Non-metallic Crystals Research Articles

Determination of the charge distribution in nonmetallic crystals. I. Theory and application to tetrahedrally coordinated solids

AbstractA SCF method using localized molecular orbitals which are built up on hybrid atomic orbitals is proposed to obtain the charges in infinite crystals. Hybrid orbitals are built up on a minimal STO basis set. The formalism has been adapted in order to take into account the periodicity of the system and its infinite size by introducing the Madelung constant. The total energy is given by an infinite sum of terms each corresponding to the energy of a bond in the crystal field. Minimizing this bond energy with respect to eigenvectors it is straightforward to obtain the electronic charges, whence the polarity, i.e., the ionicity, of bonds. In this first paper, we study and discuss the polarity of bonds in zincblende and wurtzite‐type compounds built up on first and second row elements. Our values are coherent between themselves and in agreement with other authors' results. The connection with electronegativity and polarizability is discussed.

Двойной электронно-ядерный резонанс примесных центров в неметаллических кристаллах

Двойной электронно-ядерный резонанс примесных центров в неметаллических кристаллах

On the question of the ground state of a magnetic polaron in a ferromagnetic crystal (S ≥ 1)

The energy spectrum of a nonmetallic ferromagnetic crystal is investigated in the presence of a moving conduction electron which interacts with the atomic spins of a magnet (S ≥ 1). Because of the antiferromagentic nature of this interaction under consideration, the calculation of the system ground state is performed with 2S spin deviations bound to the electron, taken into account. It is shown that an increase in the number of spin deviations does not result in the appearance of states lying below the whole system spectrum obtained in a single-magnon approximation. In other words, for any quantity S the system ground state is determined by the least value of the energy of its single-magnon spectrum.

Contribution of Internal Energy to the Elasticity of Rubberlike Materials

Abstract Rubber is an interesting material possessing some remarkable properties, the most important of which is its high elastic extensibility. Ordinary solids, such as metals, nonmetallic crystals, and glasses, can be deformed or strained elastically only to a small extent, corresponding to an extension of the order of 1%. If an attempt is made to apply a greater extension, they either break, as in the case of glass, or deform plastically, like lead.

Electronic Correlation Effects (Exciton Manifestation) in the Single‐Particle Band Structure of Nonmetallic Crystals II. Investigation of a Model

AbstractThe appearance of excitonic satellites is the most pronounced feature due to electron correlation in the band structure of nonmetallic crystals. The general theory presented in a foregoing paper [1] is applied to a model of an insulator with parameter values appropriate to the alkali halide CsI. Only four bands with tight‐binding energies and one exciton line are taken into account. Numerical results are given for self‐energy, spectral function, and density of states in the main bands and the satellite bands.

Electronic Correlation Effects (Exciton Manifestation) in the Singl—Particle Band Structure of Nonmetallic Crystals I. General Theory

AbstractThe effect of electron correlation on the single‐particle spectrum of electrons in nonmetallic crystals is investigated starting from the full Hamiltonian of the interacting electron system. By these effects excitons become manifest in the single‐particle spectrum entering Dyson's equation of the corresponding Green's functions via the screeningof the Coulomb potential in the self‐energy. Dyson's equation is solved to render the spectral function by a systematic perturbative approach recently introduced in connection with the electron gas problem. The single‐particle spectrum exhibits modified main bands accompanied by exciton satellites contributing remarkably to the density of states. This picture appears not to be fully obtained by the electronic polaron theory starting from a model Hamiltonian which is also investigated in this study.

Influence of the phonon subsystem on the probability of formation of proper point defects in nonmetallic crystals

Influence of the phonon subsystem on the probability of formation of proper point defects in nonmetallic crystals

Heat conductivity of non-metallic crystals

Abstract Some non-metallic crystals can have heat conductivities comparable with the best metals, while other non-metals are better heat insulators than most metallic alloys, and materials at both extremes have practical uses. The temperatures at which the conductivities of pure crystals are a maximum vary widely, and for any application which requires an electrical insulator with high heat conductivity, the most suitable crystal depends on the temperature range concerned. Imperfections in the lattice can mask the ideal behaviour, and materials having very disordered atomic arrangements or composed of small crystallites are used as heat insulators. The theory of phonon conductivity can explain the observed phenomena quite well, but the magnitude of the intrinsic conductivity Cannot be predicted accurately from a knowledge of other crystal properties. Some analogies can be drawn between heat flow in a crystal and gas flow in a tube, but the analogy does not cover ‘Umklapp’ processes, which are peculiar to ...

Nonmetallic crystals with high thermal conductivity

Nonmetallic crystals transport heat primarily by phonons at room temperature and below. There are only a few nonmetallic crystals which can be classed as high thermal conductivity solids, in the sense of having a thermal conductivity of > 1 W/cmK at 300K. Thermal conductivity measurements on natural and synthetic diamond, cubic BN, BP and AIN confirm that all of them are high thermal conductivity solids. Studies have been made of the effect on the thermal conductivity of nitrogen impurities in diamond, and oxygen impurities in AIN. The nitrogen impurities scatter phonons mostly from the strain field, the oxygen impurities scatter phonons mostly from the mass defects caused by aluminum vacancies. Pure A1N as well as pure SiC, BeO, BP and BeS conduct heat almost as well as does copper at room temperature, while pure natural and synthetic diamonds conduct heat five times better than copper. All of the nonmetallic solids that are known to possess high thermal conductivity have either the diamond-like, boron carbide, or graphite crystal structure. There are twelve different diamond-like crystals, a few boron carbide-type crystals, and two graphite structure crystals that have high thermal conductivity. Analyses of the rock-salt, fluorite, quartz, corundum and other structures show no candidates for this class. The four rules for finding crystals with high thermal conductivity are that the crystal should have (1) low atomic mass, (2) strong bonding, (3) simple crystal structure, and (4) low anharmonicity. The prime example of such a solid is diamond, which has the highest known thermal conductivity at 300K.

A study of the β→α phase transition in UO 2(OH) 2 by dilatometric, microcalorimetric and X-ray diffraction techniques

When β-UO 2(OH) 2 is cooled to about 5°C, it transforms rapidly into α-UO 2(OH) 2 with a volume change ΔV V of −14.7%. The volume change was measured by dilatometry. The enthalpy change accompanying the transformation was measured by a specially constructed microcalorimeter, and found to be 1.36 ± 0.22 kcal/mole. The temperature of the transition was 5.1 ± 2.3°C. The fraction of β-UO 2(OH) 2 converted to α-UO 2(OH) 2 in each run was determined both by direct weighing and X-ray diffraction methods. The transformation is unusual for a nonmetallic crystal in that it is analogous to the martensitic transformations common in metals and alloys. On the basis of the known structures of the α and β phases, and morphological changes accompanying the transition, a mechanism for the transformation is proposed, and it is suggested that hydrogen bonds are probably not broken during the transformation.

593. Space distribution of radiation defects and implanted ions in crystals irradiated by fast ions: M A Kumakhov, Radiation Physics of Nonmetallic Crystals, 3 (1), Collect, Naukovaya Dumka Kiev 1971, 132–142 ( in Russian)

593. Space distribution of radiation defects and implanted ions in crystals irradiated by fast ions: M A Kumakhov, Radiation Physics of Nonmetallic Crystals, 3 (1), Collect, Naukovaya Dumka Kiev 1971, 132–142 ( in Russian)

596. Influence of ion implantation on density of dislocations in Si: R I Garber et al, Radiation Physics of Nonmetallic Crystals, 3 (2), Collect, Naukovaya Dumka Kiev 1971, 128–133 ( in Russian)

596. Influence of ion implantation on density of dislocations in Si: R I Garber et al, Radiation Physics of Nonmetallic Crystals, 3 (2), Collect, Naukovaya Dumka Kiev 1971, 128–133 ( in Russian)

594. Investigation of microhardness of surface layers of silicon doped by ion implantation: I L Gverdtsiteli et al, Radiation Physics of Nonmetallic Crystals, 3 (2), Collect, Naukovaya Dumka Kiev 1971, 138–143 ( in Russian)

594. Investigation of microhardness of surface layers of silicon doped by ion implantation: I L Gverdtsiteli et al, Radiation Physics of Nonmetallic Crystals, 3 (2), Collect, Naukovaya Dumka Kiev 1971, 138–143 ( in Russian)

592. Radiation damage of semiconductor crystals on ion implantation: R I Garber et al, Radiation Physics of Nonmetallic Crystals, 3 (1), Collect, Naukovaya Dumka Kiev 1971, 42–65 ( in Russian)

592. Radiation damage of semiconductor crystals on ion implantation: R I Garber et al, Radiation Physics of Nonmetallic Crystals, 3 (1), Collect, Naukovaya Dumka Kiev 1971, 42–65 ( in Russian)

595. Doping of silicon by high-energy ions of boron, nitrogen and phosphorus: Yu V Bulgakov et al, Radiation Physics of Nonmetallic Crystals, 3 (2), Collect, Naukovaya Dumka Kiev 1971, 148–154 ( in Russian)

Préciser l’indication chirurgicale d’une vitrectomie chez des patients gênés par la présence de corps flottants intra-vitréens.Nous rapportons les cas de 4 patients (4 yeux), d’âge moyen 51 ans (de 42 à 65 ans), un homme et trois femmes ayant été traités par une vitrectomie à deux ou trois voies. Pour deux patients, il existait un décollement postérieur du vitré. Nous avons imposé un temps de réflexion de plusieurs mois avant l’intervention à tous les patients.L’acuité visuelle pré-opératoire était évaluée de 5/10e à 10/10e, et l’acuité visuelle post-opératoire de 5/10e à 10/10e. Aucune acuité visuelle n’a diminué après l’intervention. Pour tous les patients, la gêne visuelle a considérablement diminué voire disparu.La mesure de l’acuité visuelle ne reflète pas étroitement la gêne visuelle des patients. La vitrectomie est une technique connue depuis de nombreuses années. Cette intervention peut être proposée en dernier recours après une évaluation très précise de l’état du segment postérieur, et après une information complète donnée au patient, tout en s’assurant de son profil psychologique.To clarify vitrectomy indications in the treatment of vitreous floaters.This is a retrospective study of four patients, four eyes (age, 42-65 years), who underwent vitrectomy for the treatment of vitreous floaters. Two patients had a total posterior vitreous detachment. All patients were required to think about the intervention for several months before consenting.Preoperative visual acuity ranged from 20/40 to 30/30 and after the operation it ranged from 20/40 to 10/10. Minimum follow-up was 1 year. Visual acuity did not decrease after treatment in any patients. All patients expressed satisfaction with their postoperative visual function.Visual acuity measures do not always accurately reflect patients’ visual discomfort. Vitrectomy has been known for many years. It can be proposed as a last resort after thorough retina examination, after patients have received adequate information and they are psychologically ready for the procedure.

Mass-spectrometry determination of the boron impurity distribution profile in ion-implanted silicon single crystals: R I Garber et al, Radiation Physics of Non-metallic Crystals, 3, Part 2, Collection, Kiev Naukovaya Dumka 1971, 143–148 ( in Russian)

Mass-spectrometry determination of the boron impurity distribution profile in ion-implanted silicon single crystals: R I Garber et al, Radiation Physics of Non-metallic Crystals, 3, Part 2, Collection, Kiev Naukovaya Dumka 1971, 143–148 ( in Russian)

Silicon doping by ion bombardment: I B Amirkhanova et al, Radiation Physics of Non-metallic Crystals, 3, Part 2, Collection, Kiev Naukovaya Dumka, 1971, 111–122 ( in Russian)

Silicon doping by ion bombardment: I B Amirkhanova et al, Radiation Physics of Non-metallic Crystals, 3, Part 2, Collection, Kiev Naukovaya Dumka, 1971, 111–122 ( in Russian)

Wear of fluorapatite single crystals. I. A method for quantitative evaluation of wear.

A quantitative method for characterizing the surface failure of nonmetallic single crystals under sliding was evaluated. It was found that strain rate, load, and slider design influenced the wear of natural fluorapatite single crystals. High loads and slider designs with small radiuses and sharp angles resulted in the deepest penetration.

Exchange energy as functional of electronic density from Hartree-Fock theory of inhomogeneous electron gas

In a previous paper, the validity of gradient corrections to the Dirac-Slater exchange energy was discussed within the density functional theory, for a local potential. This local potential, by definition, generated the exact charge density in the (infinite) system considered. The present work considers the full Hartree-Fock theory within the same density functional framework. In particular, using a perturbation expansion of the Dirac density matrix, we obtain the exchange energy as an expansion in the displaced charge Δ H . F . ( r ) = ρ H . F . ( r ) − ρ 0 , , p0 being the average electron density and p H.F. (r) the Hartree-Fock spatially dependent density. In contrast to the local potential theory, a basic integral equation must be solved to express the above relation in explicit form. No exact solution of this equation exists to date, but using the work of Kleinman and Langreth on the vertex function we are able to find an explicit approximation for the exchange energy to second-order in Δ H.F. The difficulties encountered in defining gradient series in a local potential theory are not removed in the full Hartree-Fock theory. However, it is possible to sum sub-series of gradient terms to infinite order to give a useful generalization of Dirac-Slater theory. Although it is known in metals that correlation effects have a major influence on the Hartree-Fock results, the results obtained here may well be useful in atomic and molecular problems with a sufficiently large number of electrons, as well as in non-metallic crystals. The influence of electronic screening in the case of metals is briefly referred to.

Effects of Self-Ionization ofβ-Active Elements in Nonmetals

A new mechanism is proposed for the formation of stable substitutional or interstitial diatomic molecules in nonmetallic crystals in which self-ionization of $\ensuremath{\beta}$-active ions occurs. In particular, the probability of self-ionization of tritium ions in LiT and LiH: LiT crystals due to "shake-off" following $\ensuremath{\beta}$ decay is calculated. It is found that the probability of obtaining a ${\mathrm{He}}^{++}$ ion is 0.05, and for a ${\mathrm{He}}^{+}$ ion 0.32; and that the presence of these ions can lead directly to the formation of hydrogen molecules without diffusion. Using a model of LiH which includes noncentral forces, the energetics of this process, which is essentially temperature independent, and the rates of formation of hydrogen molecules are calculated. If, as experiments suggest, ${\mathrm{H}}^{0}$ and $\mathrm{H}_{2}^{}{}_{}{}^{\ensuremath{-}}$ are indeed absent in irradiated LiH, then the formation of ${\mathrm{H}}_{2}$ molecules (which accounts for the initial stage of swelling of LiH: LiT) is diffusion controlled above 200-300 \ifmmode^\circ\else\textdegree\fi{}K but is due to the "shake-off" mode at lower temperatures.