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Olive leaf extract pretreated by enzymolysis was selected for studying the thermal stability at different heating rates by thermogravimetric coupled with infrared spectroscopy (TG–FTIR) and nonisothermal theoretical analysis, and products analysis by pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS). Results revealed that thermal decomposition of ethyl acetate extract from olive leaf extract pretreated by enzymolysis (EAEEH) consisted of two steps and had a lower thermal stability. The nonisothermal decomposition kinetics mechanism of EAEEH was D3 model (three-dimensional diffusion, Jander equation, the first step) and F3 model (chemical reaction, n = 3, the second step). TG–IR showed –CH3, –C=C–, –OH, –C–O were continuously accelerated to produce, and evolved gas were CO2, water, CO and CH4 at the end. In addition, it was found that there were mainly polyphenols with phenolic hydroxyl groups in the pyrolysis products by Py–GC–MS, and it was also explained for the thermal stability results. This will provide a theoretical guidance for thermal application of EAEEH in the safe natural food additives, cosmetic and medicine.

First-principles density-functional linearized augmented plane-wave linear-response calculations are presented of the lattice dynamics of tetragonal ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}.$ Phonon frequencies and eigenvectors are obtained throughout the Brillouin zone. Generally good agreement is achieved with experiment, but frequencies of the lowest-lying branches, which involve anharmonic motion of the apical oxygen atoms parallel to the ${\mathrm{CuO}}_{2}$ planes, are underestimated. The octahedral tilt mode at the X point is found to be the most unstable mode throughout the Brillouin zone, consistent with the observed phase transition to the orthorhombic structure at low temperature. The calculated dispersion of the highest frequency ${\ensuremath{\Sigma}}_{3}$ branch is in good agreement with experiment, showing that a proposed large renormalization of the phonon spectrum by a Jahn-Teller electron-phonon interaction is unlikely.

Molecular dynamics simulations of the perovskite oxide KNbO3 are performed with a first-principles effective Hamiltonian. They reveal the prevalence of local polar distortions with short-range chain-like correlations, present even in the paraelectric phase far above Tc. The ordering of these dynamically fluctuating distortions yields the observed temperature sequence of ferroelectric phases. The simulations also reproduce the essential features of diffuse x-ray scattering measurements and the weak temperature dependence of diffuse streak patterns observed by Comes et al. These local distortions suggest an order-disorder character for the transitions. Softening of optical phonon branches is observed in the same simulations not only near q = 0, suggesting a displacive character for the transition, but also over large regions of the Brillouin zone. Dynamic real-space chains thus provide a unified framework for understanding both the order-disorder and displacive characteristics of these phase transitions.

We performed first principles LAPW linear response calculations for KNbO3 in the cubic, experimental tetragonal, and theoretical ground state rhombohedral structures. We found that the Born effective charges and dielectric constants strongly depend on the small atomic displacements involved in the ferroelectric phase transitions. We also calculated the zone center phonons for the cubic structure and compared them with previous LDA results.

The Born effective charge ${\mathrm{Z}}^{\mathrm{*}}$ and dielectric tensor ${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{\ensuremath{\infty}}}$ of ${\mathrm{KNbO}}_{3}$ are found to be very sensitive to the atomic geometry, changing by as much as 27% between the paraelectric cubic and ferroelectric tetragonal and rhombohedral phases. Subtracting the bare ionic contribution reveals changes of the dynamic component of ${\mathrm{Z}}^{\mathrm{*}}$ as large as 50%, for atomic displacements that are typically only a few percent of the lattice constant. ${\mathrm{Z}}^{\mathrm{*}}$, ${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{\ensuremath{\infty}}}$, and all phonon frequencies at the Brillouin zone center were calculated using the ab initio linearized augmented plane-wave linear response method with respect to the reference cubic, experimental tetragonal, and theoretically determined rhombohedral ground-state structures. The ground-state rhombohedral structure of ${\mathrm{KNbO}}_{3}$ was determined by minimizing the forces on the relaxed atoms. In contrast to the cubic structure, all zone-center phonon modes of the rhombohedral structure are stable, and their frequencies are in good agreement with experiment. In the tetragonal phase, one of the soft zone-center modes in the cubic phase is stablized. In view of the small atomic displacements involved in the ferroelectric transitions, it is evident that not only the soft-mode frequencies but also the Born effective charge and dielectric constants are very sensitive to the atomic geometry. \textcopyright{} 1996 The American Physical Society.

First principles LAPW linear response calculations of the lattice dynamics of cubic KNbO 3 reveal planar instability in the Brillouin zone. In real space, these correspond to chains along the 〈100〉 directions of Nb atoms coherently displaced along the chain direction. Zone-center calculations in the rhombohedral phase yield stable modes (and no unusually soft modes) consistent with experiment. The similarity of the ferroelectric phases in BaTiO 3 suggests such chain instabilities also occur. It is likely that the ferroelectric phase transitions in these materials will depend on the dynamics of such chains.

CuCl is known to exhibit large anharmonic effects and possibly a complicated multiwell Born-Oppenheimer surface reminiscent of the instabilities in perovskite ferroelectrics and cuprate superconductors. However, we have determined its phonon dispersion from first principles calculations and find it to agree well with low temperature experimental results. The calculations were carried out using a linear response formalism based on the linearized augmented plane wave method within the local density approximation. The calculated Born effective charges are within 10% of experiment and the high frequency dielectric constant ${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{\ensuremath{\infty}}}$ is about 30% larger than the experimental value. This level of agreement is comparable to that attained in simpler semiconductors.

Using the first-principles linear response method implemented with an LAPW basis [1], we have investigated the lattice dynamics of cubic SrTiO3 [2] at the LDA equilibrium lattice parameter. Consistent with the antiferrodistortive (AFD) phase transition to the tetragonal structure at 105 K, we found phonon instabilities along the R-M-R line in the Brillouin zone. In addition, a ferroelectric-type (FE) instability was found at the Γ-point, although no FE structural phase transition is observed. Working in the tetragonal phase, we determine the theoretical equilibrium twist angle for the oxygen octahedra, and show that as the lattice parameter is reduced, the AFD instability is enhanced, while the FE instability is diminished. Furthermore, we find that the tetragonal structure is marginally stable against FE distortions along the c-axis.

The pressure dependence of the Born effective charge, dielectric constant and zone-center LO and TO phonons have been determined for $3C$-SiC by a linear response method based on the linearized augmented plane wave calculations within the local density approximation. The Born effective charges are found to increase nearly linearly with decreasing volume down to the smallest volume studied, $V/V_0=0.78$, corresponding to a pressure of about 0.8 Mbar. This seems to be in contradiction with the conclusion of the turnover behavior recently reported by Liu and Vohra [Phys.\ Rev.\ Lett.\ {\bf 72}, 4105 (1994)] for $6H$-SiC. Reanalyzing their procedure to extract the pressure dependence of the Born effective charges, we suggest that the turnover behavior they obtained is due to approximations in the assumed pressure dependence of the dielectric constant $\varepsilon_\infty$, the use of a singular set of experimental data for the equation of state, and the uncertainty in measured phonon frequencies, especially at high pressure., Comment: 25 pages, revtex, 5 postscript figures appended, to be published in Phys. Rev. B

Based on an analysis of the wavevector dependence of lattice instabilities in KNbO3, the existence of a real-space chain-like instability was deduced[1] To further study this instability, we have constructed an ab initio effective Hamiltonian for KNbO3 that reproduces the low-energy Born-Oppenheimer surface. Classical molecular dynamics (MD) simulations yield the observed sequence of ferroelectric phase transitions. The transition temperatures are lower than experiment, especially for the higher temperature transitions. Simulations of the static structure factor reveal features consistent with chain-like correlations and qualitatively reproduce diffuse X-ray scattering measurements. The calculated dynamic structure factor reveals anharmonically stabilized TO vibrational modes that soften along the entire Γ − X line in the Brillouin zone, as the phase transition is approached from above. A real-space analysis of the intercell correlations of the soft-mode coordinate, unambiguously establishes the ...

We have applied our recently developed LAPW linear response method to the cubic perovskite KNbO3 to calculate the phonon dispersion curves and to study the wave vector dependence of the ferroelectric instability. These are first principles calculations, performed within the framework of the local density functional approximation (LDA), and the results are as accurate as conventional total energy supercell studies. The density functional linear response approach, however, permits investigation of phonons of arbitrary wave vector. Our principle finding is that the cubic structure is unstable for wave vectors over wide regions of the Brillouin zone (BZ) and suggests chain-like structures of distorted Nb-O octahedra that are uncorrelated in the perpendicular directions. This has implications regarding whether the ferroelectric phase transition is driven by a soft-mode instability or is of order/disorder type.