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Transport processes are investigated by means of the lattice gas model with nearest and nextnearest neighbour attractive and repulsive interactions. The temperature and concentration dependences of self-diffusion coefficient and electroconductivity in the disordered homogeneous phase of these systems are calculated by the cluster approximation. The regions of deviation from the Arrhenius law are determined. The spatial-time distributions of migrating particles are obtained. The influence of strong short-range interactions on the relaxation time of the diffusion process is investigated. Several experimental results of diffusion in systems with strong shortrange correlations are discussed by the theory proposed. Die Transportprozesse in Kristallen vom KDP-Typ werden im Rahmen eines Gittergasmodells beschrieben. AbstoBende und anziehende Wechselwirkungen zwischen nachsten und ubernachsten lonen der Gitterpunkte werden in Betracht gezogen. Die Temperatur- und Konzentrationsab-hangigkeiten der Selbstdiffusions- und Leitfahigkeitskoeffizienten der ungeordneten homogenen Phase dieser Systeme werden mit der Methode der Clusternaherung berechnet. In diesen Kristallen werden Bereiche gefunden, die Abweichungen vom Arrhenius Gesetz aufweisen. Die zeitliche und raumliche Teilchenverteilungen werden fur diese Systeme bestimmt. Der Einflus einer Nahordnung auf die Relaxationszeit des Diffusionsprozesses wird behandelt. Die experimentellen Diffusions-ergebnisse werden im Rahmen der entwickelten Theorie analysiert.

In the mode–mode coupling approximation the critical exponente (CE) are calculated for the damping and frequency shift of soft optic modes and acoustic phonons at the structural transition point for crystals with dimensions d = 1, 2, 3. The CE are traced back to the anharmonic interaction of phonons taking into account the intrinsic dynamic central peak of any nature. Anharmonisms of all order are considered. [Russian Text Ignored].

The damping and frequency shifts of the longitudinal and transverse phonons are derived in the right neighbourhood of the Curie point using the microscopic theory for ferroelectrics described by the pseudospin Ising model with transverse field. The scattering of phonons both on the soft modes and on the central peak gives the same contributions to the critical anomalies. The long-range contributions to the effective “exchange integral” are also taken into account. The results coincide with predictions of the phenomenological theory. [Russian Text Ignored.]

In first-order perturbation theory it is shown with respect to the Hamiltonian of the magnon–magnon interaction that for a linear ferromagnetic XY chain with large spins the frequency renormalization is proportional to the energy of noninteracting magnons. In higher order with respect to the same interaction the momentum and temperature dependence of the magnon damping is obtained and weakly damped magnons are shown to exist, provided their wavelength is much shorter than the correlation length. [Russian Text Ignored.]

In the mode—mode coupling approximation the critical exponents (CE) are calculated for the dampings and frequency shifts of soft optic modes and acoustic phonons (in uniaxial ferroelectrics) and for the dampings of soft acoustic phonons (in cubic crystals) in the structural transition point. The CE are traced back to the anharmonic interaction (of all orders) of soft phonons with the intrinsic dynamic central peak of any nature. [Russion Text Ignored]

The momentum and temperature dependence of the central peak (CP) width is found for the Ising model with a transverse field in the right neighbourhood of the Curie point. The width of CP in the critical and hydrodynamic limits is described by a scaling-type relation. During deuteration the width of CP diminishes as a squared tunneling frequency. [Russian Text Ignored].

An expression for the spectrum of critical inelastic scattering of Mossbauer radiation is derived at the structural phase transition. It is assumed that the central peak (which determines such characteristics of scattered radiation as the intensity and the width) is due to slow motions of point defects. The temperature dependence of the effective line width is found to arise from vacancy diffusion in BaTiO3 to the right of the transition temperature. Qualitative agreement with corresponding experimental data is obtained. [Russian Text Ignored].

In the random phase approximation the imaginary part of the phonon polarization operator is calculated in the left vicinity of the Curie point as well as the renormalization of the sound velocity due to the spin-phonon interaction of exchange origin in a cubic ferrodielectric. [Russian Text Ignored].

The imaginary part of the phonon polarization operator and the renormalization of the sound velocity due to the spin-phonon interaction are calculated in a cubic ferrodielectric for large atomic spins in the spin-wave temperature region. [Russian Text Ignored].

The limits of applicability are established for the different decoupling procedures in the Green's function equations of the spin system of an Heisenberg ferromagnet. Es werden die Grenzen fur die Anwendbarkeit der verschiedenen Entkopplungsverfahren auf die Gleichungen fur die Greenschen Funktionen des Spinsystems eines Heisenbergschen Ferromagneten ermittelt.