稀有金属(英文版) 2021,40(01),113-122

Insight into crystal growth and upconversion luminescence property of tetragonal Ba₃Sc₂F₁₂ nanocrystals

Juan Xie Guang-Chao Zheng Yang-Ming Hu Farhat Nosheen Zhi-Cheng Zhang Er-Jun Liang

School of Physics and Microelectronics,Zhengzhou University

Division of Science and Technology,Department of Chemistry,University of Education

Tianjin Key Laboratory of Molecular Optoelectronic Sciences,Department of Chemistry,School of Science,Tianjin University and Collaborative Innovation Center of Chemical Science and Engineering

作者简介：Zhi-Cheng Zhang e-mail:zczhang19@tju.edu.cn;Er-Jun Liang e-mail:ejliang@zzu.edu.cn;

收稿日期：13 August 2020

基金：financially supported by the National Natural Science Foundation of China (Nos.11904323, 11874328 and 211902148);the Certificate of Postdoctoral Research Grant in Henan Province (No.1902014);

Insight into crystal growth and upconversion luminescence property of tetragonal Ba₃Sc₂F₁₂ nanocrystals

Juan Xie Guang-Chao Zheng Yang-Ming Hu Farhat Nosheen Zhi-Cheng Zhang Er-Jun Liang

School of Physics and Microelectronics,Zhengzhou University

Division of Science and Technology,Department of Chemistry,University of Education

Tianjin Key Laboratory of Molecular Optoelectronic Sciences,Department of Chemistry,School of Science,Tianjin University and Collaborative Innovation Center of Chemical Science and Engineering

Abstract：

Sc-based nanomaterials have attracted considerable attention due to their unique optical properties different from those of Ln/Y-based nanomaterials.However,studies on Sc-based nanomaterials are far from comprehensive.Particularly,nanoscale alkaline(Ca,Sr and Ba)scandium fluorides were almost ignored for their stringent synthetic conditions.Herein,we synthesize high-quality tetragonal phase Ba₃ Sc₂ F₁₂ nanocrystals with uniform morphology and good dispersibility by carefully tailoring the reaction conditions,such as the molar ratio of reactants,temperature and reaction time.Then,the upconversion(UC) luminescence property of Ba₃ Sc₂ F₁₂:Yb/Er(Ho)samples is investigated in detail.The doping concentrations of sensitizer(Yb³⁺) and activator(Er³⁺ and Ho³⁺) are optimized for the strongest UC luminescence,of which the corresponding energy transfer processes are also discussed.Moreover,tetragonal Ba₃ Sc₂ F₁₂ nanocrystals can gradually transform into hexagonal Ba₄ Yb₃ F₁₇ nanocrystals with the increase in Yb³⁺ doping content.This work provides a novel type of Sc-based nanomaterials with strong red UC emissions which are promising in high-resolution 3-dimensional color displays,laser,bioimaging and biolabels.

Keyword：

Ba₃Sc₂F₁₂ nanocrystals; Crystal growth; Upconversion; Sc-based nanomaterials;

Received： 13 August 2020

1 Introduction

Lanthanide-doped upconversion nanoparticles (UCNPs)have attracted considerable attentions as a result of their outstanding luminescent performance compared with traditional photoluminescent (PL) nanomaterials such as florescent organic dyes and quantum dots (QDs) [ 1, 2, 3, 4, 5, 6, 7] .Owning to their fine photostability,low photobleaching,long lifetimes,minimal phototoxicity and deep penetration depth [ 8, 9, 10] ,UCNPs are promising in three-dimensional (3D) flat panel displays [ 11] ,white light emitting diodes [ 12, 13] ,solar cells [ 14] ,and especially in biolabels and bioimaging [ 15, 16, 17, 18] .However,UCNPs remain suffering from weak luminescent intensity,low efficiency,high power excitation and a confined excitation wavelength during UC process [ 19, 20, 21, 22, 23] .During last decades,a lot of efforts have been made to enhance UC intensity,such as broadband sensitization [ 24, 25] ,surface passivation [ 26, 27, 28, 29] ,energy transfer modulation [ 30, 31] ,surface plasmon coupling [ 32] .In addition,exploring new host matrix is also an efficient method,because the local crystal field of host matrix and the interaction between host and doping ions have a strong impact on the UC process.As widely confirmed,host lattice determines the distance,spatial position,coordination numbers and the type of surrounding anions of doping ions [ 33, 34] .

As a recently prevalent topic,Sc-based nanomaterials have become hotspots.Sc belongs to the rare earth family.However,it has no 4f electrons and behaves more like transition metals,which endows Sc-based nanomaterials unique properties,such as different UC luminescence behaviors from those of Y/Ln-based nanomaterials and remarkable negative thermal expansion constant [ 35, 36, 37, 38, 39] .Currently,alkali (Na,K,Rb,Cs)scandium fluorides with various morphologies and size including M₂NaScF₆ (M=K,Rb,Cs),NaScF₄,Na₃ScF₆and KSc₂F₇ have been synthesized and investigated [ 40, 41, 42, 43, 44] .However,alkaline earth ions (Ca²⁺,Sr²⁺and Ba2+) owning closed ionic radii with lanthanide ions were almost ignored.Recently,Yang's group have successfully synthesized SrSc₂F₇ and Ba₃Sc₂F₁₂ crystals with various morphologies by hydrothermal process and investigated the fluorescence of the new host,which indicates that SrSc₂F₇ and Ba₃Sc₂F₁₂ are excellent host material for UC luminescence [ 45, 46] .However,the research on synthesis and UC luminescence properties of nanoscaled alkaline rare earth fluorides with uniform morphology,and good dispersibility is still absent to the best of our knowledge.

Herein,we report the precise synthesis of Ba₃Sc₂F₁₂nanocrystals by thermolysis method.The synthesis conditions,including temperature,reaction time and ratio of reactants,are optimized to control the crystal growth process and to get high-quality products.Then,we carefully explore the UC luminescence properties of Yb/Er and Yb/Ho doped Ba₃Sc₂F₁₂ nanocrystals and the corresponding energy transfer mechanisms.In order to obtain the strongest UC luminescent intensity,we modify the doping concentration of sensitizer ions Yb³⁺from 10 mol%to98 mol%and activator ions Er³⁺or Ho³⁺from 1 mol%to8 mol%.We also notice that with Yb³⁺doping concentration increasing,pure tetragonal phase Ba₃Sc₂F₁₂ can gradually transform to hexagonal phase Ba₄Yb₃F₁₇nanocrystals.

2 Experimental

2.1 Materials

Sc₂O₃ (99.99%),Yb₂O₃ (99.99%),Er₂O₃ (99.99%),Ho₂0₃(99.99%) and Ba(CH₃COO)2 (99.99%) were purchased from HWRK Chem Co.Ltd.(Beijing China).CF₃COOH(99%) was obtained from Lingfeng Chemical Reagent Co.Ltd.(Shanghai China).Aleic acid (OA,90%) and 1-octadecene (ODE,90%) were purchased from SigmaAldrich.All the materials were used directly without further purification.

2.2 Characterization

Powder X-ray diffraction (XRD) data were recorded via a Rigaku D/max 2550 with Cu Ka radiation(λ=0.15418 nm).The morphology and size of nanocrystals were analyzed by the Hitachi 7700 transmission electron microscope (TEM) with an acceleration voltage of100 kV.High-resolution TEM (HRTEM) images were obtained through Tecnai G2 F20 field emission transmission electron microscope (FESEM) equipped with an energy-dispersed X-ray system (EDX) operating at200 kV.Inductively coupled optical emission spectrometry(ICP-OES) analysis was performed on a PE Optima5300DV spectrometer.The UC luminescence spectra were collected using an Edinburgh F920 spectrometer with an external continues wave diode 980 nm laser at room temperature.All luminescence studies were carried out under identical conditions.

2.3 Synthesis of RE(CF3COO)3 precursors (RE=Sc,Yb,Er,Ho)

In a typical synthesis,RE₂O₃ (2 mmol) was dissolved in2 ml deionized water under vigorous magnetic stirring.CF₃COOH was added to above mixture at room temperature.Then,the solution was heated to 100℃and kept for12 h until the solution became transparent.The final products were dried in a vacuum oven at 60℃for 24 h.

2.4 Synthesis of Ba3Sc2F12:Yb/Er nanocrystals

In a typical synthesis,0.200 mmol Ba(CH₃COO)2,0.312 mmol Sc(CF₃COO)3,0.080 mmol Yb(CF₃COO)3and 0.008 mmol Er(CF₃COO)3 were added into a 100 ml three-necked round-bottom flask containing 17 ml oleic acid and 3 ml octadecene.The mixture was heated to120℃and kept for 30 min with magnetic stirring under an argon atmosphere.Subsequently,the mixture was heated to310℃and maintained for 50 min before cooling down to room temperature.Finally,the as-prepared samples were precipitated by addition of ethanol,collected by centrifugation at 9000 r-min^-1 for 5 min,washed with water and ethanol several times,and finally redispersed in cyclohexane for further characterization.

3 Results and discussion

Ba₃Sc₂F₁₂ and lanthanide ions doped Ba₃Sc₂F₁₂nanocrystals were prepared via the modified thermolysis method [ 47] .The crystal growth process is clearly presented in Fig.1,where aleic acid (OA) was used as surfactant and noncoordinating 1-octadecene (ODE) as reaction solvent for its high boiling point.The carboxyl group of OA can coordinate the rare earth elements,while the long hydrocarbon chain acts as a passivating ligand which can prevent the final nanoparticles from agglomeration.Then,by carefully tailoring experimental conditions,high-quality Ba₃Sc₂F₁₂ nanocrystals with great dispersibility,narrow size distribution and good crystallinity could be readily acquired.

Fig.1 Scheme of growth process of Ba₃Sc₂F₁₂ and Ba₃Sc₂F₁₂:Ln (Ln=Yb,Er,Ho) nanocrystals via thermolysis method

Figure 2a 1-el shows the effect of Ba²⁺content on the morphology and crystal structure of Ba₃Sc₂F₁₂:Yb/Er nanocrystals.At low Ba²⁺content,most of the products were irregular nanoparticles and only few tetragonal nanosheets were observed (Fig.2a 1).XRD data indicate that the products are mixture of cubic phase ScF₃ and tetragonal phase Ba₃Sc₂F₁₂ (Fig.2el).With the increase in Ba²⁺content,pure tetragonal phase Ba₃Sc₂F₁₂ could be acquired.When the molar ratio of Ba²⁺to Sc³⁺was 0.50,uniform and well dispersed Ba₃Sc₂F₁₂ nanosheets were synthesized and the particle size was about 35 nm×35nm.However,at high Ba²⁺content (Ba:Sc=2.00),majority of products were tetragonal nanosheets with the size of 5 mm×5 nm.These results indicate that Ba²⁺content has considerable influence on the shape and phase evolution of Ba₃Sc₂F₁₂ nanocrystals and that appropriate Ba²⁺content is crucial to the synthesis of uniform and pure Ba₃Sc₂F₁₂ nanocrystals.

The morphology and phase development caused by Ba²⁺content can be explained by the crystallization speedcontrolled model [ 48, 49, 50] .The nanocrystal growth was initiated with the nucleation process of Ba²⁺,Sc³⁺and F^-.In addition,the change of monomer concentration in the solution is mainly caused by the nucleation process,while the following growth process has relatively slight impact.The precursors in our experiments were Ba(CH₃COO)2 and Sc(CF₃COO)3,while the ratio of F^-to Sc³⁺was set as 3,so the ratios of F^-to Ba²⁺and Ba²⁺to Sc³⁺changed depending on the amount of Ba²⁺added.Low concentration of B a²⁺shall decelerate the particle crystallization rate and produce relatively small number of nuclei,leading to a high monomer concentration,so highly uniform tetragonal Ba₃Sc₂F₁₂ nanosheets were obtained under Ba²⁺:Sc³⁺=0.50.However,according to the mechanism of chemical reaction equilibrium,Ba²⁺content was too low to form Ba₃Sc₂F₁₂ when the ratio of Ba²⁺to Sc³⁺was 0.25,so the cubic phase ScF₃ was obtained.On the contrary,high Ba²⁺content can enhance the crystallization speed which results in rapid depletion of RE(CF₃COO)3 in the solution.When the monomer concentration was reduced to a critical level rapidly,Ba₃Sc₂F₁₂ nanocrystals with smaller size and worse erystallinity appeared.

Then,the effect of the reaction temperature on the morphology and crystal structure of Ba₃Sc₂F₁₂ nanocrystals were explored.Corresponding results are listed in Fig.2a2-e2.At 280 and 290℃,most of the products were of irregular morphology and only little tetragonal Ba₃Sc₂F₁₂ nanosheets were obtained.XRD results in Fig.2e2 show that the products were not pure tetragonal phase.As the reaction temperature increased to 300℃,pure tetragonal phase was acquired.However,the morphology was not uniform and many small round Ba₃Sc₂F₁₂nanosheets were still obtained.XRD results show that the products were only pure tetragonal phase.Further increase the temperature to 310℃,uniform tetragonal nanosheets with an average size of 35 nm x 35 nm were acquired.Herein,310℃was suitable temperature to synthesize uniform Ba₃Sc₂F₁₂ nanocrystals.

Fig.2 Ba₃Sc₂F₁₂ nanocrystals synthesized under different reaction conditions:al-dl TEM images and el XRD patterns of Ba₃Sc₂F₁₂nanocrystals synthesized under different ratios of Ba to Sc (0.25,0.50,1.00,2.00);a2-d2 TEM images and e2 XRD patterns of Ba₃Sc₂F₁₂nanocrystals synthesized at 280,290,300 and 310℃,respectively;a3-h3 TEM images and i3 XRD patterns of Ba₃Sc₂F₁₂ nanocrystals synthesized at different reaction time (0,10,20,30,40,50,60,90 min)

In order to track the morphology and phase evolution of Ba₃Sc₂F₁₂ nanocrystals,we monitored the nanocrystal growth process.As is shown in Fig.2a3-h3,when the reaction time was set as 0,10 or 20 min,the products were composed of irregular small nanoplates and little small tetragonal nanosheets.Corresponding XRD results indicate the existence of impurities (Fig.2i3).When the reaction was prolonged to 30 min,pure tetragonal phase Ba₃Sc₂F₁₂nanocrystals could be obtained,which can be seen from the XRD patterns.TEM images show fewer small irregular and more uniform tetragonal nanosheets as the time increased.However,when the reaction time was prolonged to 90 min,the corners and tips of tetragonal nanosheets were smoothed and TEM images show irregular nanoparticles.TEM images of Ba₃Sc₂F₁₂ nanocrystals at different reaction time in Fig.2a3-h3 obviously demonstrate a typical Ostwald ripening process [ 51, 52] .In addition,50 min was enough for Ba₃Sc₂F₁₂ nanocrystals ripening process.

Based on the above results and discussion,we knew that the suitable ratio of Ba²⁺to Sc³⁺was 0.5,the best reaction temperature was 310℃and 50 min was enough for the synthesis of Ba₃Sc₂F₁₂ nanocrystals in current system.

To explore the UC luminescence property and to get the strongest UC luminescence intensity of Ba₃Sc₂F₁₂nanocrystals,we synthesized a series of Ba₃Sc₂F₁₂:Yb/Er(x/2 mol%) nanocrystals under above optimized conditions of which the ratio of Ba²⁺to Sc³⁺is 0.50,reaction temperature is 310℃and reaction time is 50 min.The doping content of sensitizer ion Yb³⁺was tuned from 10 mol%to98 mol%firstly.Corresponding TEM images and XRD patterns are listed in Fig.3.When the concentrations of Yb³⁺were 10 mol%-30 mol%,the products obtained were pure tetragonal phase B a₃Sc₂F₁₂ nanocrystals with uniform size.With the concentration of Yb³⁺further increasing to40 mol%-50 mol%,the products became big tetragonal nanosheets mixed with irregular small nanosheets,which were tetragonal phase Ba₃Sc₂F₁₂ and tetragonal phase Ba₄Yb₃F₁₇,respectively.When the concentration of Yb³⁺was more than 60 mol%,the products were pure tetragonal phase Ba₄Yb₃F_17.Inductively coupled plasma mass spectroscopy (ICP-MS) analysis was carried out to characterize the actual concentration of Yb³⁺doped into the as-synthesized Ba_mSc_1-xF_n:Yb/Er (x/2 mol%) nanocrystals,which are listed in Table S1.Considering the same charge and adjacent ionic radii of Sc³⁺(0.0745 nm) and Yb³⁺(0.0868 nm),we believe that Yb³⁺has substituted Sc³⁺.Figure S1 shows the evolution of (310) diffraction patterns of B a₃Sc₂F₁₂:Yb/Er (x/2 mol%).As Yb³⁺substitutes Sc³⁺,the unit cell of Ba₃Sc₂F₁₂ would swell,leading to the shift of XRD peak to small angles according to the Bragg's law,which is consistent with XRD results.

To prove the transformation process from pure tetragonal phase Ba₃Sc₂F₁₂ to hexagonal Ba₄Yb₃F₁₇,TEM and high-resolution TEM (HRTEM) images of 20 mol%,40 mol%and 80 mol%Yb³⁺doped samples are presented in Fig.S2.The HRTEM image of 20 mol%doped sample reveals a highly crystalline nature and the interplanar distance between the adjacent lattice fringes is 0.299 nm,which matches well with (310) lattice planes of tetragonal phase Ba₃Sc₂F_12.The HRTEM image of 80 mol%Yb³⁺doped product shows that the interplanar distance between the adjacent lattice fringes is 0.294 nm,which agrees well with (214) lattice planes of hexagonal phase Ba₄Yb₃F_17.While 40 mol%Yb³⁺doped products are the mixture of tetragonal phase Ba₃Sc₂F₁₂ and hexagonal Ba₄Yb₃F₁₇nanocrystals.The two crystal structures grow independently and exist as a physical mixture rather than,for example,as core-shell particles [ 39] .

Similar results were also observed when 2 mol%Ho³⁺ions were used as activator.Corresponding TEM images and XRD patterns are listed in Fig.S3,which indicate that Ba_mSc₁-xF_n:Yb/Ho (x/2 mol%) nanocrystals transform from pure tetragonal Ba₃Sc₂F₁₂:Yb/Ho to Ba₄Yb₃F₁₇:Sc/Ho with Yb³⁺concentration varying from 0 mol%to98 mol%.

Figure 4a displays UCPL emission spectra of Ba_mSc_1-xF_n:Yb/Er (x/2 mol%) nanocrystals with variable Yb³⁺concentration from 0 mol%to 98 mol%.All samples exhibit strong red UC luminescence,which is contrary to NaYF₄:Yb/Er (20/2 mol%) nanocrystals that generate strong green UC emission under 980-nm laser excitation.The UC emission intensity of the products also changed with the variation of Yb³⁺concentration.When Yb³⁺concentration varied from 0 mol%to 25 mol%,the UC luminescence intensity of Ba₃Sc₂F₁₂:Yb/Er nanocrystals increased gradually.As is known to us all,Yb³⁺has a sufficient absorption cross section matching well with common 980-nm laser excitation source.Herein,we reason that introduction of an elevated amount of Yb³⁺dopants in the Ba₃Sc₂F₁₂ host lattice decreases Yb³⁺-Er³⁺interatomic distance and thus facilitates energy transfer from Yb³⁺to Er³⁺.In addition,high Yb³⁺contents help sustain multi-step excitation without depletion of the intermediate,which will benefit UC luminescence process.However,as the Yb³⁺ions content further increased from 25 mol%to98 mol%,the luminescent intensity fell down quickly as a result of concentration quenching phenomenon,which means that a high Yb³⁺content leads to an increased possibility of long-distance energy transfer that takes excitation energy to lattice or surface defects instead of luminescence [ 53, 54] .So,for Ba₃Sc₂F₁₂:Yb/Er system,the optimum Yb³⁺concentration for efficient UC under980-nm excitation was 25 mol%.

Then,we optimized Er³⁺concentration.ICP-MS analysis of the actual concentration of Er³⁺doped into the assynthesized Ba₃Sc₂F₁₂:Yb/Er (25/x mol%) nanocrystals is given in Table S2.Figure 4b shows the UC luminescence spectra of Ba₃Sc₂F₁₂:Yb/Er (25/x mol%) nanocrystals with variable Er³⁺contents ranging from 1 mol%to 8 mol%.As an elevated amount of Er³⁺dopants will increase the amount of UC emission centers,the UC intensity becomes stronger and stronger as the Er³⁺contents increase from1 mol%to 4 mol%.However,more than 4 mol%Er³⁺doping would lead to the decrease in intensity because high doping level could inevitably lead to localized concentration quenching of activator emissions caused by enhanced cross relaxation between the lanthanide ions.Herein,the optimal Er³⁺content is 4 mol%.

Fig.3 a-1 TEM images and m,n XRD patterns of Ba_mSc_1-xYb_xF_n:Er (2 mol%) with different amounts of Yb³⁺doped (x=10 mol%-98 mol%)

Fig.4 a UC emission spectra of Ba_mSc_1-xYb_xF_n:Er (2 mol%)(x=0,10,15,20,25,30,40,50,60,70,80,90 and 98,mol%);b UC emission spectra of Ba₃Sc₂F₁₂:Yb/Er (25 mol%/x mol%)(x=1,2,4,6 and 8,mol%) under the excitation of 980-nm NIR diode laser at room temperature

For the case of activator Ho³⁺,we also optimized the doping content of Yb³⁺.The UC luminescence spectra of Ba₃Sc₂F₁₂:Yb/Ho (x/2 mol%) nanocrystals are exhibited in Fig.S4a,which shows that the optimal Yb³⁺content was30 mol%.Then,we changed the concentration of Ho³⁺from 1 mol%to 8 mol%,finding that the optimal Ho³⁺content for Ba₃Sc₂F₁₂:Yb/Ho (30/x mol%) was 4 mol%,of which the results are shown in Fig.S4b.The actual concentrations of Yb³⁺and Ho³⁺doped into the as-synthesized Ba₃Sc₂F₁₂:Yb/Ho samples are listed in Tables S3,S4,respectively.

Fig.5 a 1g-lg plots of UC emission intensity in 648 nm (red) and 548 nm (green) intensity for Ba₃Sc₂F₁₂:Yb/Er (25 mol%/4 mol%)nanocrystals;b luminescent decay curves of Er³⁺in 648 nm (red) and 545 nm (green);c proposed UC mechanism under excitation of 980-nm NIR diode laser at room temperature (E:energy)

To further understand the underlying UC mechanism of Ba₃Sc₂F₁₂:Yb/Er (25/4 mol%),we explored the number of photons involved in the UC luminescence process (Fig.5a)and UC lifetime (Fig.5b).According the equation I∞Pⁿ,where I represents the UC luminescence intensity,P is the pumping power,and n is the number of required pumping photons;^theemission intensity (1) ^is proportional to Pⁿ.The logarithmic curves between the UC emission intensity and pumping power are shown in Fig.5a,where the slope of each curve denotes the number of photons required.Based on the value of n presented in Fig.5a,the UC emissions of 545 and 654 nm all follow a two-photon process.The decay time of energy transition from ⁴S_3/2 and4F_9/2 to ⁴I_15/2 for Er³⁺is 0.54 and 0.41 ms,respectively.Figure 5c shows the proposed mechanism of UC emission spectra of Ba₃Sc₂F₁₂:Yb/Er (25/4 mol%) nanocrystals.Under 980-nm laser excitation,Yb³⁺can be excited to ⁴F_5/2level,from where it transfers its energy to the nearby Er3+41_11/2 state and returns to its ground state.Populating on the metastable level ⁴I_11/2,Er³⁺can be either excited to its higher state ⁴F_7/2 via absorbing another photon or relax to its lower state ⁴I_13/2 via non-radiative transition from the level ⁴I_13/2,Er³⁺can also be excited to its higher state ⁴F_9/2by absorbing a 980-nm photon.The transition of Er³⁺at4F_9/2 state to the ground state gives rise to a 654-nm photon,while the Er³⁺at ⁴F_7/2 state will relax to ²H_11/2^and4S_3/2,from where they emit 525,545 nm photons and transit back to the ground state ⁴I_15/2.For Ho³⁺,the red(654 nm) and green (545 nm) emission originates from the characteristic radiative transition from ⁵F₅ and ⁵F₄ of Ho³⁺to its ⁵I₈ level and their decay time are 0.35 and 0.25 ms,respectively,as depicted in Figure S5.

4 Conclusion

In summary,high-quality tetragonal phase Ba₃Sc₂F₁₂nanosheets with uniform morphology and good dispersibility have been obtained via the thermal decomposition method.Then,we adjusted the doping contents of sensitizer Yb³⁺and activator ions Er³⁺or Ho³⁺to get the strongest UC luminescent intensity,demonstrating that the optimum doping concentrations for Yb³⁺/Er³⁺and Yb³⁺/Ho³⁺are 25 mol%/4 mol%and 30 mol%/4 mol%,respectively.Interestingly,when the Yb³⁺doping concentration is more than 60 mol%,final products could transform from pure tetragonal phase Ba₃Sc₂F₁₂ to hexagonal Ba₄Yb₃F₁₇ nanocrystals.This study would provide a new kind of UC nanomaterials with strong red emission,and further complements the investigation of Scbased nanomaterials,which may have promising applications that the Ln/Y-based materials don't possess.

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