Cyclic Phase Transition from Hexagonal to Orthorhombic Then Back to

Article pubs.acs.org/JPCC

Cyclic Phase Transition from Hexagonal to Orthorhombic Then Back to Hexagonal of EuF3 While Loading Uniaxial Pressure and under High Temperature Zhilei Sui,†,⊥ Jizhou Wu,†,⊥ Xiangqi Wang,† Rucheng Dai,‡ Zhongping Wang,‡ Xianxu Zheng,*,§ and Zengming Zhang*,‡,∥ †

Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China The Centre for Physical Experiments, University of Science and Technology of China, Hefei, Anhui 230026, China § Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, Sichuan, 621900, China ∥ Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China ‡

S Supporting Information *

ABSTRACT: The structure and photoluminescence properties are investigated under high pressure and high temperature for pure orthorhombic and hexagonal EuF3 nanocrystals. Under hydrostatic compression, the hexagonal EuF3 remains stable at pressures up to 26 GPa. Under nonhydrostatic compression, a cyclic phase transition from hexagonal to orthorhombic and then back to hexagonal is observed for the first time. When loading uniaxial compression, the pure hexagonal EuF3 partly transforms to orthorhombic at 70 MPa, then the orthorhombic EuF3 transforms to hexagonal at about 3 GPa, and the transition is completed at about 10 GPa. The cyclic phase transition is also observed during the heating process; the hexagonal transforms to orthorhombic at 550 °C and then to hexagonal at 855 °C. The content phase diagrams are obtained under high pressure and at high temperature.

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INTRODUCTION

enhanced due to the great irradiative transition rate of the D0 level induced by the lower symmetry environment of Eu3+ in hexagonal phase La3+:EuF3. Wang et al.18,19 prepared a pure EuF3 nanocrystal with either orthorhombic or hexagonal structure using the coprecipitation approach. The emission spectrum of the Eu3+ ion strongly depends on the local environment surrounding the Eu3+ ion. The split number and/or position of the emission peak from the same transition are sensitive to the crystal structure, so the photoluminescence of the Eu3+ ion is widely employed as the probe to analyze the structure of the sample due to the convenient observation. Some research groups used Eu3+ PL as the probe to investigate the structure phase transition of crystals at high temperature and/or under high pressure.20,21 This gave us the motivation to use the PL of the Eu ion as the probe to investigate the structure transformation of EuF3 under high pressure and high temperature. Actually, the hexagonal structure is considered as the hightemperature phase of EuF3 and is a kinetic product at room 5

Fluorides have been widely applied in many fields due to their excellent properties such as upconversion and photoluminescence (PL).1−5 Europium trifluoride (EuF3), as a member of the rare-earth trifluorides (REF3) family, has been recently noted due to its low phonon energy and stable multiphase structures.6−10 The structure of the REF3 family depends on the rare-earth ionic radius. It possesses an orthorhombic phase for a smaller RE ionic radius and a hexagonal structure for a larger one.11−13 Because the radius of Eu3+ is intermediate compared to the radii of other rare-earth family members, both orthorhombic and hexagonal phases of EuF3 are stable at ambient conditions. Unfortunately, it is difficult to obtain pure phase EuF3, because EuF3 often exists as mixed phases.14,15 The PL properties are strongly dependent on the structures of the materials; many attempts have been devoted to get the pure phase EuF3. On the basis of the first principles calculation, Wang et al.16 concluded that the larger rare-earth ionic radius can reduce the activation energy barrier to form the hexagonal REF3. Tian et al.17 successfully achieved the phase transition of a EuF3 nanocrystal from orthorhombic to hexagonal phase by doping the lanthanide ion with a larger ionic radius. They also observed that the luminescence intensity is significantly © 2016 American Chemical Society

Received: June 11, 2016 Revised: August 4, 2016 Published: August 7, 2016 18780

DOI: 10.1021/acs.jpcc.6b05907 J. Phys. Chem. C 2016, 120, 18780−18787

Article

The Journal of Physical Chemistry C temperature.11,12The higher temperature can help the surface energy to overcome the activation energy barrier16 and drive the phase transition from orthorhombic to hexagonal. Some groups reported the reversible phase transition from orthorhombic to hexagonal of EuF3 at high temperature,12,22−25 but there exists a discrepancy in the phase transition temperatures. Spedding et al.22 reported that the phase transition occurs at 697 °C, whereas Greis’s group25 reported that the phase transition temperature is 852 °C. The discrepancy should be resulted from the oxide contamination at the high-temperature environment.24 Besides high temperature, high pressure also plays an important role in phase transition because it can reduce the interatomic distance to modify the crystal structure and electronic orbital.26−34 Li et al.14 modulated the phase transition of EuF3 from mixed phases of orthorhombic and hexagonal to pure hexagonal under high pressure using an in situ high-pressure synchrotron X-ray diffractometer (XRD) and luminescence. They found that the EuF3 transforms from orthorhombic to hexagonal between 2.7 and 7.8 GPa and that the mechanism of transition involves the variations of Eu positions in one layer and changes in the coordination number of Eu from 9 to 11. Their results reveal that the poor pressure condition makes the phase transition sluggish. Gong et al.35,36 observed phase transition from orthorhombic to hexagonal of YF3 without and with Eu3+ dopant under high pressure, and the hexagonal phase inversed to the orthorhombic phase after releasing the pressure to the ambient environment. However, the pressure behavior of pure phase EuF3 is hardly reported. In this paper, we performed the hydrostatic pressure and uniaxial pressure on pure orthorhombic and hexagonal EuF3 nanocrystals. We observed a cyclic phase transformation from hexagonal to orthorhombic and back to hexagonal during loading pressure for the first time. The same cyclic phase transition is also found during the heating process. The content phase diagrams of EuF3 under high pressure and high temperature are obtained.

double-grating Jobin Yvon spectrometer, equipped with a thermoelectric-cooled charge coupled device detection system. The line 514.5 nm of an argon laser was used as the Raman excitation source. An Olympus microscope lens with a focal distance f = 20 mm and a numeric aperture of NA = 0.35 was used to focus the laser beam on the sample surface. The high temperature was achieved by temperature-controlled stages (LinkamTS 1500) which can be heated to 1500 °C. All spectra were measured in the backscattering geometry.

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RESULTS AND DISCUSSION Figure 1 shows the XRD patterns for presynthesized EuF3 samples. All peaks in Figure 1a are indexed to the hexagonal

Figure 1. X-ray diffraction patterns of EuF3 in orthorhombic and hexagonal.

structure of EuF3 [space group: P3̅c1 (165)] with lattice constants a = 6.9200 Å, b = 6.9200 Å, c = 7.0860 Å, while all peaks in Figure 1b are identified as the orthorhombic structure [space group: pnma (62)] with lattice constants a = 6.6200 Å, b = 7.0150 Å, c = 4.3960 Å. Patterns of Figure 1 show fine accordance with the standard indexing (JCPDS Nos. 032-0373 and 033-0542). All the peaks exhibit pure single phase crystallinity without any substantiation of second phase. Figure 2 is the SEM images for EuF3 samples. The orthorhombic EuF3 is the nanorod morphology as seen in Figure 2a. The transmission electron microscope (TEM) image (inset in Figure 2a) shows that the average size of the nanorods is about 100 nm in length and 50 nm in width. Figure 2b represents clusters of hexagonal phase nanocrystals in the form of flattened balls with 300 nm. The average size of the nanocrystals is about 50 nm. According to the Scherrer’s equation, D = 0.9λ/β cos θ, where λ is the X-ray wavelength 0.15405 nm, β is the full-width at half-maximum of the diffraction line, and the grain sizes of orthorhombic and hexagonal EuF3 are 46 and 53 nm, respectively. The estimated sizes from XRD pattern are in complete agreement with SEM and TEM measurements. Figure 3 displays the PL emission of Eu3+ 5D0 → 7F0−4 transitions in hexagonal and orthorhombic EuF3 excited by a 488 nm laser. There exists a clear difference between the two spectra. For the 5D0 → 7F1 transition, the first peak centered at 587 nm for the orthorhombic phase is slightly shifted toward shorter wavelength compared with the hexagonal phase, leaving a noticeable distance with the next two peaks, while all three

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EXPERIMENTAL SECTION The pure phase EuF3 samples in orthorhombic or hexagonal were synthesized by the methods reported in some literatures.18,19 The synthesis hexagonal phase EuF3 involved the solution-phase reaction between Eu(NO3)3 and KF at room temperature. An aqueous solution of 50 mL containing 3 mmol of KF and 1 mmol of Eu(NO3)3 was stirred for 3 h. The resulting white solid precipitates were collected and washed several times with distilled water and ethanol. The final products were dried at 70 °C for 3 h. A similar synthetic procedure was employed for orthorhombic phase EuF3, which used NH4F instead of KF while other reaction conditions were kept constant. Powder XRD (Rigaku, TTR III) with Cu−Kα radiation was used for crystal phase identification. The scan was performed in the 2θ range of 20−70°. Scanning electron microscope (SEM) images were obtained using a field emission scanning electron microscope (Hitachi S-4800) to characterize the particle size of the samples. High-pressure experiments were performed using a diamond-anvil cell (DAC) with 500 μm diameter culets and a 769YP-10Dmini desk powder tablet compression machine. The sample was placed in the 150 μm diameter hole of the stainless steel gasket that preindented to a thickness of 80 μm. Pressure was calibrated by the shift of the ruby R1 fluorescence line.37 The high-pressure Raman spectra were measured with a 18781


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The Journal of Physical Chemistry C

bands of 5D0 → 7F2 and 5D0 → 7F3 transitions between orthorhombic and hexagonal phases. These emission bands can be employed as the fingerprints to characterize the two phases by PL spectra. For convenient analysis, the PL bands from 5D0 → 7F1 and 5D0 → 7F4 transitions are used as the probe to determine the phase structure. It is easy to distinguish the orthorhombic and hexagonal phases due to the obvious characteristic between the two emission spectra. The emission from 5D0 to 7F0 transition for both orthorhombic and hexagonal phases shows a single peak, as seen in the inset of Figure 3. The single peak reveals a pure phase structure for both orthorhombic and hexagonal EuF3. The PL spectra and the corresponding peak position shifts for orthorhombic phase EuF3 are shown in Figure 4 using silicone oil as pressure transmitting medium (PTM). All the emission peaks display a red shift with the increasing pressure. The orthorhombic structure stability is kept until 4.7 GPa; then, four peaks arise at 589, 592, 613, and 681 nm as the fingerprints of the hexagonal structure, notifying the commencement of phase transition from orthorhombic to hexagonal. The peaks at 613 and 615 nm begin to lose their strength, while the peak intensity at 617 nm rises up with increasing pressure. Emission lines at 587 and 594 nm completely disappear, and the peaks at 614 and 614.5 nm merge into a single one as the pressure rises to 9.5 GPa, as seen in Figure 4. The results indicate that the pressure-induced structural phase transition starts at 4.7 GPa for nanosized EuF3 and completes at 9.5 GPa. This shows that the phase transition occurs at high pressure in the case of pure orthorhombic nanocrystal EuF3 as compared to the bulk mixed phase EuF3, which is earlier reported.14 During releasing pressure, emission bands from the hexagonal phase are maintained until 0.6 GPa; then, the emission peak from the orthorhombic structure appears in the PL spectrum. After coming back to ambient pressure, there still exists a part of the hexagonal phase and the sample turned to mixed phases including orthorhombic and hexagonal as seen in Figure S1. This result reveals that the phase transition is not completely reversible. The PL spectra of hexagonal phase EuF3 are shown in Figure 5 under high pressure with silicone oil as PTM and without any PTM. The PL spectra under high pressure strongly depend on the PTM. All emission bands maintain the characteristic of hexagonal phase EuF3 under high pressure with PTM of silicone oil as seen in Figure 5a. This shows that the hexagonal EuF3 keeps a stable structure under high pressure up to 26.2 GPa using silicone oil as PTM. In contrast to hydrostatic pressure, the nonhydrostatic pressure hugely changes the emission spectra. The PL spectra of hexagonal phase EuF3 under high pressure up to 27.5 GPa without any PTM are displayed in Figure 5b. The orthorhombic fingerprint is observed using the DAC setup even at a small nonhydrostatic pressure of about 0.1 GPa. The PL spectra in Figure 5b show that, with the increasing nonhydrostatic pressure, the hexagonal phase content decreases initially and increases with the further increase in nonhydrostatic pressure and turns to hexagonal phase completely again at 10.3 GPa. In other words, hexagonal phase EuF3 undergoes a cycle phase transition under nonhydrostatic pressure. The above discussion shows that the nonhydrostatic compression plays an important role in driving the phase transition from hexagonal to orthorhombic. To clarify this observation further, we produced the nonhydrostatic compression by using a desk powder tablet compression machine. After

Figure 2. SEM images of EuF3 samples: (a) orthorhombic phase; (b) hexagonal phase.

Figure 3. PL spectra of EuF3 in orthorhombic and hexagonal with an excitation wavelength of 488 nm at ambient conditions.

peaks are comparatively nearer in the case of hexagonal. The similar trend is also found from the 5D0 → 7F4 emission band for the hexagonal phase. The other differences in shape and split number of peaks can also be observed from emission 18782


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Figure 4. Luminescence spectra of orthorhombic EuF3 at various pressures (a, b) and pressure dependence of the emission peaks (c, d).

Figure 5. Luminescence spectra of hexagonal EuF3 at various pressures: (a) PTM: silicone oil; (b) no PTM.

releasing nonhydrostatic compression, the XRD and PL measurements were taken for the samples, and results are shown in Figure 6. The hexagonal samples were compressed to a slice with 5 mm in diameter and 0.5 mm in thickness. Both XRD patterns and PL spectra indicate that the pure hexagonal EuF3 is induced orthorhombic at only 70 MPa nonhydrostatic compression. The phase diagrams of content ratio vs pressure on comprehensive PL spectra of both hexagonal and orthorhombic under high pressure are expressed in Figure 7. The content

ratio is obtained by decomposition of hexagonal and orthorhombic from PL spectra. The phase diagram reveals that the content from the pure orthorhombic sample monotonously decreases with the increasing pressure and completely disappears and transforms to hexagonal EuF3 at about 10 GPa. The hexagonal content first decreases and then increases with increasing pressure and reaches pure hexagonal again at about 10 GPa. The phase transition from orthorhombic to hexagonal starts at about 3 GPa. On loading uniaxial pressure up to 3 GPa, the hexagonal content decreases due to the phase 18783


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Figure 6. X-ray diffraction patterns and luminescence spectra of hexagonal EuF3 after various uniaxial pressures.

Figure 7. Phase diagrams of content ratio of hexagonal and orthorhombic at various pressures.

transition from hexagonal to orthorhombic. The generated orthorhombic phase transforms to hexagonal by loading pressure above 3 GPa, which dominates the phase transition trend and results in the enhancing hexagonal phase content under higher pressure. The hydrostatic pressure can drive the EuF3 transformation from orthorhombic to hexagonal while it stops the reverse phase transition. The total energies of orthorhombic and hexagonal EuF3 at different pressures are calculated using Vienna Ab initio Simulation Package (VASP) simulations, and the results are presented in Figure 8a. At ambient pressure, the total energy of orthorhombic EuF3 is lower than that of the hexagonal. Although the orthorhombic and hexagonal EuF3 can coexist, the orthorhombic EuF3 is more stable at ambient pressure. The total energies for the two phases increase with increasing pressure, but it increases more rapidly for the orthorhombic. Therefore, when pressure is higher than P0, the orthorhombic phase transforms to the hexagonal. The value of P0 is larger than the experimental result. The reason is that the VASP software cannot precisely consider all facts from pressure and temperature.38,39 The energy barrier stops the transition from hexagonal to orthorhombic under hydrostatic pressure lower than P0. Structures of both orthorhombic and hexagonal phases are constituted by the overlapped layers of europium atoms which are coordinated by the surrounding fluorine ions,14 as shown in Figure 8c,d. During loading uniaxial pressure on hexagonal EuF3, the europium layers easily slide and trend to stabilize in the orthorhombic structure due to

Figure 8. Total energies (a) and the difference of total energy (b) for orthorhombic and hexagonal EuF3 under various pressures. The structure diagrams of EuF3 in (c) hexagonal and (d) orthorhombic; (e, g) layer slip of Eu atoms of hexagonal EuF3 at different directions under nonhydrostatic pressure; (f, h) the corresponding structure diagrams after layer slips. The big colored balls are Eu atoms, and the small gray ball is a F atom.

stress, as seen in Figure 8e−h. In other words, the stress makes it easier to overcome the energy barrier between the phases, so as a result, the hexagonal EuF3 transforms to orthorhombic 18784


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increases after transformation to orthorhombic phase, which shows that photoluminescence stability of the orthorhombic phase is better than that of hexagonal EuF3. Between 630 and 800 °C, the enhancing intensity of emission is ascribed to the grain grown. The profile and intensity of the emission changed enormously from 850 to 855 °C. According to the Greis group’s result, the temperature-dependent phase transition takes place at 852 °C. Therefore, this indicates that the orthorhombic phase transforms to hexagonal EuF3 again. The phase transition and the PL quenching of hexagonal EuF3 are responsible for the huge change in the spectra shown in Figure 9. Some extra luminescence peaks from EuOF reveals that EuF3 is easily oxidized in the cooling process as seen in Figure 9. The PL spectrum in Figure S3 at ambient conditions identifies that the hexagonal EuF3 transforms to the pure orthorhombic after annealing at 900 °C for 3 h in the environment without O2. In contrast to the thermodynamic behavior of the hexagonal EuF3, the orthorhombic keeps its phase stable during heating up to 800 °C as seen in Figure S4a. The PL spectra in Figure S4b reveal that the orthorhombic phase EuF3 is oxidized into EuOF during the cooling process. The phase diagram of content ratio vs temperature is shown in Figure 10.

EuF3 at very low uniaxial pressure. According to other reported works,40 phase transition often occurs by nucleation at defects; the plastic deformation caused by nonhydrostatic pressure can continuously produce defects. Those dislocation defects are generated and piled up against grain boundaries, which creates a strong concentrator of the stress tensor and may lead to an obvious reduction of threshold pressure for phase transitions. For instance, the BN transformation from rhombohedral to cubic phase starts at 5.6 GPa under nonhydrostatic pressure, whereas it occurs at 55 GPa under hydrostatic conditions.41,42 The evolution of the luminescence spectra of hexagonal EuF3 is shown in Figure 9 with the increasing temperature in an

Figure 10. Phase diagrams of content ratio of hexagonal and orthorhombic at various temperatures. The red circle and blue square are the content ratio with increasing temperature, starting from pure hexagonal and orthorhombic, respectively.

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CONCLUSION Pure phase EuF3 orthorhombic and hexagonal are prepared. PL spectra under hydrostatic pressure show that a phase transition from orthorhombic to hexagonal takes place from 4.7 to 9.5 GPa. The hexagonal phase of EuF3 maintains the structure stability under hydrostatic pressures up to 26.4 GPa. In contrast to hydrostatic pressure, the hexagonal phase of EuF3 undergoes a cyclic phase transformation to orthorhombic at 70 MPa, to hexagonal at about 3 GPa, and completely back to hexagonal at about 10 GPa under nonhydrostatic pressure. The similar cyclic phase transformations are also observed during heating. With the increasing temperature, the pure hexagonal EuF3 transforms to orthorhombic at 550 °C, and then back to hexagonal at 855 °C. The content phase diagrams are achieved under high pressure and at high temperature.

Figure 9. Luminescence spectra of hexagonal EuF3 at various temperatures.

argon atmosphere. Hexagonal EuF3 shows structural stability until 550 °C; then, a new peak at 586.5 nm appears and the peaks at 588.5 and 681 nm become weaker and disappear completely above 630 °C. The PL spectra indicate that the hexagonal phase completely transforms to orthorhombic at 630 °C. The PL intensity of hexagonal EuF3 strongly decays with the increasing temperature. However, the emission intensity

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b05907. 18785


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Pressure-dependent PL spectra, temperature-dependent PL spectra, and PL spectrum after annealing (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86 551 63607671 (Z.Z). *E-mail: [email protected] (X.Z). Author Contributions ⊥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 11304300 and 11404320) and 909 project of the China Academy of Engineering Physics (Grant No: 991912).

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