Cui Qiliang

A New Method for Preparation of Nanocrystalline Molybdenum Nitride

Nanocrystalline molybdenum nitride (γ-Mo2N) with the cubic structure is prepared by the direct-current arc discharge method in N2 gas, using metal Mo or W rod as a cathode. The x-ray diffraction (XRD) and transmission electron microscopy (TEM) are used to characterize the product. It is found that the conversion of Mo to γ-Mo2N and affinity of Mo to N2 are determined by the nitrogen pressure. Moreover, we compare the effect of Mo and W rod as a cathode for preparing γ-Mo2N. The average size of γ-Mo2N particles is about 5 nm. The rapid quenching mechanism can be used to explain the formation of nanocrystalline γ-Mo2N.

Raman Investigation of Sodium Titanate Nanotubes under Hydrostatic Pressures up to 26.9 GPa

High pressure behavior of sodium titanate nanotubes (Na2Ti2O5) is investigated by Raman spectroscopy in a diamond anvil cell (DAC) at room temperature. The two pressure-induced irreversible phase transitions are observed under the given pressure. One occurs at about 4.2 GPa accompanied with a new Raman peak emerging at 834 cm−1 which results from the lattice distortion of the Ti-O network in titanate nanotubes. It can be can be assigned to Ti-O lattice vibrations within lepidocrocite-type (H0.7Ti1.825V0.175O 4H2O)TiO6 octahedral host layers with V being vacancy. The structure of the nanotubes transforms to orthorhombic lepidocrocite structure. Another amorphous phase transition occurs at 16.7 GPa. This phase transition is induced by the collapse of titanate nanotubes. All the Raman bands shift toward higher wavenumbers with a pressure dependence ranging from 1.58–5.6 cm−1/GPa.

Structural phase transformations of ZnS nanocrystalline under high pressure

In-situ energy dispersive x-ray diffraction on ZnS nanocrystalline was carried out under high pressure by using a diamond anvil cell. Phase transition of wurtzite of 10 nm ZnS to rocksalt occurred at 16.0 GPa, which was higher than that of the bulk materials. The structures of ZnS nanocrystalline at different pressures were built by using materials studio and the bulk modulus, and the pressure derivative of ZnS nanocrystalline were derived by fitting the equation of Birch-Murnaghan. The resulting modulus was higher than that of the corresponding bulk material, which indicates that the nanomaterial has higher hardness than its bulk materials.

Ferroelectric Phase Transition in Nanocrystalline Lead Zirconatitanate System by Raman Scattering

The Raman spectra of PbZr0.3Ti0.7O3 for different grain size as well as at high temperature and high pressure have revealed that its structure is transformed from tetragonal to quasi-cubic phase with decreasing grain size. The phase transition temperature drops down to a lower temperature and the pressure-induced phase transition can be correlated with the diffuse phase transition.

In-situ high pressure X-ray diffraction studies of orthoferrite SmFeO3

The high-pressure behaviors of SmFeO3 are investigated by angle-dispersive synchrotron X-ray powder diffraction under a pressure of up to 40.3 GPa at room temperature. The crystal structure of SmFeO3 remains stable at up to the highest pressure. The different pressure coefficients of the normalized axial compressibility are obtained to be βa = 0.60 × 10−3 GPa−1, βb = 0.79 × 10m3 GPa−1, βc = 1.28 × 10−3 GPa−1, and the bulk modulus (B0) is determined to be 293(3) GPa by fitting the pressure-volume data using the Birch—Murnaghan equation of state. Furthermore, the larger compressibility of the FeO6 octahedra suggests the evolution of the orthorhombic structure towards higher symmetry configuration at high pressures.

Influence of grain size on the soft mode and the phase transition in the nanocrystalline PbTi1−xZrxO3 system

Abstract The Raman spectra of nanocrystalline PbTi 0.75 Zr 0.25 O 3 show that its perovskite phase occurred at a higher crystallization temperature during its sol-gel process than the perovskite PbTiO 3 phase. The effects of grain size on the ferroelectric phase transition and the soft mode have been investigated. The phase transition temperature T c is shifted to a lower temperature with decreasing grain size. The possibility that the additional mode occurring in the low frequency Raman spectra might be due to zone boundary transverse acoustical phonons which couple to the q ∼ 0 soft TO phonon mode is confirmed. The Raman spectra, as a function of pressure, reveal that the phase transition may be correlated with Diffuse Phase Transition (DPT).

High-Pressure and High-Temperature Behaviour of Gallium Oxide

High-temperature and high-pressure behaviours of β-Ga2O3 powder are studied by energy-dispersive x-ray diffraction in a diamond anvil cell (DAC). It is found that the phase transition from the monoclinic β-Ga2O3 to the trigonal α-Ga2O3 occurs at around 19.2 GPa under cold compression. By heating the powder to 2000K at 30 GPa, we confirm that α-Ga2O3 is the most stable structure at the high pressure. Furthermore, the structural transition from β-Ga2O3 α-Ga2O3 is irreversible. After laser heating, the recrystallized Ga2O3 has a preferable (012) orientation. This interesting behaviour is also discussed.

High pressure raman spectra of melamine (C3H6N6) and pressure induced transition

Abstract High pressure Raman spectra of Melamine (C3H6N6) have been studied up to 8.7 GPa. The pressure effect on Raman spectra reveals that the discrepancy between intra and inter-molecule bonds tends to shrink with increasing pressure. The experimental results show that Melamine probably undergoes two structural transitions at 2 and 6 GPa respectively and the structure of the new phase has C1 1 i symmetry probably at 2GPa

Crystal structure and ionic conductivity of Mg-doped apatite-type lanthanum silicates La10Si6−xMgxO27−x (x = 0–0.4)

Lanthanum silicates La10Si6?xMgxO27?x (x = 0?0.4) were prepared by solid state synthesis to investigate the effect of Mg doping on crystal structure and ionic conductivity. Rietveld analysis of the powder XRD patterns reveals that Mg substitution on Si site results in significant enlargement of channel triangles, favoring oxide-ion conduction. Furthermore, an increase of Mg concentration significantly influences the linear density of interstitial oxygen, which plays an important role in ionic conductivity. The Arrhenius plots of La10Si6?xMgxO27?x (x = 0?0.4) suggest that Mg-doped samples present higher conductivity and lower activation energy than non-doped La10Si6O27, and La10Si5.8Mg0.2O26.8 exhibits the highest conductivity with a value of 3.0?10?2 S ?cm?1 at 700 ?C. Such conductive behavior agrees well with the refined results. The corresponding mechanism has been discussed in this paper.

In-situ high-pressure behaviors of double-perovskite Sr2ZnTeO6

Structural and spectroscopic properties of Sr2ZnTeO6 (SZTO) were investigated by angle-dispersive synchrotron X-ray powder diffraction and Raman spectroscopy in a diamond anvil cell up to 31 GPa at room temperature. Although SZTO remained stable up to the highest pressure, the different pressure coefficients of the normalized axial compressibility were obtained as βab = 8.16 × 10−3 GPa−1 and βc = 7.61 × 10−3 GPa−1. The bulk modulus B0 was determined to be 190(1) GPa by fitting the pressure-volume data using the Birch-Murnaghan equation of state. All the observed Raman modes exhibited a broadening effect under high pressure. The vibrational band v1 around 765 cm−1, which is associated with the Te—O stretching mode in the basal plane of the TeO6 octahedron had the largest pressure coefficient, and the Gruneisen parameters for all the observed phonon modes were also calculated and presented. These parameters could be used to measure the amount of uniaxial or biaxial strain, providing a fundamental tool for monitoring the magnitude of the shift of phonon frequencies with strains.