Morphology-Tuned Phase Transitions of Horseshoe Shaped BaTiO3

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Article

Morphology-Tuned Phase Transitions of Horseshoe Shaped BaTiO Nanomaterials Under High Pressure 3

Huafang Zhang, Benyuan Cheng, Quanjun Li, Bingbing Liu, and Yanli Mao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00211 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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The Journal of Physical Chemistry

Morphology-Tuned Phase Transitions of Horseshoe Shaped BaTiO3 Nanomaterials under High Pressure Huafang Zhang†, §,‖ ,‡ Benyuan Cheng‖ ,⊥,‡, Quanjun Li‖ ,*, Bingbing Liu‖ , Yanli Mao†,§,* †

School of Physics and Electronics, Henan University, Kaifeng 475004, China

§

Institute of Micro/Nano Photonic Materials and Applications, Henan University, Kaifeng

475004 China ‖

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China

⊥

China Academy of Engineering Physics, Mianyang, Sichuan 621900, China

ACS Paragon Plus Environment

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ABSTRACT: Exploring new physical properties of nanomaterials with special morphology have been important topics in nanoscience and nanotechnology. Here we report a morphology-tuned structural phase transition under high pressure in the horseshoe shaped BaTiO 3 nanomaterials with an average diameter of 26±4 nm. A direct structural phase transition from the tetragonal to the cubic phase without local rhombohedral distortion was observed at about 7.7 GPa by in-situ high-pressure X-ray diffraction and Raman spectroscopy, which is clearly different from the phase transition processes of the BaTiO 3 bulks and nanoparticles with different size. Additionally, bulk modulus of the tetragonal and cubic phases were determined to be 125.0 GPa and 211.7 GPa, respectively, obviously smaller than the estimated values for BaTiO 3 nanoparticles with the same grain size. Further analysis shows that the unique phase transition process and the enhanced structural stability of the tetragonal horseshoe shaped BaTiO3 nanomaterials, may be attributed to the similar axes compressibility. Comparing with the highpressure study on BaTiO3 nanoparticles, this study suggests that the morphology plays an important role in the pressure-induced phase transition of BaTiO3 nanomaterials.


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1. INTRODUCTION As a classic material, perovskites have been widely used in many fields like energy converters, photo-detectors, multilayer ceramic capacitors and redox-catalysts1-3. Recently, perovskites have attracted much enthusiasm as a fundamental material in solar cells.4-8 As the size of electronic devices becomes smaller, the grain size required for the perovskites is down to nanometers. At such small scale, new physical properties begin to appear and show great effect on the device performance.9,10 Therefore, exploring the physical properties of nanometer sized perovskites has become an important subject in both condensed matter physics and materials science. Pressure is an important tuning parameter, which can reduce atomic distance and change band gaps, providing an effective way to explore the novel properties of materials. Nanomaterials, owing to its special morphology and size effect, always show unique high-pressure behaviors, typical examples are the morphology effect induced direct anatase-to-baddeleyite transition in TiO2

nanowires11

and the

size-dependent

crystalline-amorphous transition in Y2 O3

nanoparticles.12 For the high pressure researches on perovskite nanomaterials, lead zirconate titanate nanomaterials undergo the phase sequence of rhombohedra + tetragonal phases → tetragonal + cubic phases → cubic phase, the transition pressures increase with decreasing particle size.13 And the nanocrystalline lead titanate present a diffused tetragonal-cubic phase transition instead of the normal second order one occurred in their bulk counterparts.14 These results suggested that grain size plays an important role on both structural phase transition sequence and transition pressure of perovskite nanomaterials. Unfortunately, few studies have been reported on the morphology effect on the high-pressure behavior in perovskites. It is well


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known that morphology and size effects coexist in most cases15 , making it more difficult to understand the morphology effects on the high pressure behaviors.11, 16 Therefore, selecting an ideal model perovskite system, for which size effects had been deeply understood, to investigate the morphology effects on high pressure behaviors is very important. Barium titanate (BaTiO3) is a typical perovskite-type material and an important functional material for diverse potential applications in electronic industry. 17 The phase transition process of BaTiO3 in different sizes has been well understood. For bulk materials, a tetragonal-cubic phase transition in long range order occurs at about 2.3 GPa, but tetragonal nanodomains remain up to 9.3 GPa. 18-23 For nanoparticles smaller than 120 nm, the original quasi-cubic phase (cubic symmetry in macroscopic crystal structure and a distorted cubic symmetry in local structure) transforms into the cubic phase at 5-9 GPa.24-26 The 20 nm BaTiO3 nanoparticles are in a mixed tetragonal and orthorhombic phase at ambient conditions, it transforms to an orthorhombic phase at about 10.9 GPa firstly, then to a new high-pressure phase at about 19.8 GPa.27 For the nanoparticles smaller than 5 nm, the original hybrid structure of tetragonal and orthorhombic phases transforms into a pure tetragonal phase at 7.5 GPa and then transforms to the cubic phase at around 17.3 GPa.28 In addition, nano-sized BaTiO3 with different morphologies, including nanowires, pyramid-shaped nanostructures, hollow cylinders, shows obvious morphology-tuned physical properties at ambient conditions.29,

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These published results indicate that BaTiO 3

nanomaterial with special morphology is an ideal model for exploring morphology effects on high pressure behaviors of perovskite nanomaterials. Horseshoe shape nanomaterials show enhanced local field, making the structure promising in applications. 31-33 In this paper, we synthesized 26±4 nm horseshoe shaped BaTiO3 nanomaterials and investigated their highpressure behaviors using synchrotron X-ray diffraction and Raman scattering techniques. The


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tetragonal phase was stabilized to ambient conditions in the 26±4 nm horseshoe shaped BaTiO3 nanomaterials, and a direct structural phase transforms from tetragonal into a well ordered cubic phase was observed at about 7.7 GPa, indicating an obvious morphology-tuned high pressure behaviors. 2. EXPERIMENTAL SECTION 2.1 Synthetic procedures. The horseshoe shaped BaTiO3 nanomaterials were synthesized through hydrothermal reaction. In a typical procedure, 0.05g H 2Ti3O7 nanotubes and 0.202g Ba(OH) 2·8H2O were directly added to a 14 mL Teflon-lined autoclave holding 6 mL deionized water and 5 mL alcohol, and stirred until a homogeneous solution was formed. Then the Teflonlined autoclave was held at 100 ℃ for 24 h. After cooled to room temperature, the precipitate was collected by vacuum filtration and dried in oven at 50 ℃ for 6h. The synthesis details in controlling the size and morphology of the particles are shown in supporting information. 2.2 CHARACTERATION. Samples were characterized by XRD with Cu-kα radiation (λ=0.15418 nm) and transmission electron microscopy (TEM) (H-8100). High-pressure experiments were carried out by using diamond anvil cells (DACs) with 300 μm culet size. The T301 stainless-steel gaskets were preindented to 40 μm and drilled a center hole of 100 μm in diameter as a sample chamber. Silicone oil was used as pressure-transmitting medium. In situ angle-dispersive synchrotron XRD measurements under high pressure were performed at the Advanced Photon Source in the USA (λ=0.4246). High-pressure Raman spectra were recorded on a Renishaw spectrometer with an Ar+ excitation laser (wavelength of 514.5 nm). The ruby fluorescence technique was used to calibrate pressures. All the measurements were performed at room temperature.


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3. RESULTS AND DISCUSSION

Figure 1. (a) TEM images and (b) XRD pattern of the horseshoe shaped BaTiO3 nanomaterials, the bottom lines correspond to the diffraction peaks of the tetragonal phase BaTiO3 taken from PDF NO.81-2201, (c), (d) and (e) are the size distributions of dA , dB and dC. Figure 1a shows the TEM images for the as synthesized horseshoe shaped BaTiO3 nanomaterials with an average diameter of about 26±4 nm. As shown in figure 1(b), the XRD pattern displays clearly peak splitting at about 45°and 65°, all the diffraction peaks can be indexed to the tetragonal phase (PDF No.81-2201) with a space group of P4mm. We obtained the lattice constants a=b=4.018Å, c=4.0413Å and the unit cell volume V0 = 65.2 Å3 , slightly


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larger than the values observed for bulk materials. These results are different with that reported on BaTiO3 nanoparticles with particle size less than 120 nm, for which the tetragonal phase transforms into the cubic phase at ambient conditions. 24 According to early study about the dependence of crystal structure on particle size of BaTiO 324, the decreases of the tetragonal-cubic phase transition temperature is caused by the increased internal pressure (P) in BaTiO 3 nanoparticles, which is given by the expression of P = 2γ/R, where R is the radius of the particles, γ is the surface tension stress that related to the morphology. 34 In our case, any dimensions of the horseshoe shaped BaTiO3 nanomaterials (shown in the inset of figure 1a) are smaller than the critical particle size (120 nm) for triggering tetragonal-cubic phase transitions in BaTiO3, thus it can be deduced that the stabilization of the T phase at ambient conditions could mainly result from the special morphology. The pressure evolution of the XRD patterns for the horseshoe shaped BaTiO3 nanomaterials is shown in Figure 2a. Upon compression, the diffraction peaks broaden gradually, and the diffraction peaks of (002) and (200), (202) and (220) spacing merge into one broad peak, respectively, at about 1.7 GPa. With further compression, the diffraction peaks shift toward smaller d-spacings, without modification of the overall diffraction pattern. In high-pressure region, all the diffraction peaks can be indexed to the cubic phase (PDF No.74 -1963). These results indicate that the tetragonal phase transforms into a high-pressure cubic phase, and the cubic phase is stable up to the highest pressure of 32.7 GPa. Unfortunately, as the diffraction peaks are greatly broadened and overlapped under pressure, the critical pressure of tetragonalcubic transition cannot be determined. We plot the variation of relative d-spacings with applied pressure in figure 2b. The pressure evolution of the relative d-spacings shows obvious changes at 7.7 and 14.2 GPa, which may correspond to the start and finish pressure of tetragonal-cubic


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phase transition (more evidences are shown in our Raman study below). The phase transition pressure is much higher than that of corresponding bulk materials (1.8-3 GPa).19-22 After released to ambient conditions, all the diffraction peaks shift to larger d-spacing without new peaks emerging (figure 2a). However, the crystal structure of the released samples, tetragonal or cubic phase, cannot be determined because the diffraction peaks are greatly broadened and the tetragonal splitting of the diffraction peaks cannot be distinguished due to overlapping.

Figure 2. (a) XRD patterns and (b) variation of relative d-spacings of BaTiO3 at selected pressures. The solid/dash black, magenta and dark yellow lines are drawn as a guide to eye. Figure 3 shows the pressure-dependent variations of normalized unit cell lengths (a/a0 and c/c0) for the horseshoe shaped BaTiO3 nanomaterials within the pressure range of 1.7-8 GPa, and


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for the bulk BaTiO3 materials within the pressure range of 0-5 GPa reported in previous study18 . In the horseshoe shaped BaTiO3 nanomaterials, the compressibility of the c-axis length is found to be similar with that of a-axis length. This is quite different with that observed in corresponding bulks, for which the c-axis length is more compressible than a-axis length in the tetragonal phase, suggesting a morphology-tuned anisotropic compressibility in BaTiO 3 nanomaterials.

Figure 3. Variation of lattice parameters (a/a0 and c/c0) for horseshoe shaped BaTiO3 nanomaterials from this study (filled square and triangles), and corresponding bulk materials from M. Malinowski et al. (open square and triangles) in ref 18. The pressure versus volume data for the horseshoe shaped BaTiO3 nanomaterials is plotted in Figure 4. The volume decreases gradually with increasing pressure, but illustrates abrupt change within the pressure region of 7.7-14.2 GPa. According to early reports, the pressure-volume relation for BaTiO3 nanoparticles with different size also show similar discontinuity under pressure, and the transition pressure increases with decreasing particle size. 25 We list the transition pressures for both of our sample and different size BaTiO 3 nanoparticles in table 1. It is interesting to note that the transition pressure for the horseshoe shaped BaTiO3 is lower than


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that of nanoparticles with the diameter of 30 nm. The data within the pressure region before and after the transition, 1.7-7.7 GPa (L) and 14.2-32.7 GPa (H), were fitted to the third-order BirchMurnaghan equation of states, 7

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2

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

where P, V0 and V are the applied pressure, volume at ambient pressure and volume under pressure P, respectively. B0 and 𝐵0′ are the bulk modulus and its first pressure derivative. 𝐵0′ is fixed to 4. The obtained bulk modulus in the low pressure region is 125.0 GPa and in the high pressure region is 211.7 GPa (shown in Table 1). And both of the values are smaller than the estimated values for BaTiO3 nanoparticles with same grain size (shown in the inset of figure 4).

Figure 4. Pressure versus volume date for the horseshoe shaped BaTiO3 nanomaterials. The boundary of phase change is marked by the shadow region. Inset: The B 0 of horseshoe shaped BaTiO3 nanomaterials and nanoparticles with different size25 . Blue/red star and square denote the B0 of horseshoe shaped BaTiO3 nanomaterials and nanoparticles with different size in low/high pressure region.


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Table 1. The transition pressure (P) for horseshoe shaped BaTiO3 nanomaterials and BaTiO3 nanoparticles with different size.

size (nm) P (GPa)

horseshoe shaped (this study) 26±4 nm 7.7

Nano particles 25 10 10.1

30 9.1

50 6.8

To explore the local structure transition of horseshoe shaped BaTiO3 nanomaterials under pressure, we carried out high pressure Raman measurements in the spectral range from 100 to 800 cm-1 . Figure 5 shows the Raman spectra of the horseshoe shaped BaTiO3 nanomaterials measured during compression and after released to ambient pressure. The Raman spectrum measured before compression shows three broad peak centered near 250 cm-1 [A1(TO)], 520 cm-1 [A1 , E(LO)], 715 cm-1 [A1, E(LO)], and two sharp peaks at about 184 cm -1 [A1(LO)], 305 cm1

[B1, E(TO+LO)], which agrees well with the Raman spectrum reported on the BaTiO 3

nanocrystalline in tetragonal phase35. With increasing pressure, the intensity of all the Raman peaks gradually decreases, and all the Raman peaks show blue shift except for the broad Raman peak at about 520 cm-1 . When the applied pressure increased to 7.7 GPa, the intensities of the broad Raman peaks decrease to very small values, whereas the sharp Raman peaks remain intense. At about 14.1 GPa, all the Raman peaks disappear. We note that these two critical change pressures agree well the start and finish pressure of tetragonal-cubic phase transition observed in our XRD study. According to early reports, the two broad Raman peaks centered at about 250 and 520 cm -1 have been shown to persist in the cubic phase 19, 20 , which could mainly resulted from the disorder in the position of the Ti atoms23 . In contrast, and the sharp Raman peaks near 305 cm-1 and 715 cm-1 disappear completely at tetragonal-cubic phase transition (shown in Table 2), suggesting that presence of sharp Raman peaks indicates a tetragonally


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distorted structure. In our case, both of the sharp and broad Raman peaks disappear at the same pressure of 14.1 GPa, thus demonstrating that the tetragonal phase transform into a cubic phase with Ti atoms at the center of the oxygen octahedrons in the horseshoe shaped BaTiO3 nanomaterials. The structural transition start from 7.7 GPa and finished at about 14 GPa, with the tetragonal and cubic phases coexistence spanning about 6 GPa, much wider than that of BaTiO 3 bulks (1-2 GPa)21 , which may related with the relative wider particle size distributions (shown in Figure 1c-d). After released to ambient condition, all the Raman peaks revert back to the tetragon form, indicates a reversible phase transition process under pressure.

Figure 5. Raman spectra of horseshoe shaped BaTiO3 nanomaterials measured during compression and after released to ambient pressure. Table 2. XRD characterized structure and optical phonon frequencies (ω) in BaTiO3. XRD structure tetragonal cubic

ν1 187±4 182±2

ν2 256±3 248±3 264±3 225±5 227±4 243±7

Raman vibration (cm-1) ν3 ν4 ν5 311±2 517±2 721±3 310±1 512±3 718±2 310±2 525±2 719±3 535±3 540±6 521±4

Ref. [19] [35] [20] [19] [20] [35]


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The [B1+E(3)], [A1 (3)+E(4)] and [E(4)(LO)+A1(3)(LO)] Raman modes frequency (～305 cm-1 , 520 cm-1 and 715 cm-1) of the horseshoe shaped BaTiO3 as functions of pressure are shown in Figure 6. Noticeable changes in slope were observed in the pressure range of 4.5-7.7 GPa. The pressure dependences of these Raman mode frequencies (dν/dp) have been obtained by linear fit the data in the pressure range below and above 7.7 GPa, respectively. The obtained pressure coefficient of the [B1+E(3)] is 1.36 cm-1/GPa below 7.7 GPa, and increases to 1.77 cm-1 /GPa above 7.7 GPa. The pressure coefficient of the [A1(3)+E(4)] is -2.33 cm-1/GPa below 7.7 GPa, and suddenly increases to 4.90 cm -1/GPa above 7.7 GPa. The pressure coefficient of the [E(4)(LO) + A1 (3)(LO)] is 5.46 cm-1/GPa below 7.7 GPa, then decreases to 4.11 cm-1 /GPa above 7.7 GPa. For comparison, we fit the corresponding pressure-Raman mode frequencies data of BaTiO 3 nanoparticles

reported

in

ref.

28,

the

obtained

values

for

[B1+E(3)]/[A1(3)+E(4)]/[E(4)(LO)+A1(3)(LO)] are 1.08/5.12/2.16 cm-1 and 0.98/1.10/3.33 cm-1 below and above 7.7 GPa, respectively. It worth noting that, the coefficient changing is more pronounced in the horseshoe shaped BaTiO3 compared with nanoparticles, which could be closely related with their special morphology. As is well-known, these [B1+E(3)], [A1(3)+E(4)] and [E(4)(LO)+A1(3)(LO)] Raman mode arises from O-Ti-O bond vibrations along c axis and in ab plane (shown in figure 6).36 The pressure coefficient changing maybe arise from a decrease of the displacive component of the Ti atom in the TiO 6 octahedra as the applied pressure is increased above 7.7 GPa (schematic illustration is shown in figure 5).


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Figure 6. Pressure dependence of the frequency of (a) the [B1+E (3)], (b) [A1(3)+E(4)] and (c) [E(4)(LO)+A1(3)(LO)] Raman modes, LO: longitudinal optical mode, (d) The Sketches of the optical vibrational modes for B1, E31 , E32, A1 3, E41 and E42 . It is interesting to note the tetragonal phase transforms into a cubic phase without disorder in the position of Ti atoms in the horseshoe shaped BaTiO3 nanomaterials. This is new recognition for the tetragonal phase BaTiO3. It is well known that the high-pressure behaviors of nanomaterials mainly depend on size. For bulk BaTiO 3 materials, A.K. Sood et al and Uma D. Venkateswaran et al. reported that the tetragonal phase transforms into the cubic phase at about 2 GPa, whereas and the strong broad Raman peaks at about 260 cm-1 and 520 cm-1 are still persist, thus, the presence of disorder in the high-pressure cubic phase was suggested.19, 20 Evidences of the pressure induced disappearance of the disorder were observed by XAS study when pressure increased to about 10 GPa.22 For BaTiO3 nanomaterilas, Jin et al.

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shown that BaTiO3 with

average particles size of 5 nm undergo a phase transition process of T+O →(7.5 GPa) T→(17.3


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GPa) C under pressure, and the cubic phase also show the typical distort cubic Raman spectrum of BaTiO3, indicating that the displacement of Ti atoms also persist in high-pressure cubic phase of BaTiO3 nanoparticles. These are obviously different from the results observed in our horseshoe shaped BaTiO3 nanomaterials, thus the size-effect on local structure transition can be excluded. Previous studies have indicated that morphology is another importance factor in tuning the high pressure behavior of nanomaterials.11, 16 In our case, similar compressibility of the a and c axes was found in horseshoe shaped BaTiO3 nanomaterials. As is well known, the value of tetragonal lattice parameter c is larger than a, and the value of cubic lattice parameter a and c are the same.18 Thus, it can be deduced that the observed similar compressibility of the a and c axes in T phase of the horseshoe shaped BaTiO3 nanomaterials may inhibit the structural transition from the T phase into the distorted C phase in low pressure region, leading to the direct structural transition into the C phase without disorder in the position of the Ti atoms under higher pressure. Compared with the phase transition process of BaTiO3 nanoparticles, these results demonstrate that the selected phase transition under high pressure could be achieved with tuning the morphology. 4. CONCLUSIONS In summary, we synthesized tetragonal BaTiO3 horseshoe shaped nanomaterials with an average diameter of about 26±4 nm, and investigated their high-pressure phase transition behaviors using in situ angle-dispersive synchrotron XRD and Raman spectroscopy. A morphology-tuned structural phase transition from tetragonal directly into the cubic phase with Ti atoms in the center of oxygen octahedron was found under high pressure in the BaTiO3 horseshoe shaped nanomaterials. The structural phase transition starts at 7.7 GPa and completes at about 14.2 GPa, and transforms into the original structure after releasing to ambient pressure.


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Bulk modulus of the tetragonal and cubic phases was determined to be 125.0 GPa and 211.7 GPa, respectively, which are smaller than the estimated values for BaTiO 3 nanoparticles with same grain size. Further analysis shows that the unique phase transition sequence and increased phase transition pressure could be mainly related to the similar axes compressibility in the tetragonal phase of BaTiO3 horseshoe shaped nanomaterials. ASSOCIATED CONTENT Supporting Information The synthesis details in controlling the size and morphology of the particles (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

[email protected]

Notes The authors declare no competing financial interest. Author Contributions ‡These authors contributed equally. ACKNOWLEDGMENT This work was financially supported by the NSFC (11374120 and 51320105007), National Basic Research Program of China (2011CB808200), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1132). The NSFC-Henan Province Joint Fund (U1604144),

Science

and

Technology

Development

Project

of

Henan

Province


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16. Liu, B.; Yao, M.; Liu, B.; Li, Z.; Liu, R.; Li, Q.; Li, D.; Zou, B.; Cui, T.; Zou, G., et al. Highpressure studies on CeO2 nano-octahedrons with a (111)-terminated surface. J. Phys.Chem. C, 2011, 115, 4546-4551. 17. Haertling, G. H. Ferroelectric ceramics: history and technology. J. Am. Ceram. Soc., 1999, 82, 797-818. 18. Malinowski, M.; Lukaszewicz, K.; Asbrink, S. The influence of high hydrostatic pressure on lattice parameters of a single crystal of BaTiO3. J. Appl. Cryst., 1986, 19, 7-9. 19. Sood, A. K.; Chandrabhas, N.; Muthu D. V. S.; Jayaraman, A. Phonon interference in BaTiO3: High-pressure Raman study. Phys.Rev. B, 1995, 51, 8892-8896. 20. Venkateswaran, U. D.; Naik, V. M.; Naik, R. High-pressure raman studies of polycrystalline BaTiO3. Phys.Rev.B, 1998, 58, 14286-14260. 21. Pruzan, Ph.; Gourdain, D.; Chervin, J.C.; Canny, B.; Couzinet, B.; Hanfland, M. Equation of state of BaTiO3 and KNbO3 at room temperature up to 30 GPa. Solid State Commun., 2002, 123, 21-26. 22. Itié, J. P.; Couzinet, B.; Polian, A.; Flank A. M.; Lagarde, P. Pressure-induced disappearance of the local rhombohedral distortion in BaTiO 3. Europhys. Lett., 2006, 74, 706-711. 23. Ehm, L.; Borkowski, L. A.; Parise, J. B.; Ghose, S.; Chen, Z. Evidence of tetragonal nanodomains in the high-pressure polymorph of BaTiO3. Appl.Phys. Lett., 2011, 98, 021901. 24. Uchino, K.; Sadanaga, E.; Hirose, T. Dependenceof the crystal structure on particle size in barium titanate. J. Am. Ceram. Soc., 1989, 72, 1555-1558.


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25. Li, F.Y; Jin, C. C.; You, S.J.; Xiao, C.J; Yu, R.C.; Wang, X. H.; Liu, J.; Li, X.D,; Li, Y.C.; Chen, L.C. Pressure-induced phase transition in BaTiO3 nanocrystals. Chin. Phys. Lett, 2006, 23, 1249. 26. Frenkel, A. I.; Frey, M. H.; Payne, D. A. XAFS analysis of particle size effect on local structure in BaTiO3. J. Synchrotron Radiat., 1999, 6, 515-517. 27. Zhang, W.; Chen, L.; Jin, C.; Deng, X.; Wang, X.; Li, L. High pressure Raman studies of dense nanocrystalline BaTiO3 ceramic. J. Electroceram., 2007, 21, 859-862. 28. Han, W.; Zhu, J.; Zhang, S.; Zhang, H.; Wang, X.; Wang, Q.; Gao. C.; Jin, C. Phase transitions in nanoparticles of BaTiO3 as functions of temperature and pressure. J. Appl.Phys., 2013, 113, 193513. 29. Ma, W.; Harnagea, C.; Hesse. D.; Gösele, U. Well-ordered arrays of pyramid-shaped ferroelectric BaTiO3 nanostructures. Appl. Phys. Lett., 2003, 83, 3770-3772. 30. Tang, H.; Lin, Y.; Sodano, H. A. Synthesis of high aspect ratio BaTiO3 nanowires for high energy density nanocomposite capacitors. Adv.Energy Mater., 2013, 3, 451-456. 31. Du, Z.; Hu, B.; Ma. F.; Liu, J. Local field enhancement tuning of horseshoe-shaped nanoparticles. J. Electron. Sci. Technol., 2016,15,259-262. 32. Fahmi, A.; D’Ale´o, A.; Williams, R. M.; Cola, L. D.; Gindy, N.; Vo¨gtle, F. Converting self-assembled gold nanoparticle/dendrimer nanodroplets into horseshoe-like nanostructures by thermal annealing. Langmuir. 2007, 23,7831-7835.


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33. Panda, R.; Rathore, V.; Rathore, M. K.; Shelke, V.; Shripathi, T.; Jain, D.; Ganesan, V. Selfassembled horseshoe type shape transitions in morphology of Cu substituted ZnO. AIP Conference Proceedings, 2017, 1832, 080013. 34. Shi, L.; Dai, Y. Synthesis, formation mechanism and photoelectric properties of GeS nanosheets and nanowires. J. Appl. Crystallogr., 2014, 47, 527-531. 35. Shiratori, Y.; Pithan, C.; Dornseiffer, J.; Waser, R. Raman scattering studies on nanocrystalline BaTiO3 Part I—isolated particles and aggregates. J. Raman Spectrosc. 2007, 38, 1288-1299. 36. An, W.; Liu, T.H.; Wang, C.H.; Diao, C.L.; Luo, N.N.; Liu, Y.; Qi, Z.M.; Shao, T.; Wang, Y.Y.; Jiao, H. Assignment for vibrational spectra of BaTiO3 ferroelectric ceramic based on the first-principles calculation. ACTA Phys.Chem. Sin. 2015, 21, 1059-1068


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Toc Graphic


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Figure 1. (a) TEM images and (b) XRD pattern of the horseshoe shaped BaTiO3 nanomaterials, the bottom lines correspond to the diffraction peaks of the tetragonal phase BaTiO3 taken from PDF NO.81-2201, (c), (d) and (e) are the size distributions of dA, dB and dC. 119x118mm (300 x 300 DPI)



Figure 2. (a) XRD patterns and (b) variation of relative d-spacings of BaTiO3 at selected pressures. The solid/dash black, magenta and dark yellow lines are drawn as a guide to eye.


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Figure 3. Variation of lattice parameters (a/a0 and c/c0) for horseshoe shaped BaTiO3 nanomaterials from this study (filled square and triangles), and corresponding bulk materials from M. Malinowski et al. (open square and triangles) in ref 18. 80x56mm (300 x 300 DPI)



Figure 4. Pressure versus volume date for the horseshoe shaped BaTiO3 nanomaterials. The boundary of phase change is marked by the shadow region. Inset: The B0 of horseshoe shaped BaTiO3 nanomaterials and nanoparticles with different size25. Blue/red star and square denote the B0 of horseshoe shaped BaTiO3 nanomaterials and nanoparticles with different size in low/high pressure region. 80x59mm (300 x 300 DPI)


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Figure 5. Raman spectra of horseshoe shaped BaTiO3 nanomaterials measured during compression and after released to ambient pressure. 80x59mm (300 x 300 DPI)



Figure 6. Pressure dependence of the frequency of (a) the [B1+E(3)], (b) [A1(3)+E(4)] and (c) [E(4)(LO)+A1(3)(LO)] Raman modes, LO: longitudinal optical mode, (d) The Sketches of the optical vibrational modes for B1, E31, E32, A13, E41 and E42. 119x69mm (300 x 300 DPI)


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Morphology-Tuned Phase Transitions of Horseshoe Shaped BaTiO3

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