A note on the mechanics of diffusion of defects in solids

This paper deals with diffusion of point defects due to lattice deformations around the defects. The continuum theory of lattice defects is applied to derive suitable diffusion equations taking into consideration the fields of self-stresses caused by the drift of defects. The model is applied to defects near surfaces and interfaces. The interesting phenomenon of “near-surface Cottrell atmospheres” is discussed. The authors show that Cahns theory of spinodal decomposition is included in the model as a special case.     

NQR data on the activation energy of orientational defects in crystal lattice

The nuclear quadrupole spin-lattice relaxation related to the formation of orientational defects in crystals has been analyzed. The defects arise because of thermally activated reorientational motion of molecules and their fragments between the nonequivalent potential wells. These molecular reorientations are considered for double-and triple-well potentials, both periodic and bounded by external side walls. It was shown that, for such potentials, the temperature dependence of the relaxation time provides the data on the activation energy for transitions of molecules in the crystal lattice from the equilibrium ground-state positions to those corresponding to metastable states. We systematized the known NQR data on the activation energies for orientational defects arising in the intracrystal rotations of molecules and atomic groups possessing the imperfect two-, three-, and sixfold axial symmetries.     

Influence of the model parameters on the Monte Carlo simulation of the initial aggregation of point defects in solids

The kinetic process to reach the equilibrium state of lattice defects is characterized by an aggregation of these defects. The formation of the precursors of the aggregates, the so called critical nuclei, can be investigated by Monte Carlo simulation. But in many cases the exact interaction potential between the movable defects in the lattice and the nucleus is unknown. Therefore such simulations were carried out for different typical interaction parameters. Conclusions concerning the general behaviour were derived. The calculations allow especially an estimation of the influence of strength of the potential and temperature on the number of defects forming a critical nucleus.     

Stoichiometric defects and volume diffusion in epitaxial PbTe-SnTe multilayers

The effect of annealing on multilayer structures based on epitaxial layers of PbTe-SnTe was investigated by X-ray diffraction profile changes. Decrease of satellite reflections intensity and conversion of pattern asymmetry were found. Profiles of interplanar distance and electron density deviations were determined by solving both the direct problem (numerical computation of Takagi-Taupin equations) and the inverse problem (optimisation task) according to the average lattise model. Computation showed the presence of interlayer area of PbSnTe composition containing up to 8% of stoichiometric vacancy type defects between PbTe and SnTe layers. Two diffusion mechanisms are proposed: “fast” diffusion connected with occupation of stoichiometry defects and “slow” substitutional diffusion.     

A dynamic correlation function for amorphous solids

A report is given of a new inelastic neutron scattering measurement for vitreous SiO₂ made with a high energy resolution and a wide and continuous range of momentum transfer. The use of a real-space correlation function in the interpretation of the Q-dependence of the scattering function is investigated.     

Trapped electrons in amorphous solids: A method for estimating the concentration of molecular vacancies

The densities for n-propanol and n-butanol in their glassy and crystalline forms have been measured at 77 K. The density difference of ca. 0.11 cm³ g^?1 is comparable with that normally found between liquids and solids at their melting points. The maximum yield of trapped electrons in the γ-irradiated glassy alcohols has been estimated and, if it is assumed that they are exclusively trapped at molecular-sized vacancies present only in the glasses, then such vacancies account for no more than ca. 1.0% of the volume difference between the crystalline and glassy materials. Much of the remaining difference can be accounted for in terms of long and bent hydrogen bonds in the glasses.     

The role of surface and volume defects in the fracture of glass under quasi-static and dynamic loadings

In this paper, the role of surface and volume defects on fracture in soda-lime glass is analyzed when samples are submitted to quasi-static or dynamic loadings. To investigate fracture, different experiments are carried out, namely, quasi-static compression of glass spheres and edge-on-impacts. The first test series aims at studying crack initiation. Different surface treatments are performed to study their influence on the failure load, from which it is concluded that initiation of cracks occurs in the vicinity of the contact surface. The second series is concerned with the examination of crack patterns under dynamic loadings with two different strikers (i.e., soft/flat and hard/perforating projectiles). Crack initiation under dynamic load histories is investigated near and far from the impact zone and it is concluded that it cannot take place within the volume, except in a very small zone close to the impact point. Conversely, initiation of damage from the surface, at a location far from the impact point, is possible and clearly present with a soft projectile.     

On the Urbach rule in non-crystalline solids

A comparative analysis of temperature behavior of optical absorption edge is performed for non-crystalline materials where the Urbach behavior is observed (SiO₂), with deviations from the Urbach behavior (20PbO · 80SiO₂) and with a mixed behavior (As₂S₃). For glassy As₂S₃, the Urbach behavior of the absorption edge in a limited temperature range is explained from the point of view of two components of structural disordering - static and dynamic. A parallel red shift of the absorption edge in As₂S₃ and, consequently, the temperature-independent Urbach energy at T < 300 K, are related to the lack of medium-range order in the atomic arrangement in the presence of short-range order.     

Theoretical approach to the mechanoluminescence excitation in solids

The present paper reports the results of some theoretical approach made to the studies of Mechanoluminescence (ML) of solids. It is found that the ML intensity depends on three major factors, the ML efficiency, damping factor, and elastic complience. It is concluded that the non-viscous solids with high elastic compliance possessing luminescence centres with appropriate efficiency may exhibit intense ML. It is shown that the ML emission should take place only during the time at which there will be a change of applied pressure with time. There should be a phase difference between the applied pressure and the emitted ML signal.     

Review of pulsed neutron techniques for the study of amorphous solids

This paper presents some of the advantages of neutrons and pulsed neutron methods for study of amorphous materials. It compares the kinematic constraints on neutron and other spectroscopies, and describes the main features of pulsed neutron beams. The report discusses time-of-flight wide- and small-angle neutron diffractometers, chopper and inverse-geometry spectrometers, and illustrates a method of millisecond-scale time-resolved neutron diffraction.     

Scientific opportunities for the study of amorphous solids using pulsed neutron sources

The use of pulsed-neutron-scattering techniques for the investigation of amorphous solids is reviewed in the light of current problems in amorphous solid-state physics and chemistry. The broad areas in which neutron scattering can contribute are those of structure and dynamics, both atomic and magnetic. Possible applications for the new generation of pulsed neutron sources are discussed, and it is concluded that not only will these play a vital role in future studies but also that an important step forward will be made with the extension of measurements into hitherto inaccessible regions of ω-Q space.     

A combination of oscillating slit and oscillating film techniques for the investigation of lattice defects

A combination of oscillating slit and oscillating film techniques is described. This X-ray topographical method allows reflections due to the same places on the crystal surface to be recorded first in form of a topogramme registrated on the oscillating film set in the parallel position to the crystal surface. Second on the stationary film set parallel to the crystal surface in the form of an interference line Kα₁ or Kß outgoing from the investigated area and registrated on the first topogramme at the fixed angular position of the crystal and slit and next, recorded separate on the oscillating film and on the stationary film set in the distance of 50 mm from the crystal (oscillation axis). As an example a serie of photographs corresponding to four types of pattern obtained in the case of a PbSnTe crystal with small angle boundaries are presented.     

Boson elementary excitations and the specific heat of non-crystalline solids: A short review

A simple theoretical approach to investigate the low-temperature behavior of the specific heat of a non-crystalline solid is proposed. We discuss the possibility that this behavior can be explained in analogy to what is done in the physics of superfluid helium. We assume that the low-temperature excitation spectrum of the system is formed by two ideal gases of boson quasiparticles. One of them is a phonon gas that is always present in the spectrum, leading to the Debye contribution; the other one, which is very important for very low temperature, is an ideal gas of another boson quasiparticles, whose dispersion relation is similar to the one proposed for the liquid helium. To explore in more details this analogy, we discuss the possibility to build a simple, but general framework to understand the temperature behavior of the specific heat at very low temperatures, by following a programme similar to the one developed by Bogoliubov for a system of interacting bosons.     

Paramagnetic defects in the mineral hauyne

In blue crystals of hauyne from two localities two broad signals due to Fe³⁺, a hole center of tetragonal site symmetry and an isotropic signal from an electron center were detected by electron paramagnetic resonance (EPR). According to its site symmetry the hole center is tentatively assigned to a [MgO_4/2]⁰ species on an Al site with the unpaired electron delocalized over the oxygens. The site symmetry does not change with cooling to 20 K. The large linewidths of the signals from Fe³⁺ are interpreted as a result of the non-stoichiometry and high static disorder and some dipolar broadening. The linewidths of the other EPR signals are also larger than for corresponding centers in the isotypic sodalite. The cause of the blue colour is briefly discussed.     

Estimation of point defect parameters of solids on the basis of a defect formation model of melting (III). Formation entropy and concentration of schottky defects (vacancies)

The formation entropy of Schottky defects (vacancies) is calculated from the entropy of fusion and the concentration x_L = 0.125 of vacancies within the melt at melting point T_m. The formation of vacancies is connected with a decrease of the vibrational frequency of those lattice forming particles (atoms, ions, molecules) being neighbours of the vacancies. Theoretical values of the vacancy concentrations x_s agree with those x_s obtained with the help of experimentally determined free formation enthalpies of the defects.     

Universal sound absorption in amorphous solids: A theory of elastically coupled generic blocks

Glasses are known to exhibit quantitative universalities at low temperatures, the most striking of which is the ultrasonic attenuation coefficient Q^?1. In this work we develop a theory of coupled generic blocks with a certain randomness property to show that universality emerges essentially due to the interactions between elastic blocks, regardless of their microscopic nature.     

Structural transformations in amorphous solids detected by the exoelectron emission technique

The temperature dependence of photostimulated exoelectron emission from vitreous and crystalline (grey) selenium and from vitreous boric anhydride B₂O₃ are presented. It was found that in all the cases the curves of the temperature dependence of emission intensity displayed peaks at temperatures corresponding to the structural transformations in the materials investigated. This makes exoelectron emission a very useful tool for the detection of the phase transformations in amorphous solids.