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The effective equations of motion for a surface atom in an anharmonic surface potential have been derived for dispersionless one-dimensional substrates. The system is equivalent to a non-linear damped oscillator (Duffing oscillator) with the forcing term depending on the form of the incident wave. Efficiency of harmonics generation, phonon reflection coefficients, effective local density of states, regions of chaotic motion and windows of periodic motion have been comparatively evaluated for the system subject to an oscillating external force and to the irradiation by a monochromatic phonon coming from the bulk. Comparison of the resonant desorption of the surface atom within a given time interval has been made for the same example of anharmonic surface potential in both types of perturbation.

The microscopic Ising-like model of the order-disorder phase transition in improper ferroelastic crystals is presented. For such a system, the spin-strain coupling can be written in terms of four-spin interactions. Three-dimensional Monte Carlo simulation shows that for a certain strength of the four-spin interaction the system is close to the tricritical point. The calculated temperature dependence of the order parameter is in very good agreement with the experimental data for RbSCN. The results also show that the growth of the size of precursor clusters is suppressed when approaching from below, whereas the order parameter susceptibility is increasing. This is in good agreement with the diffuse neutron scattering data on RbSCN and KSCN, which yield an increase of the diffuse intensity with increasing temperature below , whereas its width remains constant throughout the whole ordered phase. These results show that elastic effects can stabilize the precursor clusters at the order-disorder phase transition below .

A two dimensional model of KSCN crystal has been used for molecular dynamics simulations of the phase transition. Diffuse scattering function calculated during simulations shows that the correlation lenght of ordering SCN particles remains constant close below Tc but it grows when one approaches Tc from above. Order parameter maps of the system are in agreement with diffraction data.

We report on maximum entropy method study of the experimental atomic and ionic charges of LiBD4 in its low-temperature orthorhombic phase. Synchrotron radiation x-ray powder diffraction data, neutron powder diffraction data, and density functional calculations were used. The atomic and ionic charges were determined for both experimental and theoretical results using the Bader analysis for atoms in molecules. The charge transfer from the Li cation to the BD4 anion is 0.86(+/- 9) e, which is in good agreement with the ab initio calculated value of 0.895 e. The experimental accuracy was determined considering the differences between results obtained for data collected at 10 and 90 K, different experimental setups (high-resolution diffractometer or image plate diffractometer), and different structural models used for the prior density distributions needed for accurate maximum entropy calculations (refined using only synchrotron radiation x-ray powder diffraction data or combined with neutron powder diffraction data).

We have performed micro Raman measurements on LiBH4 and LiBD4 powders for temperatures between 5 and 300 K. At the lowest temperature, the peak energies agree very well with the results of a calculation within the density functional theory for the orthorhombic structure. The spectra are dominated by three separated bands: the external modes, the internal bending, and the internal stretching vibrations. Internal refers to vibrations within the BH 4 tetrahedrons, whereas external modes imply motions of Li and BH 4. The temperature dependence of the observed phonons corroborates the strong anharmonicity of the system. Due to the anharmonicity, Fermi resonances occur between the first order stretching modes and the second order bending modes of LiBH4. Moreover, the linewidths of the observed modes in LiBH4 have an Arrhenius-like component, with activation energies ranging from 250 to 500 K.

Very recently we presented puzzling results of diffuse neutron scattering experiments on KSCN and RbSCN. The data yield an increase of the diffuse intensity with increasing temperature below T(c), whereas the width remains constant. Using molecular dynamics and 3D Monte Carlo simulations, we have shown that below T(c) the width of the correlation functions can be stabilized by strain fields originating from the order parameter strain interactions. Here we construct a novel analytic model which predicts the existence of a second characteristic length scale and explains the suppression of the growth of precursor clusters by the influence of inhomogeneous strain fields.

The results of dielectric measurements ( f 510 kHz‐400 MHz) of the natural mineral lawsonite are presented in the region of the ferroelectric transition ( T2’124 K). The dielectric behavior is characterized by a small value of the Curie-Weiss constant ( CCW52220 K) typical for order-disorder ferroelectrics. This is consistent with the fact that the origin of ferroelectricity in lawsonite is related to the dynamical ordering of protons. In the paraelectric phase the dielectric dispersion is characterized by a Debye relaxation with a single relaxation time t P54.2310 210 (T2T2) 21 . A broadening in the dielectric dispersion occurs in the paraelectric phase only in a narrow temperature range (’5 K) close to T2 and can be associated with disordering processes in the lattice affected by large polarization clusters. The dielectric relaxation in the ferroelectric phase includes the order parameter dynamics and the domain wall dynamics. The relaxational domain wall dynamics is characterized by an extremely broad distribution of relaxation times. The characteristic parameters of the dielectric relaxation are determined.