Dapeng Xu

Electron spin resonance study of NiFe2O4 nanosolids compacted under high pressure

In this article, NiFe2O4 nanosolids are prepared by compacting nanoparticles, synthesized by the coprecipitation method, under different pressures. The variations of structure and interface states of the NiFe2O4 nanosolids with compacting pressures are studied by x-ray diffraction and electronic spin resonance (ESR). It is found that the crystal structure of the nanosolids has not changed under pressure up to 6.0 GPa, but the linewidth and g-factor values of their ESR spectra increased significantly with increasing pressure up to 4.5 GPa and then decreased slightly with further increase of pressure. These variations are discussed in terms of the changes of interparticle magnetic dipole interaction and superexchange interaction in NiFe2O4 nanosolids under different pressures. The experimental results suggest that 4.5 GPa is the optimum forming pressure, under which the NiFe2O4 nanoparticles can be compacted into a dense solid with their structure and nanoproperties still preserved

Effect of high pressure on the typical supramolecular structure of guanidinium methanesulfonate.

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.

Optical interband transitions in relaxor-based ferroelectric 0.93Pb(Zn1∕3Nb2∕3)O3–0.07PbTiO3 single crystal

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.

High pressure Raman scattering and X-ray diffraction studies of MgNb2O6

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.

Electrospinning synthesis and photoluminescence properties of SnO 2 : x Eu 3+ nanofibers

SnO2:xEu3+(x=0, 1%, 3%, 5%, molar fraction) fibers were synthesized by electrospinning technology. The size of the as-prepared fibers is relatively uniform and the average diameter is about 200 nm with a large draw ratio. The as-prepared Eu3+ doped SnO2 nanofibers have a rutile structure and consist of crystallite grains with an average size of about 10 nm. A slight red shift of the A1g and B2g vibration modes and an additional peak at 288 nm were observed in the Raman spectra of the nanofibers. The energies of bandgaps of the SnO2 nanofiber with Eu doping of 1% and 3% are 2.64 eV, and the energy of bandgap is 2.94 eV with Eu doping of 5%(molar fraction). There is only orange emission(5D0→7F1 magnetic dipole transition) for Eu doped SnO2 nanofibers, and no red emission could be observed. The orange emission upon indirect excitation splits into three peaks and the peak intensity at the excitation wavelength of 275 nm is higher than that at the excitation wavelength of 488 nm.

Preparation of one-dimensional SnO2–In2O3 nano-heterostructures and their gas-sensing property

Herein, we employ a combination of electrospinning and hydrothermal approaches to synthesize 1D SnO2–In2O3 nano-heterostructures with a series of morphological evolutions. Through variations in the mole ratio of Sn4+ and In3+ ions in hydrothermal condition, several 1D SnO2–In2O3 heterogeneous morphologies have been realized. The proposed growth mechanism for 1D nano-heterostructures is expected to be a nucleation-growth process. In2O3 nanofibers, as templates, provide numerous nucleation sites for the growth of SnO2 nanostructures. As the Sn4+ concentration increases, the SnO2 nucleus can start to grow from the surface of In2O3 template and extend out along the lateral direction until adjacent grains begin to be connected, forming morphological evolutions. The sensor of SnO2 nanocylinders, grown on In2O3 nanofibers (SI-3 sample), exhibits highest response value at optimal operating temperature. The sensor based on SI-3 sample displays quicker recovery capability towards ethanol gas. A rapid recovery rate can be ascribed to the spillover effect and high surface area. The gas-sensing mechanism of 1D SnO2–In2O3 nano-heterostructures has been discussed.

Floating zone growth and optical phonon behavior of corundum Mg4Ta2O9 single crystals

We demonstrate that corundum Mg4Ta2O9 crystals, free of low-angle grain boundaries and bubbles, were prepared for the first time by the infrared heating optical floating zone method. The X-ray diffraction results showed that the crystals had corundum structure, cleaved parallel to the c plane and grew along the a-axis. The temperature-dependent optical phonon behavior of Mg4Ta2O9 were also investigated in the range from 84 to 834 K.

Temperature-dependent optical phonon behaviour of a spinel Zn2TiO4 single crystal grown by the optical floating zone method in argon atmosphere

The spinel Zn2TiO4 single crystals were grown via optical floating zone technology in an argon atmosphere for the first time, the as-grown crystals were dark blue and transparent. And the as-grown crystals, free of bubbles and with low angle grain boundary, grow along the a-axis direction. The annealed crystals were colorless and transparent, and the chemical valence of as-grown and annealed single crystals was characterized by X-ray photoelectron spectra. The room-temperature polarized Raman spectra of spinel Zn2TiO4 crystal were presented and the in situ temperature-dependent Raman spectra of the crystals were also described. DSC results of Zn2TiO4 were also discussed.

Optical phonon behavior of columbite MgNb2O6 single crystals

To explore potential applications, MgNb2O6 single crystal grown previously by optical floating zone method was used as a prototype for optical phonon behavior investigation. Polarized Raman spectra obtained in adequate parallel and crossed polarization were presented. All the obtained Raman modes were identified for the MgNb2O6, in good agreement with previous theory analysis. The selection rules of Raman for the columbite group were validated. Additionally, in-site temperature-dependent Raman spectra of MgNb2O6 were also investigated in the range from 83 to 803 K. The strong four Ag phonon modes all exhibits red shift with the temperature increasing. But thermal expansion of spectra is sectional linear with inflection points at about 373 K. And the absolute value of dω/dT at high temperature is higher than the one at lower temperature.