Pressure-Induced Amorphization in BaF2 Nanoparticles - The Journal

Article pubs.acs.org/JPCC

Pressure-Induced Amorphization in BaF2 Nanoparticles Jingshu Wang,*,†,‡ Qiliang Cui,*,‡ Tingjing Hu,† Jinghai Yang,† Xiuyan Li,† Yanqing Liu,† Bo Liu,‡ Wanqi Zhao,† Hongyang Zhu,‡ and Lili Yang† †

Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, P. R. China ‡ State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: Synchrotron X-ray diffraction (XRD) is performed on BaF2 nanoparticles to study the structural phase transition up to about 30 GPa under ambient temperature. We observed that the cubic structure in BaF2 nanoparticles is stable up to 6.8 GPa, a level much higher than that in bulk BaF2. Pressure-induced amorphization (PIA) occurs in BaF2 nanoparticles under compression, which results in a high-density amorphous (HDA) form. Upon pressure release, a low-density amorphous (LDA) form is maintained at ambient conditions. This study is the first to demonstrate that PIA and polyamorphism exist in BaF2 nanomaterials and that finite grain size plays a critical role in inducing PIA and polyamorphism.

1. INTRODUCTION Barium fluoride (BaF2), which holds intrinsic optical and lattice-dynamical properties, has been widely studied as a type of important alkaline earth metal fluoride.1 In particular, the BaF2 crystal is an excellent high-density luminescent material and is extensively used for γ-ray and elementary particle detection.2 At ambient conditions, bulk BaF2 exhibits a cubic fluorite structure with a space group of Fm3m, in which Ba and F atoms occupy Wyckoff 4a and 8c positions, respectively. At high pressure, the structural phase transition follows the sequence of fluorite structure (Fm3m) to α-PbCl2-type structure (Pnma) and then to Ni2In-type structure (P63/mmc).3−6 Pressureinduced nanomaterials not only display a series of novel mechanical properties and transformations but also perform different behaviors from their bulk counterparts.7−12 However, the phase transformations of the nanoscale BaF2 system remain short of reports to date. Our previous study has reported in situ high-pressure synchrotron XRD results of 19 nm (averaged grain size) BaF2 nanocrystals.13 The phase transitions were discovered at 5.8 and 14.4 GPa; both phase transition pressures were higher than those of bulk BaF2. The α-PbCl2-type phase was retained during decompression back to 0 GPa. The phase transition pressure and routines of the nanomaterials strongly depend on their grain size. Beyond that the pressure-induced phase transition of nanoscale BaF2 has no other reports. PIA refers to a phenomenon characterized by the transformation of crystalline materials into amorphous solids upon compression, which currently becomes an intensely developing subject of research. The tetrahedrally coordinated solids, such as H2O, Si, or C, are the systems that would most probably © XXXX American Chemical Society

exhibit PIA. Although PIA has been observed in many bulk materials, only a few publications have reported such phenomenon in nanomaterials (TiO2,14,15 Y2O3,11 and PbTe16). A variety of mechanisms have been proposed to understand these transitions, and several of these mechanisms are interrelated.11 In this paper, in situ synchrotron XRD studies on the BaF2 nanoparticles with size of 14 ± 3 nm are conducted at high pressure and room temperature. Our study is the first time to show the occurrence of PIA and polyamorphism existed in BaF2 nanomaterials. High-resolution transmission electron microscopy (HRTEM) characterization demonstrates the essence of the polyamorphism. Analysis shows that grain size plays a significant role in PIA and HDA−LDA transition. The grain size strongly confines the nucleation sites to further develop into large grains and eventually generates distinct highpressure behaviors.

2. EXPERIMENTAL SECTION A liquid−solid−solution solvothermal route was used to prepare the BaF2 nanoparticles.17,18 In a typical experiment, a mixture of oleic acid, ethanol, and sodium hydroxide, together with 2 mmol of Ba(NO3)2 and 4 mmol of NaF, was thoroughly stirred and then loaded into an autoclave. The hydrothermal treating condition was at a constant temperature of 180 °C for about 24 h. Finally, we set the system to cool down to room temperature automatically and extracted products by using Received: February 24, 2016 Revised: May 18, 2016

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DOI: 10.1021/acs.jpcc.6b01858 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Representative TEM micrographs and ED pattern (inset in (a)) of the typical product: (a) low-magnification image; (b) highmagnification image. (c) Particles size distribution histograms.

centrifugation. The sample was characterized by transmission electron microscopy (TEM; 200 kV, Hitachi, H-8100IV), HRTEM (JEM-2200FS), and powder XRD with Cu Kα radiation (λ = 1.5418 Å). High pressure was realized by using a diamond anvil cell (DAC) with a culet size of 400 μm. A small amount of BaF2 nanoparticles and a tiny ruby chip were loaded into the gasketed sample hole. Silicone oil was adopted as pressure medium to maintain hydrostatic pressure. The frequency shift of the ruby R1 fluorescence line was chosen to determine the pressure. High-pressure synchrotron XRD experiments were completed at the B2 station of the high-pressure station of Cornell High Energy Synchrotron Source (CHESS), Cornell University. The incident wavelength was 0.485 946 Å. The highest pressure in this experiment was carried up to 30 GPa at room temperature. The MAR165 CCD detector was used to collect diffraction data. The FIT2D software was adopted to convert XRD image patterns into one-dimensional intensity versus diffraction angle 2θ plots. High-pressure structural information was refined by the Rietveld method in Material Studio.

3. RESULTS AND DISCUSSION TEM and electron diffraction (ED) techniques were applied to the BaF2 nanoparticles to characterize morphology and structure. Figure 1 exhibits the low/high-resolution TEM image and the histograms of the particle size distribution of the prepared sample BaF2 nanocrytals. The synthesized BaF2 nanocrytals (average size 14 ± 3 nm) were well crystallized and exhibited a narrow diameter distribution. The inserted ED pattern in Figure 1a clarifies that the nature of the polycrystal product was a cubic fluorite structure. Figure 2a presents the Rietveld refinement of the prepared BaF2 nanoparticles performed at ambient conditions; the results are in accordance with the cubic cell with space group Fm3m and the final Rietveld residual Rwp = 0.0913. The cubic fluorite phase was

Figure 2. (a) Rietveld refinement of diffraction pattern of synthesized BaF2 nanoparticles. (b) EDX spectrum of the synthesized BaF2 nanoparticles.

characterized by “Ba” atoms occupying the (0, 0, 0) position and “F” atoms occupying the (0.25, 0.25, 0.25) position. According to the Debye−Scherrer formula, the average particle size of the BaF2 nanoparticles was about 12 nm. This result was B

DOI: 10.1021/acs.jpcc.6b01858 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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diffraction peaks belonging to the fluorite structure weakened. Interestingly, when the pressure was higher than 10.2 GPa, the diffraction peaks obviously broadened, and no other new peak appeared for the high-pressure phase. The phase transformation into the orthorhombic α-PbCl2-type structure was not completed. It indicates that structural disorder of BaF2 nanoparticles set in, and the sample had started to transform into an amorphous phase above the pressure of 10.2 GPa. Beyond 21.4 GPa, almost all diffraction peaks vanished, and only two broad and weak band peaks remained and persisted to the maximum pressure value of 30 GPa. The results demonstrate that the sample underwent two structural transformations from cubic to orthorhombic to amorphous. According to the sequence of phase transformation of bulk and 19 nm nanoscale BaF2,5,6,13 the two amorphous diffraction peaks at 3.26 and 2.10 Å are most probably associated with the Ni2In-type structure. After the pressure released from 30 GPa, one broad peak was shown in the ambient pressure spectrum (Figure 3b); this is distinct from the results of the high-pressure amorphous phase at 30 GPa. Upon decompression, a new finding of low-pressure amorphous phase was discovered. The broad peak at 3.49 Å is most likely associated with the α-PbCl2type structure. The above-mentioned results imply that the intimate relationship between the HDA and LDA phase is related to the Ni2 In-type and α-PbCl 2 -type structure, respectively. This behavior is similar to the previous studies on Si19 and TiO2.15 Therefore, it can be reasonably concluded that polyamorphism exists in the BaF2 nanoparticles. Table 1 lists the critical transition pressure (PT) and transformation of fluorite-type BaF2. The comparison results demonstrate the discrepancy among the bulk and 19 nm nanoscale BaF2 and our synthesized 14 nm BaF2 nanoparticles. Both bulk and 19 nm nanoscale BaF2 disclosed two phase transformations, and the symmetry of the high-pressure phases is refined as orthorhombic α-PbCl2-type structure and hexagonal Ni2In-type structure, respectively. The orthorhombic phase is survived during decompression back to 0 GPa. Our 14 nm BaF2 nanoparticles showed distinct high-pressure behaviors. The nanoparticles did not only show that the pressure of cubic to orthorhombic phase transition was higher than that of the bulk BaF2 (3 GPa) and enhanced the phase stability but also that the transformation to an amorphous phase (PIA) occurred. In fact, the nonhydrostatic condition is a factor facilitating the occurrence of PIA and polyamorphism. Silicone oil was used as the current pressure medium in our studies. Silicone oil creates less shear stress below or near 10 GPa and can provide ideal hydrostatic condition.20 The first phase transformation of the 14 nm BaF2 nanoparticles was observed at 6.8 GPa, and the sample commenced the PIA at 10.2 GPa. Obviously, the hydrostatic condition was insufficient to contribute to PIA. Numerous previous works showed that the grain size directly affects phase stability and even phase transition routines.21−23 When the value of grain size was less than that of the critical size, the nanoscale effects of the samples were manifested. We

almost the same as the value determined from TEM and ED. Further energy-dispersive X-ray spectroscopy (EDX) was performed on the synthesized BaF2 nanoparticles to investigate the composition (Figure 2b). The EDX spectrum shows the presence of the F, Ba, C, and O elements in the sample. The C and O peaks originated from the surfactant (oleic acid), which capped on the surface of the BaF2 nanoparticles. It confirms that the formed nanoparticles were pure BaF2. Figure 3 displays some selected XRD spectra with the pressure gradually increased to 30 GPa. All diffraction peaks

Figure 3. (a) High-pressure XRD patterns of BaF2 nanoparticles up to 30 GPa at room temperature (the peak (marked with diamond) is derived from the gasket). (b) A comparison between the X-ray patterns of the as-synthesized BaF2 nanoparticles obtained at 30 GPa and recovered at ambient conditions, respectively (two weak peaks (marked with asterisk) are derived from the synchrotron XRD diffraction system).

shifted to higher degrees accompanied by the change of diffraction peaks intensities. The (111) diffraction peak became asymmetric at 6.8 GPa, indicating the appearance of a new peak for the high-pressure phase. This result implies that the cubic fluorite structure began transforming into the orthorhombic αPbCl2-type structure. Some additional diffraction peaks began to emerge at 8.8 GPa. Meanwhile, the intensity of the

Table 1. Transition Pressure (PT) and Transformation of Fluorite-Type BaF2 PT (GPa) morphology

size

fluorite-type to α-PbCl2-type

α-PbCl2-type to Ni2In-type

structure (quenched)

ref

bulk nanocrystals nanoparticles

micro 19 ± 3 nm 14 ± 3 nm

2.84−3 5.8 6.8

12−12.8 14.4

orthorhombic orthorhombic amorphous

5,6 13 this work

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DOI: 10.1021/acs.jpcc.6b01858 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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thus deduced that the enhanced stability and PIA for the BaF2 nanoparticles were due to the fact that the grain size of 14 nm was smaller than that for the nanoscale BaF2 in previous literature,13 even smaller than the critical size. In the BaF2 nanoparticles, the α-PbCl2-type phase nucleus appeared when the pressure reached 6.8 GPa. However, the grain size strongly confined the nucleation sites to further form into large grains, and the nucleation sites eventually developed into the HDA form. The HDA form internally containing the Ni2In-type nucleation sites transformed into the α-PbCl2-type nucleus during decompression. The α-PbCl2-type to Ni2In-type phase transition in the bulk material, accompanied by the reduction of volume, involved a 9 Ba−F to 11 Ba−F coordination change.5 The change of atomic coordination number contributed to the density difference between the HDA and LDA polyamorphs, thereby explaining that the phase transition of HDA−LDA occurred between the two polyamorphic forms. To further prove our deduction, HRTEM analysis was performed on the surviving sample quenched from 30 GPa. The long-range ordered structure is not displayed in the HRTEM image in Figure 4; only some random short-range

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01858. To further reveal the PIA occurred in BaF2 nanoparticles upon compression and decompression, the 2D synchrotron X-ray diffraction patterns are shown in Figure 1S (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected], Ph 86-434-3294566, Fax 86-4343294566 (J.W.). *E-mail [email protected], Ph 86-431-85168346, Fax 86-43185168346 (Q.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 11404137 and 61378085), Program for New Century Excellent Talents in University (No. NCET-13-0824), Program for the development of Science and Technology of Jilin province (Item No. 20150204085GX), and Twentieth Five-Year Program for Science and Technology of Education Department of Jilin Province (Item No. 20150221).

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Figure 4. HRTEM image of a representative section of LDA BaF2 nanoparticles as defined by the black rectangle in the inset image.

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4. CONCLUSION In conclusion, the structural transition of the BaF2 nanoparticles of 14 ± 3 nm size was studied by synchrotron XRD to about 30 GPa at ambient temperature. The cubic structure in the BaF2 nanoparticles was stable up to 6.8 GPa; the pressure was much higher than that in the bulk BaF2. As the pressure exceeded 10.2 GPa, the crystalline structure began to transform into an amorphous state. Upon pressure release, the HAD form transformed into an LDA form. The mechanism of PIA and HDA−LDA phase transition was mainly attributed to the effect of the grain size. Below the critical size, the compressive behavior began to display a notable size dependence under extreme conditions. This work not only first reports the occurrence of PIA and the HDA−LDA transition in BaF2 nanoparticles; the pressure-induced behavior also opens a new means for the synthesis of amorphous nanomaterials at ambient conditions. D

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DOI: 10.1021/acs.jpcc.6b01858 J. Phys. Chem. C XXXX, XXX, XXX−XXX