摘要:

The theoretical definition of partial ionic character is considered. The basic requirements which the author feels must be met before a satisfactory theoretical definition can be found are outlined. It is shown that current models for defining partial ionic character fall short of meeting these criteria. Suggestions are made as to the directions in which one must look for better models. In particular, it is emphasized that the dual requirements of orthogonal concepts based upon models which are invariant to arbitrary choices of basis functions must be met.

ISSN:0021-8979    

DOI:10.1063/1.1777109

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

If a lattice of point ions is embedded in a free electron gas, the resulting screening can be calculated by first‐order perturbation theory. The answer can be expressed in terms of a static wave‐number‐dependent dielectric function &egr;(q). Because the plane‐wave approximation gives good results for the valence bandwidths in semiconductors, it is interesting to apply the simple idea of dielectric screening to charge distributions in partially ionic zinc‐blende and wurtzite semiconductors. To do so one must include the effect of occupied atomic core states. This is easily done in the effective potential representation. One then finds that dielectric screening gives a good approximation to the results of band calculations for the screening of the longest and strongest Fourier component of the lattice potential. Nonlinear effects leading to the formation of covalent bonds will also be discussed.

ISSN:0021-8979    

DOI:10.1063/1.1777110

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Pure nuclear quadrupole resonance spectra have been reported for halogen nuclei in a number of metal halides crystallizing in layer structures of the CdI2or related types. Because of the extreme smallness of the quadrupole coupling constants, particularly in the case of the dihalides, the possible appropriateness of a purely ionic model to account for the electric field gradients has been investigated. Detailed calculations of the ion‐lattice field gradient have been made for a number of compounds representing several types of infinite‐layer structure. In no case is the coupling due to the ion‐lattice alone sufficient to explain the observed interactions. In fact, comparison with the experimental data suggests that empirical values of the antishielding factor &ggr;∞ about 3–5 times smaller than the theoretical values are appropriate to account for the ion‐lattice contribution to the coupling. Covalent bonding between the metal and halogen layers involving only small departures from purep‐bond configurations at the halogens can account for the experimental results. Taking the ion‐lattice gradient into account causes in general a small increase in the values ofshybridization needed in the covalent bond model.

ISSN:0021-8979    

DOI:10.1063/1.1777111

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

A brief account is given concerning the theoretical framework of the molecular orbital treatment of crystal field theory. Indirect and direct experimental evidences of electron delocalization are summarized. A historical survey is given of the theoretical treatment of cubic crystal field splitting. Finally, a molecular orbital treatment of the cubic field splitting is presented, emphasizing the importance of &pgr;‐electron bonding and itinerancy of electrons between a metal ion and ligands.

ISSN:0021-8979    

DOI:10.1063/1.1777112

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

This is a review of selected experimental and theoretical advances in the subject of lattice vibrations over the past several years. A general approach to the properties of phonon fields is presented, then the use of inelastic neutron scattering experiments and topologically consistent interpolation methods for phonon dispersion relations are reviewed. Localized modes of vibration are discussed, and a new method of ``effective moduli'' is presented.

ISSN:0021-8979    

DOI:10.1063/1.1777113

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

(1) We have calculated the binding energy of vacancy pairs in NaCl and the other five alkali halide crystals. The calculations is improved at several points compared with that by Reitz and Gammel. The calculated values of the binding energy are considerably smaller thane2/kaand differ from the result by Reitz and Gammel; for instance, 0.44 ev in NaCl, 0.37 ev in NaBr, and 0.58 ev in KCl crystals.(2) As the first step of the systematic investigations of dislocation problems, we have calculated the core energy and the Peierls‐Nabarro critical yield stress of the edge dislocation for slip in the (110) plane in NaCl type crystal [the Burgers vector isa(110)]. If we write the energy of the dislocation at the center of a cylinder with radiusRasKln (R/R0),R0is about 1 A in most of the alkali halide crystals. The results of the calculation concerning the Peierls‐Nabarro yield stress permit us to understand the well‐known fact that LiF has much larger critical yield stress compared with the other alkali halide crystals.

ISSN:0021-8979    

DOI:10.1063/1.1777114

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Transition metal impurity ions and intrinsic defects such as trapped electron and trapped hole centers are readily detected in magnesium oxide crystals by electron spin resonance. The valence state of impurity ions may readily be changed by heat treatment or by irradiation. For nickel and cobalt one may observe both the 2+ and 1+ states, while for iron, one additionally observes the 3+ state. The extra positive charge of trivalent ions is usually compensated by positive ion vacancies. Vacancies, either single or in clusters, alter the electric field symmetry about an impurity ion or intrinsic defect. Hence they may indicate their presence by altering the symmetry from cubic to tetragonal, rhombic or lower type. The impurities are generally substitutional, but one interstitial type has been found. Isolated positive ion vacancies can trap either one or two holes, in each case producing centers of axial electric field symmetry. Other hole centers are observed after extensive atomic displacements have been produced by neutron irradiation. Negative ion vacancies are difficult to produce except by such irradiation. Electrons trapped at negative ion vacancies are largely localized upon the vacancy, as is evident from the small hyperfine splitting of adjacent25Mg ions. If crystals are heated in order to lead to association of vacancies, a second type ofFcenter is formed. With protracted neutron irradiation, yet a third type ofFcenter is produced. Optical studies are being undertaken to learn more about these paramagnetic centers.

ISSN:0021-8979    

DOI:10.1063/1.1777115

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Alkaline earth impurities introduce hole traps into KCl and KBr. Holes of the typeX2−(Xis a halogen) with ⟨100⟩ orientation are observed after x‐irradiation at 195°K, and are a source of anion vacancies. Iron group ions behave as electron traps in NaF and LiF.

ISSN:0021-8979    

DOI:10.1063/1.1777116

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

A review is given of the present status of the theory ofsandpelectrons in deep traps. The theory of theFcenter in LiF is reviewed in detail including recent work on ionic displacements in the vicinity of the center. This is followed by a discussion of the work of Cohen and Heine and the general justification of the point ion approximation for the interaction of a trapped electron with a matrix composed of closed‐shell ions. Vacancies in diamond are discussed, and new results are reported for the three‐electron configuration of a vacancy in silicon. Trapped‐hole centers in the alkali halides are mentioned briefly.

ISSN:0021-8979    

DOI:10.1063/1.1777117

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The importance of the acoustical mode lattice vibration for self‐trapping of an electron is emphasized in this paper. It is shown that when the coupling constant between the electron and the acoustical vibration exceeds a certain critical value, the effective mass of the electron changes discontinuously to such an enormous value that it is practically allowed to take a localized self‐trapping state as an eigenstate, in contrast to the case of polar mode, in which the effective mass changes continuously with coupling constant. This difference is attributed to the different force range of the electron‐lattice interaction in the two cases.The self‐trapping state as well as the activated state for the hopping motion are discussed within the framework of adiabatic approximation by making use of Koster‐Slater's method. It is pointed out that drift motion is more probable than the hopping motion for the intermediate magnitude of coupling constant.

ISSN:0021-8979    

DOI:10.1063/1.1777118

出版商:AIP    

年代:1962

数据来源: AIP