Structural Phase Transition and Photoluminescence Properties of YF3

Sep 10, 2014 - High-pressure behaviors of YF3:Eu3+ nanocrystals with an average grain size of 40 nm were investigated by in situ high-pressure synchro...
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Structural Phase Transition and Photoluminescence Properties of YF3:Eu3+ Nanocrystals under High Pressure Chen Gong,† Quanjun Li,† Ran Liu,† Yuan Hou,‡ Jinxian Wang,‡ Xiangting Dong,‡ Bo Liu,† Xiao Tan,† Jing Liu,§ Ke Yang,∥ Bo Zou,† Tian Cui,† and Bingbing Liu*,† †

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China Key Laboratory of Applied Chemistry and Nanotechnology at Universities of Jilin Province, Changchun University of Science and Technology, Changchun 130022, People’s Republic of China § Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ∥ Shanghai Synchrotron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: High-pressure behaviors of YF3:Eu3+ nanocrystals with an average grain size of 40 nm were investigated by in situ high-pressure synchrotron radiation X-ray diffraction measurements up to 31.1 GPa at ambient temperature. The pressure-induced structural phase transition starts at 11.8 GPa and completes at 23.3 GPa. YF3:Eu3+ nanocrystals with a starting phase of orthorhombic structure transform into a highpressure phase, which is inferred to be hexagonal structure. The high-pressure structure returned to the orthorhombic phase after release of pressure. The transition pressure is enhanced in nanosized YF3:Eu3+ as compared to submicrometer size samples, which is due to the surface energy differences between submicrometer size and nanosized materials. The nanosized samples of high-pressure phase were easier to compress with smaller bulk modulus than the submicrometer size samples. The photoluminescence properties of YF3:Eu3+ have also been studied in the pressure range from ambient pressure to 25.0 GPa at room temperature. Accompanied by the structure transformation, the Eu3+ ion luminescence from 5D0 → 7 F1,2,3,4 transition in YF3:Eu3+ nanocrystals emerges obvious changes, which indicate the variation of the local symmetry of Eu3+ ions.

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INTRODUCTION Rare-earth trifluoride (REF3) nanomaterials have received considerable attention due to their novel optical properties that enable wide applications in optoelectronics, solid-state lasers, diagnostics, and biological labeling. In particular, lanthanide doped REF3 nanomaterials have attracted extensive research for their unique photoluminescence properties.1−5 Precise control over shapes and sizes of lanthanide doped REF3 nanocrystals enable people to manipulate their luminescence properties, such as luminescence intensity, fluorescence lifetime, and fluorescent wavelength.6−8 Lately, research found that controlling the crystal structure of lanthanide doped REF3 nanoparticles becomes a new approach to improving lightemitting properties. For example, Zhang et al. synthesized hexagonal GdF3:Eu3+ nanocrystals that emitted stronger Eu3+ luminescence than did orthorhombic ones.9 Tian et al. obtained hexagonal EuF3 nanocrystals that greatly enhanced 5D0 level of Eu3+ as compared to the sample of orthorhombic structure.10 In the family of REF3, two typical structures depend on the ionic radius of rare-earth ions at ambient conditions. One is hexagonal structure for rare-earth ions with large ionic radius, ranging from La to Nd. Another is orthorhombic (β-YF3 type) © 2014 American Chemical Society

structure for rare-earth ions with smaller ionic radius, ranging from Sm to Lu and Y.11,12 Except for GdF3 and EuF3 with mid ionic radius, for the other REF3 nanocrystals, obtaining nanocrystals with different crystal structures is still a great challenge. It is well-known that pressure is an important physical parameter that could reduce lattice spacing and change force constants between atoms, thus inducing modifications in band structures of crystals and electronic properties. It is a powerful method to modulate the structures, generate new properties, and unravel the phenomena hidden at ambient conditions. In the family of REF3, high pressure induces a phase transition from hexagonal to orthorhombic structure for bulk LaF3 and CeF3 with large ionic radius.13−18 Li et al. found that highpressure treatment of EuF3 is beneficial to the enhancement of its luminescence intensity.19 High pressure is an effective means to transform the crystal structure and tune the luminescence properties of REF3. Received: May 7, 2014 Revised: August 30, 2014 Published: September 10, 2014 22739

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Eu3+, average size of 40 nm) nanocrystal. The TEM image is shown in Figure 1.

Among various REF3, yttrium trifluoride (YF3) is a representative material with small ionic radius of rare-earth ions. The structure of YF3 is the typical structure (β-YF3 type) of a series of rare-earth REF3 with small ionic radius. It is one of the most important host crystals for lanthanide-doped phosphors, providing a wide band gap (>10 eV) and suitable Y3+ sites where Y3+ can be easily substituted by other trivalent rare-earth ions without additional charge compensation.20−23 So far, optical properties of rare-earth ion doped YF 3 nanocrystals have been investigated extensively, such as Eu3+, Tb3+, Ce3+, Dy3+, Pr3+, Er3+, Ho3+, Yb3+, Tm3+, and Nd3+.24−32 Among these doped materials, Eu3+ doped YF3 is an excellent luminescent material with unsurpassed red emitting.33−36 Recently, we have investigated the high-pressure behavior of submicrometer size YF3:Eu3+.37 We found that the β-YF3 type structure transforms into a new high-pressure phase (hexagonal structure with space group P3̅cl), which is accompanied by the enhancement and shifting of its luminescence lines.37 These new findings motived us to research the high-pressure behavior of nanosized YF3:Eu3+. It should be emphasized that the grain size of materials has significant effects on the phase transition pressure, compressibility, and even phase transition routines. The high-pressure behaviors of nanomaterials could be quite different from that of the bulk materials. Therefore, it is of great interest to explore the structural stability and luminescence properties of nanosized YF3:Eu3+ under high pressure. Up to now, there has been no report about the high-pressure transition or luminescence properties of nanosized REF3. The studies on the high-pressure behavior of nanosized YF3:Eu3+ would be quite important not only for its own fundamental and applied perspectives, but also would give new insights into the nature of the REF3 system. In this Article, we studied the phase transitions of nanosized YF3:Eu3+ under high pressure using in situ angular dispersive synchrotron X-ray diffraction (ADXD) up to 31.1 GPa. Nanosized YF3:Eu3+ shows quite different high-pressure behaviors, including the phase transition pressure and bulk modulus as compared to the submicrometer size sample. The luminescence properties of nanosized YF3:Eu3+ have also been studied under high pressure up to 25.0 GPa by in situ photoluminescence (PL) measurements. The pressure effect on luminescence properties of nanosized YF3:Eu3+ has also been discussed.

Figure 1. TEM image of YF3:Eu3+ (5 mol % Eu3+, average size of 40 nm) nanocrystal.

High-pressure measurements were carried out by using a diamond anvil cell with 300 μm diameter culets. Samples were loaded into 100 μm diameter holes drilled in T301 stainless steel gaskets, which were preindented to a thickness of 50 μm. A small ruby chip was inserted into the sample compartment for in situ pressure calibration, utilizing the R 1 ruby fluorescence method.38 Silicone oil was used as a pressuretransmitting medium. In situ angle-dispersive synchrotron XRD measurements under high pressure were performed at the BL15U1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF), the 4W2 beamline of the Beijing Synchrotron Radiation Facility (BSRF), and the X17C beamline of Brookhaven National Laboratory (BNL). Among them, SSRF is the third generation synchrotron facility, which provides cleaner beamline with higher flux intensity and smaller beam size (5 μm × 5 μm). All of our XRD patterns were integrated to give one-dimensional powder diffraction patterns using the FIT2D program,39 and individual peaks were fitted using a GSAS program.40 The in situ photoluminescence measurements under high pressure were performed on a QuantaMaster 40 spectrometer (produced by Photon Technology Inc.) in the reflection mode. The 405 nm line of a violet diode laser with a spot size of 20 μm and a power of 45 mW was used as the excitation source. The DAC with the sample was put on a Nikon fluorescence microscope to focus the laser on the sample. The emission spectra were recorded using a monochromator equipped with a photomultiplier at room temperature.

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EXPERIMENTAL SECTION YF3:Eu3+ (5 mol % Eu3+, 40 nm) nanocrystal was synthesized by a hydrothermal process. In a typical procedure, 2 mmol of Eu(NO3)3 and 38 mmol of Y(NO3)3 (Eu:Y = 0.05:0.95) were dissolved in 20 mL of deionized water under magnetic stirring to form a transparent solution I. 20 mL of DNA aqueous solution of 3.12 mg/mL was slowly added to solution I with stirring for 24 h to form solution II. Next, 0.4103 g of NH4F was added to 20 mL of deionized water to form solution III. After vigorous stirring for an hour, solution III was dropped slowly in solution II. After additional vigorous stirring for 2 h, the as-obtained suspension was transferred into a 100 mL Teflon stainless steel autoclave, which was sealed and then maintained at 160 °C for 12 h. As the autoclave cooled to room temperature naturally, the white precipitate was separated by centrifugation and washed with distilled water and ethanol in sequence. Finally, the precipitate was dried in a vacuum condition at 60 °C for 12 h to obtain the YF3:Eu3+ (5 mol %

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RESULTS AND DISCUSSION The high-pressure diffraction patterns of nanosized YF3:Eu3+ (40 nm) at various pressures are shown in Figure 2. The diffraction patterns collected at low pressures correspond well to the orthorhombic phase of YF3 with space group of Pnma. With increasing pressure, all diffraction peaks trend to shift toward higher 2θ angles, and some peaks gradually disappeared while new peaks emerged. The pure orthorhombic phase can be stable up to 9.0 GPa. A new diffraction peak [arrowed in 22740

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one correspondence. So, we infer that YF3:Eu3+ nanocrystals have the same high-pressure phase, hexagonal structure with space group P3̅cl. Figure 4 illustrates the Rietveld refinement of

Figure 2. XRD patterns of YF3:Eu3+ nanocrystal. The asterisk (*) denotes the diffraction from the stainless-steel gasket in the experiment.

Figure 4. Rietveld refinement of the diffraction pattern of YF3:Eu3+ nanocrystal at 31.1 GPa.

Figure 2] appears at 11.8 GPa, indicating the occurrence of the phase transition. The transition is rather sluggish that the coexisting of the two phases kept up to 20.2 GPa. The orthorhombic phase completely transformed into high-pressure phase at 23.3 GPa. The high-pressure phase is stable up to 31.1 GPa with some of the Bragg peaks merged into broad diffraction peaks, and the intensity of some peaks diminished. The high-pressure structure returned to the orthorhombic phase after release of pressure. Figure 3 shows the diffraction patterns of submicrometer size YF3:Eu3+ at 24.5 GPa and nanosized YF3:Eu3+ at 27.6 GPa in

the diffraction pattern of YF3:Eu3+ nanocrystals at 31.1 GPa, the highest pressure we performed using GSAS program. The refinements yielded the following lattice parameters, a = b = 6.457(2), c = 6.626(2) Å, and cell volume, V = 239.3(1) Å3. Using the orthorhombic phase with space group Pnma and hexagonal phase with space group P3̅cl, all of the diffraction patterns were refined. Figure 5 shows the volume per molecule

Figure 5. Pressure−volume relation of nanosized YF3:Eu3+. The “■” represent the orthorhombic phase. The “▲” represent the hexagonal phase. The line represents the fit to the Birch−Murnaghan equation. Figure 3. Diffraction patterns of submicrometer size YF3:Eu3+ at 24.5 GPa and nanosized YF3:Eu3+ at 27.6 GPa.

as a function of pressure determined for YF3:Eu3+ nanocrystal. There is 3.7% volume shrinkage during the phase transition at 14.4 GPa. The P−V data were fitted based on the third-order Brich−Murnaghan equation of state fit of primitive cell volume:

this work. The diffraction peaks of nanosized YF3:Eu3+ significantly broadened as compared to the submicrometer size counterpart due to the well-known finite nanosize effect of the crystallites. It is really difficult to determine the accurate structure for high-pressure phase of YF3:Eu3+nanocrystal from its diffraction pattern. However, comparing the diffraction patterns of YF3:Eu3+ nanocrystals with submicrometer size YF3:Eu3+ materials that were selected from their high-pressure phase, we found that their diffraction peaks are almost one to

P = 3/4B0 [(V0/V )7/3 − (V0/V )5/3 ] × {1 + 3/4(B0′ − 4)[(V0/V )2/3 ]}

(1)

where V0 is the volume at ambient pressure, B0 is the bulk modulus at ambient pressure, and B0′ is a parameter for pressure derivative. For fitting purposes, only data from diffraction 22741

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Table 1. Transition Pressure (PT), Completion of Phase Transition (PC), Volume Collapse (ΔV), and Equation of State Parameters (B0 and B′0) of Orthorhombic and Hexagonal Phases YF3:Eu3+ morphology

PT (GPa)

PC (GPa)

ΔV (%)

B0 (GPa) (orthorhombic)

B0 (GPa) (hexagonal)

B0′

submicrometer size nano

8.4 11.8

16.0 23.3

3.2 3.7

146(6) 165(15)

219(12) 113(19)

4 4

patterns clearly showing phase were used to avoid errors due to overlapping peaks of the two phases. According to the literature,41 B0′ is fixed at 4 for both phases for comparison. The fitting curves are shown in Figure 5. The yielded bulk modulus values are B0 = 164(15) GPa and B0 = 113(19) GPa for the orthorhombic phase and the high-pressure hexagonal phase, respectively. In our previous work, we found that for the submicrometer size sample, B0 = 146(6) GPa for orthorhombic phase, and B0 = 219(12) GPa for hexagonal phase. The error of B0 (15 GPa, 19 GPa) is slightly larger than that of submicrometer size counterpart (6 GPa, 12 GPa), which is generally acceptable for nanomaterials according to the previous reports.42−44 It is noted that the bulk modulus of nanosized YF3:Eu3+ of high-pressure phase is much lower than that of the submicrometer size counterpart, which has not been reported in related fluoride nanomaterials. Similar phenomena were reported in other nanomaterials, such as SnO2,45 PbS,46 TiO2,47 Al2O3,48 Ni,49 and Fe50 nanocrystals. There are two possible reasons for the reduced bulk modulus of nanomaterials. One is the inverse Hall−Petch effect, and the other is the difference of compressibility between surface layer and grain core region of nanocrystalline. The inverse Hall−Petch effect often happens to the nanoparticles with very small grain size (usually smaller than 10 nm).47,51 Because the particle size of our sample is about 40 nm, which is relatively large, the inverse Hall−Petch effect could not be the reason for the reduced bulk modulus. The other reason is the difference of compressibility between surface layer and grain core region of nanocrystalline. It has been found that the specific arrangement of atoms at the surface layer of nanocrystals is a main possible reason for the different compressibility.49,50 It is generally accepted that defects are more abundant in the surface of nanoparticles as compared to the grain core region due to the large surface to volume ratio, which may also be a possible reason for the different compressibility.52 The pressure-induced structural phase transition starts at 11.8 GPa for nanosized YF 3 :Eu 3+ and 8.4 GPa for submicrometer size YF3:Eu3+, respectively. There is a significant enhancement of the transition pressure in the nanosized YF3:Eu3+ as compared to the submicrometer size material. The transition becomes more sluggish for nanocrystals as well. The results are summarized in Table 1. For nano and submicrometer size materials, the transition process ranges from 11.8 to 23.3 GPa, 8.4 to 16.0 GPa, respectively. A highenergy hindrance might be a factor that prevents the rapid formation of a high-pressure phase in nanosized YF3:Eu3+. It is well-known that the transition pressure changes between nanosized and bulk samples caused by their different surface energy. Jiang et al.41 reported that three factors, volume collapse, surface energy, and internal energy difference between nano and bulk samples, determine the change of the transition pressure. An equation was given to describe the change of transition pressure:

Pn − PB = PB[ΔVB(PB)/ΔVn(Pn) − 1] + [Unsurf (2, Pn) − Unsurf (1, Pn)]/ΔVn(Pn) + {[UB(1, PB) − UB(1, Pn)] − [UB(2, PB) − (2, Pn)]}/ΔVn(Pn) (2)

where “n” and “B” refer to the nanocrystals and bulk materials, 1 and 2 are related to the low- and high-pressure phases, P is the transition pressure, ΔV is the volume change at the transition, U is the internal energy, and Unsurf is the internal energy of the interface. From the expression, it is clear that the change of transition pressure, Pn − PB, depends on three components: (1) the volume change for bulk and nanocrystal at transitions (hereafter term 1); (2) the surface energy difference between the phases involved (hereafter term 2); and (3) the internal energy difference between the phases involved in bulk (hereafter term 3). It is found that the ratio of volume collapse was about 3% for both nanosized and submicrometer size YF3:Eu3+ samples; thus term 1 is small and close to zero. On the basis of the experimental observation that many physical parameters for the core in nanocrystals are similar to those of the corresponding bulk materials, the internal energy for a given pressure is assumed to be the same for the bulk and the core of nanocrystals. Thus, term 3 is close to zero as well. Therefore, it reveals that term 2, the surface energy difference between nanosized and bulk samples, is the main factor determining the enhancement of the transition pressure. It can also explain the significant enhancement of the transition pressure in the nanosized YF3:Eu3+ as compared to the submicrometer size material. The nanosized YF3:Eu3+ has higher surface energy than the submicrometer size sample that enhanced its transition pressure. Figure 6 is the room-temperature emission spectra of YF3:Eu3+ nanocrystal that excited at 405 nm, which shows sharp lines in the red spectrum range from 585 to 700 nm corresponding to the Eu3+ transition from the excited 5D0 level to the 7Fj (j = 1,2,3,4) levels.53,54 There is no notable shift in position of the emission peaks as compared to other Eu3+ doped systems because the 4f energy levels of Eu3+ are hardly affected by the crystal field due to the shielding effect of the 5s25p6 electrons. The most intense peaks in Figure 6 are centered at 588, 593, and 595 nm corresponding to the magnetic dipole satellites of the 5D0 → 7F1 transition, which is dominating in the inversion symmetry. The 5D0 → 7F2 transition can be observed only when the lattice environment is distorted and contains noninversion symmetry55,56 and gives rise to peaks at 615 and 620 nm in Figure 6. Therefore, comparing the intensity at 588 nm (5D0 → 7F1) with that at 615 nm (5D0 → 7F2) in Figure 6 inferred that the Eu3+ ions in YF3:Eu3+ nanocrystals occupy a site with a small deviation from the inversion symmetry. The emission peak at 651 nm, which is most likely attributed to the 5D0 → 7F3, is so weak that it is indistinct under high pressure. The bands near 693 and 700 nm can be most likely assigned to the 5D0 → 7F4 transition. 22742

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Figure 6. Luminescence spectra of YF3:Eu3+ nanocrystal excited at 405 nm.

Figure 8. Pressure-induced shifts of the luminescence peaks.

The peaks from the 5D0 → 7F2 transition become gradually weaker and broader when pressure exceeds 13.1 GPa and present a red shift with increasing pressure. It is interesting to note that the luminescence intensity ratio of 5D0−7F1 to 5 D0−7F2, IR (2/1), called the asymmetric factor depends on the covalency and/or local structure for Eu3+ ions.57−59 The higher is the value of IR (2/1), the higher are the asymmetry and covalence between Eu3+ ions and its surrounding ligands. The variation of the peak intensity ratios IR (2/1) of YF3:Eu3+ nanocrystal with pressure is shown in Figure 9. Before 13.1 GPa

Figure 7 shows the evolution of the luminescence spectra obtained during the process of loading pressure at room

Figure 7. Luminescence spectra of YF3:Eu3+ nanocrystal at various pressures. The asterisk (*) denotes the diffraction from the pressure calibration.

temperature. Because the 5D0 → 7F3 transition peak is indistinct under high pressure, we only monitored the peaks of the 5D0 → 7F1,2,4 transition. There are three peaks for the 5D0 → 7F1 transition at ambient pressure, and this situation can be stable up to 25.0 GPa, the highest pressure we carried out. The intensity of the peak at about 588 nm gets stronger with increasing pressure until 13.1 GPa and then weaker. Figure 8 shows the pressureinduced shifts of the luminescence peaks of this transition. With increasing pressure, the two emission peaks at about 595 nm present a red shift at first and then a blue shift in the range of 14.0−19.6 GPa. The position of the emission peak at about 588 nm shows a red shift until the highest pressure and shift faster above 13.1 GPa. These behaviors in luminescence spectra with increasing pressure suggest that the host materials may undergo a pressure-induced phase transition in the vicinity of 13 and 20 GPa, which corresponds with the X-ray diffraction result.

Figure 9. Variation of the peak intensity ratios IR (2/1) of YF3:Eu3+ nanocrystal with pressure.

and above 20.5 GPa, the value of IR (2/1) tends to decrease with increasing pressure, indicating a better symmetry. The ratio has a trend to increase in the range of 13.1−20.5 GPa, suggesting larger deviation from the inversion symmetry. X-ray diffraction patterns show that the two phases are both present in this pressure area where the peaks of the starting phase are weaker and those of the new phase are stronger with increasing pressure. Therefore, we assume that the coexistence of the two phases causes deviation from symmetry, and the effect vanished when the structure phase transition has completely finished. Contrasting high-pressure XRD data, we observed pressureinduced reduction in the value of IR (2/1) for the pure phase, 22743

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Education of China (20102216110002, 20112216120003), the Science and Technology Development Planning Project of Jilin Province (Grant nos. 20130101001JC, 20070402), and the Cheung Kong Scholars Programme of China.

which indicates that the higher is the pressure, the better is the symmetry around Eu3+ ions. Figure 7 shows the luminescence peak of ruby that measured the pressure during the high-pressure experiment at about 695 nm, and the peaks near 693 and 700 nm for the 5D0 → 7F4 transition of Eu3+ ions are submerged in the ruby band at low pressure. The small peak near 691 nm is obviously emerged at 4.0 GPa because of the difference of shift speed under pressure between the luminescence peaks of Eu3+ ions and ruby. A new peak appears at 682 nm at 14.0 GPa and becomes a little more intensive with increasing pressure, which is most likely attributed to the 5D0 → 7F4 transition. The pressure-induced shifts of the two peaks are shown in Figure 8. With increasing pressure, the peak near 691 nm presents a blue shift until 12.2 GPa and then a red shift. The position of the new peak that appears at 14.0 GPa is hardly changed under high pressure. These behaviors in luminescence spectra with increasing pressure suggest that the host materials may undergo a pressure-induced phase transition near 13 GPa, which corresponds with the analysis result of X-ray diffraction data of YF3:Eu3+ nanocrystal that we mentioned above.

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CONCLUSION The high-pressure behaviors of YF3:Eu3+ nanoparticles (40 nm) were investigated using angle-dispersive synchrotron radiation X-ray diffraction and photoluminescence measurements. The starting orthorhombic phase transforms into a high-pressure phase, which is identified as hexagonal structure with space group P3̅cl. The phase transition began at 11.8 GPa and completed at 23.3 GPa. The transition pressure of nanosized YF3:Eu3+ is higher than that of submicronmeter size sample, which is due to the surface energy differences between submicrometer size and nanosized materials. The bulk moduli of orthorhombic and hexagonal phases were estimated at 164(15) and 113(19) GPa. The bulk modulus for the hexagonal phase of the nanosized YF3:Eu3+ is much lower than that of submicrometer size samples B0 = 219(12) GPa. The changes on Eu3+ ion luminescence from the 5D0 → 7F1,2,4 transition in YF3:Eu3+ were observed at 13.1 GPa, which correspond with the pressure-induced phase transition of YF3:Eu3+ from orthorhombic to hexagonal structure. The relative luminescence intensity ratio IR (2/1) tends to decrease for pure phase, which indicates that the higher is the pressure, the better is the symmetry around the Eu3+ ions.

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ASSOCIATED CONTENT

S Supporting Information *

Complete author list of references. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-431-85168256. Fax: 86-431-85168256. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Basic Research Program of China (2011CB808200), the NSFC (10979001, 51025206, 51032001, 21073071, 11374120, and 50972020), Ph.D. Programs Foundation of the Ministry of 22744

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The Journal of Physical Chemistry C

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