Dapeng Xu

Jilin University, Yung-chi, Jilin Sheng, China

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Publications (10)22.28 Total impact

  • Ya Fan, Jing Zhou, Dapeng Xu
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    ABSTRACT: With diamond anvil cell as a high pressure apparatus, the high pressure Raman spectra of the 25% (V/V) oleic acid solution with CCl4 as solvent were measured up to 0.84 GPa. Its phase transition process was investigated by analyzing the changes of the Raman bands at different pressures. The results show that oleic acid in 25% (V/V) solution transforms from liquid phase to the crystalline α phase in the 0.15–0.20 GPa pressure range. The degree of the molecular conformational order increases in this transition. In the 0.28–0.40 GPa pressure range, oleic acid in solution further transforms to the crystalline γ phase. This transition is a disorder–order type. In contrast to pure oleic acid, the solidifying pressure of oleic acid in solution is much lower and there is one more intermediate phase (the crystalline α phase) during compression.
    High Pressure Research 01/2014; 34(2). · 0.90 Impact Factor
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    ABSTRACT: Mg4Nb2O9 single crystals were prepared by the optical floating zone method. While the as-grown were boules polycrystalline and cracked with dimensions of ∅ 5–8 mm×L 91 mm, they contained single crystal regions as large as ∅ 4 mm×L 15 mm. The X-ray diffraction analysis showed that the crystals had the corundum structure, grew along the a-axis and cleaved parallel to the c plane. Polarized light microscopy, in a cross-transmission configuration measurement, showed that the crystal was free of low-angle grain boundaries and bubbles. The optical interband transitions were measured from the absorption spectrum of the crystal along the c-axes at room temperature in the range of 200–800 nm. Based on the theory of band-to-band transitions, the direct band gap was determined for the first time to be Eg=5.09 eV. The photoluminescence spectra exhibited a broad and strong blue emission band centered at 450 nm.
    Journal of Crystal Growth 01/2014; 402:109–112. · 1.55 Impact Factor
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    The Journal of Physical Chemistry C 01/2014; · 4.84 Impact Factor
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    ABSTRACT: High pressure X-ray diffraction and Raman scattering studies have been carried out on magnesium niobate (MgNb2O6) in a diamond anvil cell (DAC) at room temperature up to 40 GPa. A pressure-induced phase transition was observed at pressures above 10 GPa accompanied by a softening of the internal ν9 (B3g) modes. Raman scattering results reveal that the distortion of the NbO6 octahedra decreases under high pressure. According to the X-ray diffraction data, the high pressure phase can be indexed with a monoclinic unit cell. The initial orthorhombic phase changes to a monoclinic phase possibly due to the rotation of the NbO6 octahedra, similar to some perovskite ABO3 structures.
    RSC Advances 06/2013; · 3.71 Impact Factor
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    ABSTRACT: Polarized Raman spectroscopy was performed to investigate the local lattice structure and phase transitions of unpoled 0.24Pb(In1/2Nb1/2)O3–0.43Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 (0.24PIN–0.43PMN–0.33PT) single crystal in the temperature range from 30 °C to 260 °C. MA- and MC-type monoclinic phases were detected by micro-Raman spectra measured in different micro areas. Temperature dependence of Raman intensities, frequency shifts, mode merge and intensity ratios in the VV and VH geometries were investigated. Our results indicated that the monoclinic–tetragonal (M–T) phase transition of the ternary relaxor-based ferroelectric single crystal 0.24PIN–0.43PMN–0.33PT occurs at 85 °C, which is verified by the mode merging from 520 cm−1 and 580 cm−1 to 500 cm−1, and the tetragonal–cubic (T–C) phase transition happens at 200 °C based on the vanishing mode at 780 cm−1 measured in the VH polarization.
    Journal of Alloys and Compounds 02/2013; 551:98–100. · 2.73 Impact Factor
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    ABSTRACT: A high-quality Zn2TiO4 crystal was used as a prototype for determination of the optical interband transitions by measuring the absorption spectrum of the crystal along the a-axes at room temperature in the range of 200–1000 nm. Based on the theory of band-to-band transitions, the indirect energy gap and the phonon energy were determined as Egi = 3.29 eV and Ep = 0.09 eV, respectively. Also, this phonon was just the vibrational mode at 721 cm−1 in the Raman spectra, which participated in the indirect transitions of the Zn2TiO4.
    physica status solidi (a) 12/2012; 209(12):2596-2599. · 1.21 Impact Factor
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    ABSTRACT: With cetyltrimethylammonium bromide (CTAB) as a soft template, magnesium borate (Mg2B2O5) one-dimensional micro/nanorods were synthesized. The products prepared in the absence of CTAB were Mg2B2O5 nanoparticals and needles. However, using CTAB as a soft template the products were Mg2B2O5 whiskers (diameter: 200 ± 10 nm, length: 1–2 μm). The formation mechanism was discussed. In addition, the experimental and theoretical Raman spectra of Mg2B2O5 were reported for the first time, and the possible vibrations modes of Mg2B2O5 crystals were assigned based on the calculation results.
    Materials Research Bulletin. 11/2012; 47(11):3650–3653.
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    ABSTRACT: We report the high-pressure response of guanidinium methanesulfonate (C(NH(2))(3)(+)·CH(3)SO(3)(-), GMS) using in situ Raman spectroscopy and synchrotron X-ray diffraction (XRD) techniques up to the pressures of ~11 GPa. GMS exhibits the representative supramolecular structure of two-dimensional (2D) hydrogen-bonded bilayered motifs under ambient conditions. On the basis of the experimental results, two phase transitions were identified at 0.6 and 1.5 GPa, respectively. The first phase transition, which shows the reconstructive feature, is ascribed to the rearrangements of hydrogen-bonded networks, resulting in the symmetry transformation from C2/m to Pnma. The second one proves to be associated with local distortions of methyl groups, accompanied by the symmetry transformation from Pnma to Pna2(1). The cooperativity of hydrogen bonding, electrostatic, and van der Waals interactions, as well as mechanisms for the phase transitions is discussed by means of the local nature of the structure.
    The Journal of Physical Chemistry B 03/2012; 116(10):3092-8. · 3.61 Impact Factor
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    ABSTRACT: Manganese oxyborate (Mn2OBO3) necklace-like nanofibers have been successfully synthesized by an electrospinning technique. First, poly(vinyl pyrrolidone) PVP/[Mn(CH3COO)2 + H3BO3] composite fibers (average size of 300 ± 10 nm in diameter) were fabricated by an electrospinning method using a solution that contained PVP and manganese acetate and boric acid. Manganese oxyborate (Mn2OBO3) nanofibers were prepared by sintering the above composite fibers in air at 600, 700, and 800 °C, and the calcined nanofibers were characterized by X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. After sintering at 600 °C, monoclinic phase Mn2OBO3 nanofibers of 180 ± 20 nm in diameter were successfully obtained. Morphology analysis showed that the surface of the 700 °C-sintered Mn2OBO3 nanofibers (average size of 250 ± 20 nm in diameter) was rough and they had fiberlike structure with packed particles. The 800 °C-sintered Mn2OBO3 fibers have necklace-like morphology. The crystallite size of the nanoparticles contained in nanofibers increases from 100 to 200 nm with increasing calcination temperature between 700 and 800 °C. The morphology changes of the Mn2OBO3 nanofibers can be attributed to the grain growth during the calcinations process. The Raman spectrum of Mn2OBO3 was provided and assigned.
    Physica Status Solidi (A) Applications and Materials 10/2010; 208(1):114 - 117. · 1.53 Impact Factor
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    ABSTRACT: The optical transmission spectrum of [111]c poled relaxor-based ferroelectric single crystal 0.93Pb(Zn1/3Nb2/3)O3–0.07PbTiO3 (PZN–0.07PT) was measured in the range of ultraviolet to near infrared. The optical absorption edge has been determined and the wavelength dependence of the absorption coefficient was calculated. The direct energy gap Egd = 3.144 eV, indirect energy gap Egi = 2.915 eV, and phonon energy Ep = 0.097 eV (or 782 cm−1) were determined based on the theory of band to band transitions. It was also confirmed by Raman spectra that the indirect transition for the [111]c poled PZN–0.07PT single crystal is mainly due to the contribution of 780 cm−1 phonon corresponding to the Nb–O–Zn bond stretching mode.
    Journal of Applied Physics 06/2010; 107(11). · 2.21 Impact Factor