CrystEngComm

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PAPER View Article OnlineView Journal | View Issue

a State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China.

E-mail: hpyou@ciac.ac.cnbUniversity of the Chinese Academy of Sciences, Beijing 100049, PR China

Electronic supplementary information (ESI) available: Additional graphics.See DOI: 10.1039/c3ce41598h

9930 | CrystEngComm, 2013, 15, 99309937 This journal is The Ro

Cite this: CrystEngComm, 2013, 15,9930

Received 11th August 2013,Accepted 2nd October 2013

DOI: 10.1039/c3ce41598h

www.rsc.org/crystengcomm

Tysonite type Gd1yCayF3y solid solution:hydrothermal synthesis and luminescence properties

Qi Zhao,ab Baiqi Shao,ab Wei L,a Yongchao Jia,ab Wenzhen Lv,ab Mengmeng Jiaoab

and Hongpeng You*a

The monodisperse submicroplates of Gd1yCayF3y solid solution were successfully synthesized by a simple and

facile hydrothermal method for the first time. The structure of the as-synthesized Gd1yCayF3y was determined to be

tysonite-type instead of the usual fluorite-type via a Rietveld refinement method with GSAS. Detailed contrast exper-

iments revealed that the addition amounts of ethylene glycol, calcium chloride, and sodium fluoroborate played

critical roles in the formation of the Gd1yCayF3y solid solution. Desirable multicolor outputs were successfully

realized by doping activator ions (Eu3+, Tb3+, and Dy3+) in the Gd1yCayF3y crystals where the Gd3+ ions served as an

energy intermediate. Furthermore, the energy transfer from the Gd3+ to Eu3+ ions in the obtained Gd1yCayF3y solid

solution was more efficient than that in the orthorhombic GdF3.

Introduction

Inorganic fluoride is an important group of functional mate-rials in metallurgy, isotope separation, catalysis, and optics.1

The AEF2REF3 systems (AE = alkaline-earth element, RE = rareearth element) are of particular significance among the otherfluorides due to their roles in solid-state lasers, scintillators,fiber optics, high resolution color displays, etc.2 In recent years,nano- and submicro-scale alkaline-earth lanthanide manifoldfluorides have drawn increasing attention as an efficient lumi-nescent materials because of their prospective applications inbio-labeling, bio-imaging, drug delivery, and so on.38 Themost famous example was the nanoparticles of BaREF5 whichhave been prepared by thermolysis of trifluoroacetates inorganic solvents,9,10 hydro/solvothermal method,11,12 thermaltreatment of glasses,13,14 etc. Besides, Wang's group synthe-sized nanocubes of AEF2REF3 solid solution (RE = La, Ce, Pr;AE = Ca, Sr, Ba) via the liquidsolid-solution (LSS) technique.15

Other micro-scale AE1xRExF2+x solid solutions have also beenprepared, although their morphologies were usually irregular.16

The alkaline-earth lanthanide fluorides nanocrystals previouslyreported were almost all fluorite structure which was formedby dissolving REF3 in AEF2. As is known, the fluorite, or CaF2,structure (Fm3m space symmetry group) has a good isomor-phous capacity to absorb heterovalent admixtures and the

solubility limits of alkaline-earth fluorides in CaF2 type struc-tures are high.2,17 However, solid solution RE1yAEyF3y whichwas obtained through solving AEF2 in REF3 matrix was lessinvestigated because the composition interval of homogeneitywas much narrower compared with the CaF2-type.

18

In the earlier literature, the RE1yAEyF3y solid solution hasbeen fabricated by annealing and quenching methods andgrowth of single crystals.17,19 These procedures have severaldisadvantages such as energy- and time-consuming procedure,complex experimental setups, higher operating temperature,and uncontrolled size of the products. By contrast, thesolution-based method shows technical superiority becauseof the energy economy, inexpensive facilities, lower reactiontemperature, and size- and morphology-selective growth. Inview of these advantages, the solution-based route was usuallyused in the synthesis of high-quality nano- or micro-particles.However, as a result of the complex chemical environmentand intricate growth mechanism, compounds that can besynthesized in a controlled manner via the solution methodwere limited. To the best of our knowledge, there was noreport on the synthesis of RE1yAEyF3y solid solution via thesolution-based method.

In the present work, we synthesized uniform submicro-plates of Gd1yCayF3y solid solution through a facile hydro-thermal process. The obtained Gd1yCayF3y was determinedto be tysonite-type structure by a Rietveld refinement methodwith GSAS. A series of control experiments were carried outto investigate the effect of the ethylene glycol, calciumamount, and fluorine amount on the phase and morphology.Furthermore, Eu3+, Tb3+, and Dy3+ were singly doped into thecrystals as the sensitizer to investigate the luminescenceproperties of as-synthesized Gd1yCayF3y.

yal Society of Chemistry 2013

http://dx.doi.org/10.1039/C3CE41598Hhttp://pubs.rsc.org/en/journals/journal/CEhttp://pubs.rsc.org/en/journals/journal/CE?issueid=CE015046

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Experimental sectionReagents

The rare-earth oxides RE2O3 (RE = Gd, Eu, Tb, and Dy)(99.99%) were purchased from Shanghai Yuelong Non-FerrousMetals Limited. The other analytical chemicals (calciumchloride, ethylene glycol, and sodium fluoroborate) were pur-chased from Beijing Chemical Co. and used as received with-out further purification. Rare-earth chloride stock solutionswere prepared by dissolving the corresponding metal oxidein hydrochloric acid under heating with agitation. The excesshydrochloric acid was evaporated until the pH value was 4.

Preparation

In a typical procedure, 1 mL of GdCl3 (1 mol L1) and 1 mL

CaCl2 (1 mol L1) aqueous solution were added to mixed

solution of ethylene glycol (20 mL) and water (10 mL). Aftervigorous stirring for 10 min, 8 mL of aqueous solutioncontaining 1.5 mmol (0.15 g) of NaBF4 was introduced intothe solution. When the amount of ethylene glycol and CaCl2was varied, the addition of water was changed accordinglyto ensure 40 mL of the whole volume of the solution. Afteradditional agitation for 15 min, the feedstock was transferredto a 50 mL Teflon-lined stainless autoclave and heated at160 C for 6 h. When the autoclave was cooled to room tem-perature naturally, the precursors were separated by centrifu-gation, washed with ethanol and deionized water severaltimes, and dried at 60 C in air.

A similar process was employed for preparing lanthanide-doped Gd1yCayF3y. A stoichiometric amount of LnCl3 (Ln =Eu, Tb, and Dy) was added at an initial stage with GdCl3 andother processes the same afterwards.

The detailed addition amount in the contrast experimentswas listed in Table S1, ESI.

Fig. 1 Rietveld refinement of the powder X-ray diffraction profile. The red solid linesare calculated intensities, and the black crosses are the observed intensities. The short

vertical lines show the position of Bragg reflections of the calculated pattern. The

bottom line in blue shows the difference between observed and calculated data.

Characterization

The samples were characterized by powder X-ray diffraction(XRD) performed on a D8 Focus diffractometer (Bruker). Thesize and morphology of the samples were inspected using afield emission scanning electron microscope equipped withan energy-dispersive spectrometer (EDS) (FE-SEM, S-4800, Hitachi,Japan). Photoluminescence (PL) excitation and emission spec-tra were recorded with a Hitachi F-4500 spectrophotometerequipped with a 150 W xenon lamp as the excitation sourceat room temperature.

Elemental analyses of Gd and Ca in the solid sampleswere carried out using inductively coupled plasma-opticalemission spectroscopy (ICP-OES) (iCAP 6300, Thermo Scientific,USA). Gd(III) and Ca(II) were analyzed at wavelengths of 335.0and 396.8 nm, and the quantitation limits were 0.01 and0.001 g mL1, respectively. The solid samples were treatedas follows: (1) 0.05 g powder sample was mixed with 2 mLHClO4. (2) This mixture was heated at 180 C until whitesmoke disappeared. (3) The residue was dissolved by 2 mL

This journal is The Royal Society of Chemistry 2013

HNO3. (4) The solution was made up to 25 mL with deion-ized water and stored at 4 C until analysis.

Results and discussionCrystal structure and morphology

To give an insight into the structural characteristics, the energydispersive X-ray spectroscopy (EDS) measurement of the typicalsample was initially carried out to confirm the presence ofthe Ca, Gd, and F (Fig. S1d) and the mole ratio of Gd : Cawas determined to be 0.9003 : 0.0997 by ICP spectrometry.Accordingly, a typical sample of the ternary fluoride can bepresumed as a composition of Gd0.9Ca0.1F2.9. The crystalstructure and phase purity were identified by powder X-raydiffraction analysis. The XRD patterns were not indexedin orthorhombic GdF3 or a cubic fluorite-type structure.Unexpectedly, all of these peaks were coincident with thetysonite-type REF3 (RE = LaNd) except slight shifting towardsthe higher 2 side. Taking account of the contraction of the cellparameter resulting from the smaller radius of Gd3+ ions, weassumed that the Gd0.9Ca0.1F2.9 was CaF2GdF3 solid solutionwhich was characterized by the unordered distribution ofthe Gd3+ and Ca2+ ions in the cation sites of the tysonite-typestructure. To verify our hypothesis, Rietveld refinement wasperformed by the least-squares method using the GSAS soft-ware suite with an initial model of LaF3 (ICSD 34108). Theobserved and calculated plots of the Rietveld refinement, aswell as the difference between them, are shown in Fig. 1. Therefinement confirms that the single-phase nature of the com-pound is hexagonal in space group P63/mmc (194). The Ca

2+

and Gd3+ statistically occupy the 2c sites and there are twodifferent kinds of crystallographically distinguishable sitesfor fluorine, namely 2b and 4f. The partial substitution ofthe Gd3+ ion by the lower valent Ca2+ creates a correspond-ing F deficiency in the anion sublattice. Details of the crystalstructure and the equivalent isotropic parameters are givenin Table 1.

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Table 1 Details of the crystal structure and equivalent isotropic parametersa

Atom Site x y z Occupancy Uiso (2)

Gd 2c 1/3 2/3 1/4 0.9 0.0250Ca 2c 1/3 2/3 1/4 0.1 0.0250F1 2b 0 0 1/4 0.98 0.0023F2 4f 1/3 2/3 0.059059 0.96 0.0531

a Space group: P63/mmc (194); lattice parameters: a = b = 3.9730(2) ,c = 7.0656(3) , = = 90, = 120, V = 96.59(1) 3; Rp = 1.83%, Rwp= 2.52%.

Fig. 2 (a, b) SEM, (c) TEM, and (d) HRTEM images of a typical Gd0.9Ca0.1F2.9sample.

Fig. 3 XRD patterns of samples prepared at different EG/H2O ratios: (a) 40/0, (b)10/30, (c) 15/25, (d) 25/15, (e, f) 30/10.

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The representative panoramic SEM image shown in Fig. 2ademonstrates that the product was composed of monodispersesubmicroplates with diameter of 0.71.1 m. The high-magnification SEM and TEM images (Fig. 2b, c) further showthat some raised dots grow out from the centers of thesubmicroplates. The HRTEM image reveals the highly crys-talline nature and shows an interplanar spacing of 0.345 nmcorresponding to the {1120} planes, confirming the hexagonalstructure.

Fig. 4 SEM images of samples prepared at different EG/H2O ratios: (a) 40/0, (b)10/30, (c) 15/25, (d) 25/15, (e, f) 30/10.

Effect of the ethylene glycol

Ethylene glycol with two hydroxyl (OH) groups is an impor-tant addictive in the synthesis of nanomaterials, since it cancombine with various metal cations and release the cationsgradually in the reaction process, thus having great influenceon the crystal growth.20,21 When no ethylene glycol was added,orthorhombic GdF3 was yielded which displays microplateswith rough surface (Fig. 3a and 4a). A close observation revealsthat the microplates have irregular holes in their centers. If10 mL ethylene glycol was introduced, the products turnedout as hexagonal Gd1yCayF3y (Fig. 3b). The correspondingSEM image shows that the morphology transforms to well-dispersed plates with the diameter of 1.01.3 m with dotsin their centers (Fig. 4b). Further increasing the EG/H2O ratioto 15/25 and 25/15, the products both exhibited submicroplateswith hexagonal structure (Fig. 3c, d and Fig. 4c, d). One canobserve that the surfaces were smoother and the sizes of the

9932 | CrystEngComm, 2013, 15, 99309937

plates decreased to 0.81.1 m and 600750 nm, respectively.The addition of ethylene glycol may increase the viscosity of thereaction system and inhibit the diffusion of the solutes, therebysuppressing the crystal growth and eventually resulting insmall size. However, more ethylene glycol (30 mL) resulted inthe mixed phases of cubic CaF2 and hexagonal Gd1yCayF3y(Fig. 3e). The SEM images show micropolyhedrons mixed withnanoparticles (marked by the arrow), confirming the presenceof two different phases (Fig. 4e, f). As presented in Fig. 3e, theXRD peakss of CaF2 are much sharper than those of the Gd1yCayF3y, indicating that the size of the former was larger.Accordingly, the micropolyhedrons were supposed to be CaF2and the nanoparticles were identified as Gd1yCayF3y.

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On the basis of the analysis above, it is concluded that theethylene glycol played an important role in the formation ofsolid solution Gd1yCayF3y. As is known, the solubility prod-uct constant (Ksp) of GdF3 (5.0 10

18)22 is much lower thanthat of CaF2 (3.9 10

11).23 Thus, the precipitation of GdF3was predominant in aqueous solution when a fluorine sourcewas added. After ethylene glycol was introduced to the sys-tem, the Gd3+ and Ca2+ ions were surrounded and protectedby EG molecules through forming complexes, decreasing theconcentration of free cations in the initial solution.24 Uponhydrothermal treatment, the F ions from the hydrolysis ofNaBF4 would attack the complexes and replace the EG mole-cules to form Gd1yCayF3y solid solution. During this pro-cess, the slow release of metal ions favored the simultaneousprecipitation of the Gd3+ and Ca2+ by F ions, contributing tothe formation of Gd1yCayF3y solid solution.

The decomposition rate of the complex of metal and ethyl-ene glycol was strongly dependent on the addition amount ofEG and the stability of the complex. Obviously, the more EGmolecules the cations were surrounded by, the less vulnera-ble the complex would be. Furthermore, the complex of Gd3+

and ethylene glycol was supposed to be more stable thanthe complex of Ca2+ and ethylene glycol, thus the Ca2+ ionswere more easily released than the Gd3+ ions. This issupported by the fact that separated CaF2 appeared when theEG addition was up to 30 mL (Fig. 3e). Summarily, the hexag-onal Gd1yCayF3y could form when the EG/H2O ratio was inthe interval of 10/3025/15.

Effect of the calcium amount

With regard to the synthesis of complex fluorides, the ratioof starting materials has a significant impact on the chemicalcomposition and phase structure of the product.25 In ourcase, orthorhombic hollow-plate-like GdF3 was obtained in theabsence of calcium (Fig. 5a and 6a), which is similar to the pre-vious report.26 With addition of 0.2 mmol CaCl2, the products

Fig. 5 XRD patterns of samples obtained with different amounts of CaCl2:(a) 0 mmol, (b) 0.2 mmol, (c) 0.4 mmol, (d) 0.8 mmol, (e) 1.5 mmol, (f) 2 mmol.

This journal is The Royal Society of Chemistry 2013

were orthorhombic GdF3 which showed aligned submicrodisksarrays in the face-to-face tightly stacking style (Fig. 5b and 6b).The self-assembly of the disks may be attributed to the excessCa2+ adsorbed on the surface.27 It should be noted that therelative intensity of the diffraction peak corresponding to (020)planes was much stronger than the standard line, indicatingthat the growth of the (020) planes was enhanced. This isbecause more calcium cations preferentially adsorb on thelattice planes where the density of fluoride atoms is higher,inhibiting the reactant ions diffusion and resulting in theanisotropic growth.27 The phenomenon was also observedin the situation where orthorhombic GdF3 was obtainedwith EG/H2O ratios of 0/40 (Fig. 3a). By contrast, the relativeintensities of reflection lines showed no difference from thestandard profiles when no CaCl2 was added, confirming thatthe calcium cations caused the oriented growth. When theCaCl2 amount increased to 0.4 and 0.8 mmol, the samplechanged to pure hexagonal structure (Fig. 5c, d) and the mor-phology transformed to submicroplate with dots (Fig. 6c, d).The presence of Ca was verified by the EDS analysis (Fig. S1).This indicated that the submicroplates crystallized in solidsolution Gd1yCayF3y when the CaCl2 addition was morethan 0.4 mmol. However, cubic CaF2 was formed together withGd1yCayF3y if the calcium amount was further increased to1.5 and 2 mmol (Fig. 5e, f). The SEM observations show thatsome nanoparticles with a diameter of about 10 nm areintermixed with the submicroplates for the specimen with cal-cium at 1.5 mmol (Fig. 6e). When the calcium addition was2 mmol, these submicroplates became irregular and somemicropolyhedrons were clearly observed in the SEM image(Fig. 6f). Themixedmorphologies indicated the biphase natureof the products. The elemental analysis shows that the actual

Fig. 6 SEM images of samples obtained with different amounts of CaCl2:(a) 0 mmol, (b) 0.2 mmol, (c) 0.4 mmol, (d) 0.8 mmol, (e) 1.5 mmol, (f) 2 mmol.

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Fig. 8 XRD patterns of samples prepared with different reagent concentrations(denoted with the amount of GdCl3): (a) 0.3 mmol, (b) 0.5 mmol, (c) 2 mmol, and

(d) 3 mmol.

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mole ratio of Gd : Ca was 0.9317 : 0.0683, 0.9199 : 0.0801, and0.9003 : 0.0997 for the CaCl2 amount of 0.4, 0.8, and 1.0 mmol,respectively. The composition interval of calcium in thesubmicroscale Gd1yCayF3y is much narrower compared withthe broad interval up to 33 mol% in bulk material.19 Highenergy barrier was needed to overcome if substituting Ca2+ ionsfor the Gd3+ ions in the hexagonal GdF3. Our hydrothermalprocedure was too mild to incorporate more Ca2+ ions in GdF3.Therefore, addingmore CaCl2 will increase the calcium contentof the Gd1yCayF3y in a certain range, but more calciumaddition than 1.5 mmol would result in the formation of CaF2.

Effect of the fluorine amount

Since the NaBF4 worked as a precipitant, its amount has agreat influence on the crystal nucleation and growth, therebypossibly impacting the shape and phase of the product.28

When the NaBF4 was 0.5 mmol, microcolumns stackedby plates face to face were produced which were indexed inorthorhombic GdF3 (Fig. 7a, d). Hexagonal Gd1yCayF3ysubmicroplates can be obtained when the fluorine source was1 mmol (Fig. 7b, d). However, increasing the fluorine amountto 2 mmol led to the morphological heterogeneity. From theSEM images, one can see the product was composed of severalnanospindles as well as the typical submicroplates with centraldots (Fig. 7c). The XRD pattern shows that two types of crystalswere both pure hexagonal structure (Fig. 7d). According to theexperimental results, it is concluded that the product tendedto crystallize into orthorhombic GdF3 if the fluorine wasinsufficient because the GdF3 enjoys greater ease of nucleationthan CaF2 in the solution. In contrast, an adequate amount ofNaBF4 can facilitate the formation of hexagonal Gd1yCayF3ysolid solution. At the same time, high concentration of F willaccelerate the nucleation and result in small particles.29

Effect of the reagent concentrations

As is known, the concentration of the reactant ions hassignificant impact on the crystal nucleation and growth in

Fig. 7 SEM images of samples synthesized with different amounts of NaBF4:(a) 0.5 mmol, (b) 1 mmol, (c) 2 mmol, and (d) their corresponding XRD patterns.

9934 | CrystEngComm, 2013, 15, 99309937

the solution. In our work, the initial concentration of chemi-cal species was important to form the hexagonal Gd1yCayF3ysolid solution. If the amount of the Gd3+ and Ca2+ wasdecreased to 0.3 mmol (other reactants were varied propor-tionally, see Table S1), orthorhombic GdF3 submicroplateswith holes were obtained (Fig. 8a and 9a). When the amountof the reagents was increased to 0.5 mmol, the producttransformed to hexagonal Gd1yCayF3y submicroplates(Fig. 8b and 9b). However, addition greater than 2 mmolresulted in multiple morphologies. Although the XRD patternsof the samples were indexed to pure hexagonal Gd1yCayF3y(Fig. 8c, d), the SEM images showed two types of crystalselliptical nanostructures and submicroplates with centraldots (Fig. 9c, d). Moreover, the more reagents that wereadded, the more ellipsoid-like crystals that were yielded. Asis discussed above, the precipitation of GdF3 has priority inaqueous solution because of its much lower solubilityproduct constant than that of CaF2. Therefore, when the

Fig. 9 SEM images of samples prepared with different reagent concentrations(denoted with the amount of GdCl3): (a) 0.3 mmol, (b) 0.5 mmol, (c) 2 mmol, and

(d) 3 mmol.

This journal is The Royal Society of Chemistry 2013

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concentration of chemical species was too low, orthorhombicGdF3 was formed instead of the GdF3CaF2 solid solution.Increasing the starting amounts of reactants not only favoredthe formation of hexagonal Gd1yCayF3y but also led to fastnucleation. Thus, small particles were yielded which self-assembled into elliptical structures.

Luminescence properties

Herein, we investigated the luminescence properties of theGd1yCayF3y solid solution by taking Ln

3+-doped (Ln = Eu,Tb, and Dy) samples prepared with an EG amount of 20 mLas a representative. As is shown in Fig. 10, the excitationspectra (left) are dominated by the peaks located at 277, 301,309 and 315 nm, corresponding to 8S7/2

6I7/2,8S7/2

6P3/2,8S7/2

6P5/2, and8S7/2

6P7/2 of the Gd3+ transitions, respec-

tively. The weaker lines in the long wavelength regions of320450 nm are assigned to the ff intra-configuration transi-tions of the Eu3+ ions. Excitation into the Gd3+ at 277 nmyields intense and characteristic emission patterns of theLn3+. Fig. 10a shows that the emission spectrum of theEu3+-doped sample is composed of a group of lines peakingat 524, 535, 553, 590, 613, 648, and 688 nm, which comefrom the 5D1

7FJ (J = 0, 1, 2) and5D0

7FJ (J = 1, 2, 3, 4)transitions. In Fig. 10b, the emission peaks 380, 414, 438,457, 471, 489, 542, 585, and 619 nm are explicitly assigned to5D3

7FJ (J = 6, 5, 4, 3, 2) and5D4

7FJ (J = 6, 5, 4, 3) transi-tions of Tb3+. Fig. 10c presents the typical emission peaksof Dy3+ which result from the 4F9/2

6H15/2 (477 nm) and4F9/2

6H13/2 (570 nm) transition.30 The strong excitation

peaks corresponding to the intra-transition of the Gd3+ ions

Fig. 10 Excitation (left) and emission (right) spectra of Ln3+-doped (Ln = Eu,

Tb, Dy) samples.

This journal is The Royal Society of Chemistry 2013

and the characteristic Ln3+ emission excited by 277 nm indi-cate an efficient energy transfer from the Gd3+ to Ln3+ ions.31

Considering that the excitation peak of 277 nm comesfrom the 8S7/2

6I7/2 transition of the Gd3+ ions and the peak

located at 397 nm corresponds to the transition 7F0 5L6 of

Eu3+ ions, the emission excited by 277 nm is attributed to theenergy transfer from the Gd3+ to the Eu3+ ions, whereas onlythe intra-transition in Eu3+ contributes to the PL emissionwhen the excitation wavelength is 397 nm. If we assume thatthe absorption cross-sections of the transition 7F0

5L6 of theEu3+ ions are the same, the intensity ratio of the luminescenceemission excited by 277 and 397 nm (I277/397), respectively, canbe used to evaluate the energy transfer probability from theGd3+ ions to the Eu3+ ions.32 In our work, the IO277/397 of thestrongest emission peak at 592 nm was 2.49 in the orthorhom-bic GdF3 : Eu

3+ synthesized without calcium addition(Fig. 11a). By contrast, the IH277/397 at 590 nm was estimated tobe 3.53 for the hexagonal Gd1yCayF3y solid solution(Fig. 11b). Additionally, the excitation spectra clearly show thatthe peak at 277 nm for the hexagonal Gd1yCayF3y was stron-ger than that for the orthorhombic GdF3 taking the peak at397 nm as a reference (Fig. 11c). Therefore, the energy transferof the Gd3+ Eu3+ in the hexagonal Gd1yCayF3y solid solu-tion is more efficient than that in the orthorhombic GdF3.

As is known, the intensity ratios of 5D0 7F2 and

5D0 7F1 (IR/O) are strongly dependent on the local environment

Fig. 11 Emission spectra of (a) hexagonal Gd1yCayF3y: 5 mol% Eu3+

and

(b) orthorhombic GdF3 : Eu3+

excited at 272 nm and 396 nm. (c) Excitation spectra of

hexagonal and orthorhombic samples monitored at 590 and 592 nm, respectively.

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Fig. 12 Emission spectra of 0.5 mol% and 5 mol% Eu3+-doped Gd1yCayF3y

prepared with different amount of CaCl2 (a, e) 1 mmol, (b, f) 0.8 mmol, (c, g) 0.4 mmol.

(d, h) Emission spectra of 0.5 mol% and 5 mol% Eu3+

hexagonal GdF3.

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and can be used as a sensitive parameter to understand thevariation in the symmetry around Eu3+ in the lattice.33 Thepartial replacement of the Gd3+ by Ca2+ in hexagonal GdF3can increase the lattice disorder and thus lower the symmetryof the crystal field, which is expected to increase the IR/O. Wecalculated the IR/O value of the hexagonal GdF3 (synthesizedaccording to ref. 34) and the Gd1yCayF3y prepared with dif-ferent calcium addition. As is shown in Fig. 12, the IR/O valueof these samples is almost the same both in the 0.5 mol%and 5 mol% Eu3+-doped samples, suggesting that thesubstituted Ca2+ ions have a little effect on the emissionintensities because the high ionicity of the EuF bonds allowsonly a little admixture of opposite parity states to the Eu3+

f-states, which suppresses the electric dipole transition 5D0 7F2 significantly.

35 Therefore, it is difficult to detect the intensi-ties variation caused by the change of crystal field.

Conclusion

The nonstoichiometric Gd1yCayF3y solid solution has beenprepared through a solution-based method in a hydrothermalenvironment. The SEM images showed highly uniformsubmicroplates although no surfactant was used. The XRDpattern and Rietveld refinement confirmed that theas-synthesized manifold fluoride was nonstoichiometricCaF2GdF3 solid solution which was tysonite-type structurerather than the reported fluorite-type. The Gd3+ and Ca2+ ionsdistributed disorderly in the cation sites of the tysonite struc-ture. The contrast experiments showed that the amount ofethylene glycol, calcium chloride, and fluorine had an impor-tant effect on the phase and morphology of the Gd1yCayF3ysolid solution. Besides, the multicolor emission was realizedby doping Eu3+, Tb3+, and Dy3+ in the Gd1yCayF3y crystals,respectively, in which the Gd3+ ions played an essentialintermediate role transferring energy to activator Ln3+ ions.Employing Eu3+ as a structure probe, we found that the transferfrom the Gd3+ to Eu3+ ions in the obtained Gd1yCayF3y solidsolution was more efficient than that in the orthorhombic

9936 | CrystEngComm, 2013, 15, 99309937

GdF3. Our work opened up the possibility that the solution-based technique can be used to synthesize more inorganicfunctional materials.

Acknowledgements

This work is financially supported by the National NaturalScience Foundation of China (Grant No. 21271167) and theFund for Creative Research Groups (Grant No. 21221061).

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