Chun-Gang Duan

Dielectric properties and Maxwell-Wagner relaxation of compounds ACu3Ti4O12 (A=Ca,Bi2∕3,Y2∕3,La2∕3)

We have studied the frequency and temperature dependences of permittivity and impedance of the compounds ACu3Ti4O12 (A=Ca,Bi2∕3,Y2∕3,La2∕3) in the ranges of 10−1–106Hz and −150–200°C. All compounds investigated display similar dielectric properties. Specifically, they all have a Debye-like relaxation and their dielectric constants are independent of frequency and temperature over a wide range. They all have two electrical responses in impedance formalism, indicating that there are two distinct contributions. We attribute them to grains and grain boundaries in the ceramic samples and explain the dielectric behaviors by Maxwell-Wagner relaxation arising at the interfaces between grains and their boundaries.

Dielectric permittivity and electric modulus in Bi2Ti4O11

Frequency and temperature dependences of dielectric permittivity and electric modulus of pure and Ba-doped Bi2Ti4O11 were studied in the ranges of 10−1–106 Hz and −150–350 °C, respectively. We found that the antiferroelectric phase transition temperature of Bi2Ti4O11 decreases with Ba doping. In the permittivity studies, we also observed dielectric relaxation peaks shift to higher temperature with increasing frequency. Furthermore, in the electric modulus formalism, conducting peaks were uncovered above 150 °C in addition to the dielectric relaxation peak. We discussed the mechanisms for the dielectric relaxation and conduction processes based on TiO6 octahedra distortion and a space-charge model.

Polymorphous transformations in alkaline-earth silicates

Structural phase transitions in Ca2SiO4 and Sr2SiO4 are investigated by molecular dynamics simulations. The simulations are based on the potentials calculated from the Gordon–Kim modified electron gas formalism extended to molecular ions. We successfully reproduced the transition γ−αH′−α and β−αL′−αH′−α in Ca2SiO4, and the transition β−α′ in Sr2SiO4. We find that the αL′ phase of Ca2SiO4 is an a×3b×c superstructure of the αH′ phase, while the αH′ phase has a β-K2SO4 structure, and the α phase of Ca2SiO4 has a disordered structure with space group P63/mmc.

Phase transition in single crystal Cs2Nb4O11

We studied temperature dependence of complex capacitance, impedance, and polarized Raman spectra of single crystal Cs2Nb4O11. First, we observed a sharp lambda-shaped peak at 165 degrees C in the complex capacitance, then found drastic changes in the Raman spectra in the same temperature range. Utilizing the pseudosymmetry search of structure space group, we attributed the observed anomalies to a structural change from the room temperature orthorhombic Pnn2 to another orthorhombic Imm2. We also measured room temperature polarized Raman spectra in different symmetries of normal vibrations and assigned high wavenumber Raman bands to the internal vibrations of NbO6 octahedra and NbO4 tetrahedra.

Molecular Dynamics Simulation of the Order-Disorder Phase Transition in Solid NaNO 2

We present molecular dynamics simulations of solid NaNO 2 using pair potentials with the rigid-ion model. The crystal potential surface is calculated by using an a priori method which integrates the ab initio calculationswith the Gordon-Kim electron gas theory. This approach is carefully examined by using different population analysis methods and comparing the intermolecular interactions resulting from this approach with those from the ab initio Hartree-Fock calculations. Our numerics show that the ferroelectric-paraelectric phase transition in solid NaNO 2 is triggered by rotation of the nitrite ions around the crystallographical c axis, in agreement with recent x-ray experiments [Gohda et al., Phys. Rev. B 63, 14101 (2000)]. The crystal-field effects on the nitrite ion are also addressed. An internal charge-transfer effect is found.

Theoretical study on the optical properties of polyvinylidene fluoride crystal

We report our first-principles studies on the linear and non-linear optical properties of ferroelectric polyvinylidene fluoride crystal. Calculated values of the refractive indices agree well with experiments. As regards the second-harmonic-generation coefficients, theoretical values are larger than experimental ones. Possible reasons for the discrepancy are discussed.

First-principles study on the optical properties of KNbO3

We report our studies on the electronic structure and linear and nonlinear optical (NLO) properties of KNbO3 using a first-principles method in the local density approximation (LDA). The calculated results for the refractive indices and second-harmonic-generation (SHG) coefficients agree well with experimental results. From decomposing the nonlinear susceptibility, we find that the primary contribution to the NLO behaviour comes from the hybridization of the O 2p and Nb 4d electron states. In addition, there are two different roles played by the O atoms because of their different distances from the Nb atom; thus we propose a possible way to enhance the SHG coefficients.

High-temperature phase transition in TlN3

Abstract The phase transition in TlN 3 is simulated based on the potentials calculated from the Gordon-Kim modified electron gas model extended to ion molecular crystals. It is found that TlN 3 transforms into a cubic CsCl structure at high temperature due to the rotations of the N 3 ions. Above the phase transition the orientations of the N 3 − ions are random with four preferred orientations with respect to the cubic axes.