Ta-Ryeong Park

Dielectric properties and crystal structure of Ba6-3x(Nd, M)8+2xTi18O54 (M=La, Bi, Y) microwave ceramics

Crystal structure and dielectric property of tungsten-bronze type microwave dielectric ceramics, i.e., BaOLa2O34TiO2 and Ba6-3x (Nd, M)d8+2x Ti18O54 (M = Y, Bi and x = 0.5, 0.7), are analysed. The optimum properties obtained in Ba(Nd1-xBix)2O34TiO2 were εr = 89–92, Qf = 5855–6091 GHz, and τf = −7–+7 ppm/°C x = 0.04–0.08. The Y-substitution in BaO(Nd1-xYx)2O34TiO2 reduces the dielectric constant εr. Both the Y and Bi substitutions make τεr positive. The relative dielectric constant εr and temperature coefficient τεr are 109.5 and −180 ppm/°C in BaOLa2O34TiO2, 76 and +40 ppm/°C in BaO(Nd0.77Y0.23)2O34TiO2, respectively. The crystal structures were refined by Rietveld method using x-ray and neutron diffraction data. The most reliable results were obtained by refining the cation positions using the x-ray data and the oxygens from the neutron with a superlattice structure model Pnam(c-axis ≈ 7.6 Å). The refined structures show that the a/c ratios are related to the apical oxygen displacements of the Ti–O octahedra. The substitution of the small radius atom, Y, produced a structure of severely tilted and distorted Ti–O octahedra and large a/c ratio, while the large radius atom, La, small a/c ratio. Differential scanning calorimetry analysis showed heat anomaly indicating suspected phase transition in these materials. The relation between τεr and octahedron tilting in tungsten-bronze type material is discussed in relation with complex perovskite structure.

The elastic interactions between the intercalants in layered intercalation compounds

Abstract Using continuum elasticity theory, we obtain the general expression for the host layer deformation of the layered intercalation compounds. In our model, an isolated intercalant is treated as a pair of equal δ-function-like forces acting in opposition direction, the magnitude of which is normalized to keep the bounding layer deformation at the intercalant site fixed to that of the intercalate sandwich in saturated intercalation compounds. We also calculate the intralayer and the interlayer interaction between the intercalants in intercalation compounds; our result shows an attractive intralayer interaction and an overall repulsive interlayer interaction. Between the two intercalants located in the adjacent galleries, the interlayer interaction sometimes becomes attractive depending on their lateral separation.

Photoluminescence quenching by phonons and polarons in sodium cryptand sodide

Abstract The scattering of exciton-polaritons by phonons into a spatially extended state is introduced to explain the decrease of the photoluminescence intensity from Na + (C222)Na − with increasing temperature. The temperature dependence of the spectrally and temporally integrated fluorescence intensity is derived. It is shown that the coupling of the electron in the spatially extended state and the breathing mode of the lattice leads to the non-radiative recombination. We use continuum elasticity approximation and variational principle to find the coupling. Elastic constant in Na + (C222)Na − is estimated by employing Bartram and Stonehams criterion.

Photoluminescence of terbium glass investigated by temperature and pressure dependence

The photoluminescence (PL) intensity from terbium glass either decreases or increases with temperature, depending on the wavelength of the excitation argon ion laser. The red shift of photoluminescence excitation (PLE) spectrum with increasing temperature is responsible for this behavior. The absorption spectra also indicates that the excitation resonance is shifting toward lower energy with increasing temperature. At 300 K, the PLE spectrum under 3.6 GPa broadens relative to the one under ambient pressure but with peak position nearly unchanged, indicating that the temperature dependent shift of the excitation resonance is not caused by thermal contraction of the terbium glass but by the fact that electrons have lower average ground state energy at lower temperature.

The in-plane elastic interaction energy of ternary stage-1 graphite intercalation compounds

Abstract By representing intercalants by force dipoles whose magnitudes are configuration dependent, we consider the elastic interaction energy between many intercalants in layered systems whose host layer deformation mediates the elastic interaction. The in-plane elastic interaction energy of ternary stage-1 graphite intercalation compounds, K 1− x Rb x C 8 , is calculated for 0≤ x ≤1. In the case of binary intercalation compounds, it is shown that the attractive intralayer interaction is not given by a simple quadratic function of in-plane intercalant concentration. The effective two-body attractive interaction in a Li-graphite system V 1− x Li x C 6 ( V denotes vacant site) is in good agreement with the experiment.

Effect of temperature on the exciton–polariton fluorescence of sodium cryptand sodide

Abstract We present the temperature dependent photoluminescence of sodium cryptand sodide. Both the time-integrated and time-resolved line shapes red-shift as temperature increases. At a fixed energy the fluorescence rises faster at higher temperature. Both of these effects are understood in terms of thermal widening of the exciton–polariton (EP) population. However, we report that the temporal evolution of the spectrally integrated fluorescence is independent of the temperature. Combined with the fact that the photoluminescence intensity at elevated temperatures decreases, this observation is evidence that some of the EPs annihilate very early before they begin to evolve.

A relationship between sound speed and a nonradiative relaxation process in sodium cryptand sodide

The wavefunction of the spatially extended excitonic state in sodium cryptand sodide is calculated in a semi-continuum approach. The magnitude of the lattice relaxation energy in a breathing mode induced by the electronic charge redistribution is computed in terms of the wavefunction. The sound speed in sodium cryptand sodide is estimated based on the fact that the lattice relaxes sufficiently to have a nonradiative deexcitation of the spatially extended state.