1/(kT)

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  Figure 16  Magnetization plots of GdPO₄·H₂O∶2%Sm³⁺ (a) and GdPO₄∶2%Sm³⁺(b) at 298 K

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•