Four-parameter Semi-quantum Model Research Articles

Far-infrared spectroscopy in ordered and disordered BaMg1/3Nb2/3O3 microwave ceramics

Ba ( Mg 1/3 Nb 2/3 ) O 3 ceramics with suitable microwave dielectric properties for application in wireless communications and information access technologies were studied by far-infrared spectroscopy. Samples with different B-site ordering degrees, obtained by hydrothermal synthesis followed by sintering at various temperatures, were employed in this investigation. The sixteen infrared modes predicted by factor-group analysis were observed and adjusted according to a four-parameter semiquantum model. The dispersion parameters were determined in order to calculate the real part of the dielectric permittivity and the quality factors associated with the dielectric losses in the microwave region. The materials exhibited increasing ε0 and Q values up to 1100 °C, increasing more substantially when the temperature attained 1300 °C. The B-site ordering played an important role on this behavior along with the microstructural evolution above 1100 °C (grain growth), which also increased the phonon lifetime and contributed to the Q improvement. Kramers–Kronig analyses were carried out in all experimental data and the contributions of the main optical polar modes to the dielectric and microwave properties were carefully analyzed in order to identify and attribute the Ba–BO3 external mode, the inner modes related to the O–Mg–O and O–Nb–O bending vibrations, and the stretching modes of each MgO6 and NbO6 octahedron.

Far-infrared reflectivity of GeSe2

The far-infrared reflectivity of single-crystal GeSe2 has been measured with polarized light when E//a and E//b. Using Kramers-Kronig analysis and four-parameter semi-quantum model fits of the reflectivity data, the infrared-active TO and LO modes (phonon frequencies) and damping factors have been determined. Five reststrahlen peaks were observed for the case E//a but only one for E//b. The ratio of intralayer to interlayer force constants are discussed and appeared to be between 18 and 30.

Anharmonicity in several-polar-mode crystals: adjusting phonon self-energy of LO and TO modes in Al2O3and TiO2to fit infrared reflectivity

The validity of a four-parameter semi-quantum (FPSQ) model to fit infrared reflectivity spectra of crystals having a large number of phonon branches is discussed in view of the results of anharmonic quantum theories. The form of the quantum damping function may indeed be simplified when the two-phonon density of states resembles a broad continuum in the vicinity of the resonance frequency. Infrared reflectivity data of corundum obtained at temperatures ranging from 77 to 1775K are fitted with the aid of the FPSQ model. The temperature dependence of damping of TO and LO modes is analysed and it is found that phonon lifetimes are essentially limited by anharmonic three-phonon coupling described by the cubic hamiltonian consistent with two qualitative arguments.