Diaspore from Dilln, Hungary, AlOOH, is orthorhombic with space group Pbnm, a=4.4007(6), b=9.4253(13), c=2.8452(3) A, and Z=4. The crystal structure and electron distribution have been refined from 791 graphite-monochromatized MoKα data (maximum 2θ=130°) to R=0.035 (R

 w
 =0.029). Difference maps show substantial electron density ascribed to covalent bonding in the hydroxyl group, O(2)-H, but no residual density is observed along the Al-O(1,2) bonds. An analysis of the charge distribution implies net charges of +1.47(26), −1.08(16), −0.59(13) and +0.20(5) for Al, O(1), O(2) and H respectively. Semi-empirical molecular orbital calculations of the Huckel type agree with the experimentally determined atomic charge distribution and also allow a rationalization of the observed bond length variations.

Crystal structure refinement and electron density distribution in diaspore

Experimental research on photonuclear reactions with gamma beams of intermediate energy (from the meson photoproduction threshold up to several GeV) produced by laser photons Compton backscattered on electrons (positrons) circulating in storage rings is reviewed. Common to all of the studies are the subject (interaction mechanisms of gamma-quanta with nuclei) and the experimental technique (Compton beams), which has come to be widely used in recent years. The experimental and applied possibilities offered by the high-intensity Compton beams created in recent years are also discussed.

https://iopscience.iop.org/article/10.1070/PU2004v047n04ABEH001743/pdf

Photonuclear experiments with Compton-backscattered gamma beams

In 1963 BISI et al. [5.1] demonstrated that positrons implanted in ionic crystals end in annihilation through different channels. Five years later, BRANDT et al. [5.2] observed that annihilation characteristics are sensitive to lattice defects. These two discoveries have stimulated intensive activity in experimental and theoretical research. As a consequence, modern views on positron interaction with ionic media have developed in a direction completely unforeseen at the time when FERRELL [5.3], WALLACE [5.4], and GREEN and LEE [5.5] were writing their reviews. Great attention has since been given to the formation of positron states bound to defects, as this phenomenon may provide the basis for an alternative to the classical methods of investigating microscopic imperfections in solids. Positron trapping by defects is not peculiar to ionic crystals, insofar as it is clearly apparent also in metals and semiconductors. However, no other type of material offers equal opportunities for a detailed study of the problem. The reasons are clear: I) the nature and the properties of lattice defects and color centers in ionic crystals (in particular, alkali halides) are generally well known; II) established methods of optical spectroscopy and magnetic resonance measurements allow a precise characterization of the defect structure of the sample; III) it is relatively easy to produce certain defects in controlled amounts; IV) annihilation characteristics for different states are sometimes so distinct that it is possible to attain complete separation of the associated spectral components.

Positrons in Ionic Solids

Azeitidinodiazepines 4b and 4c react with acylnitroso dienophiles 5a–c, specifically from their convexe α-side, but in a non-regiospecific way, leading thereby stereospecifically to the expected adducts 6a–d and 7a–d. The three-dimensional structures of 6a and 7a were determined by X-ray analyses which corroborated their NMR data. OsO4cis-glycolization occurred in good yield with the inverse adducts 7a and 7e and led to the rearranged products 10. These latter ones result from an intramolecular N- to O-transbenzoylation of the short-lived intermediates 9 followed by fragmentation of the animal function. X-Ray analysis of 7a showed the N(10) atom to be pyramidal, a result which demonstrates that it does not have any pronounced benzamide character; otherwise no such N- to O-transbenzoylation would have taken place. Structure and relative configuration of 10a were ascertained by X-ray analysis which confirmed its NMR data as well as the stereochemical outcome of its formation.

Stereospecific Synthesis of 2‐Oxazinyl‐4‐oxoazetidinecarbamates Starting from a 1,2‐Diazepine. A new type of intramolecular transbenzoylation

Positron annihilation 2γ angular correlation curve of NaF has been measured at temperatures ranging from 4.5 K to 300 K. The temperature dependence of the narrow peaks originating from the self-annihilation of singlet positronium (Ps) is analysed. In the lower temperature region, the peak width increases in a rate that shows the Ps is in free state with the effective mass 1.5±0.2 times heavier than the bare mass. At temperatures higher than 150 K, tlae peak width increases more rapidly than in the lower termperature region, which indicates that a number of the Ps are trapped in some localized state.

Study of the Positronium Peak in the Angular Correlation Curve of NaF Crystal

Positron annihilation in ionic crystals.

The essential features of the positron method for determining the effective charges of anions in ionic solids are presented, and papers dealing with its application are reviewed. Values of the charges of the anions are given for more than 100 compounds (alkali metal halides, oxides, etc.). In contrast to earlier work, the effective charges of the anions given in this review were determined independently of other methods. Various regular features of the behaviour of the charges in the compounds studied are noted. The bibliography contains 67 references.

http://iopscience.iop.org/0036-021X/41/4/R01/pdf/0036-021X_41_4_R01.pdf

Determination of the Effective Charges of Anions in Media of the Ionic Type by the Positron Method

The problems of the annihilation of positrons in polymers are discussed and the reactions of positrons and positronium, the effect of external electric and magnetic fields, and the influence of various kinds of radiation, temperature, pressure, phase transitions, etc. are examined. The practical value of the positron method for the investigation of polymers is demonstrated. The bibliography includes 180 references.

http://iopscience.iop.org/article/10.1070/RC1981v050n10ABEH002723/pdf

The Physics and Chemistry of the Reactions of Positrons and Positronium in Polymers

CONTENTS 1. Average lifetimes of positrons in ionic crystals 72 2. Relative counting rate of three-photon coincidences 74 3. Angular correlation of γ quanta in two-photon annihilation of positrons 74 4. Positron states in ionic crystals 77 Cited Literature 79

https://iopscience.iop.org/article/10.1070/PU1971v014n01ABEH004443/pdf

A. Z. Varisov

Papers

Positron annihilation in ionic crystals.

Determination of the Effective Charges of Anions in Media of the Ionic Type by the Positron Method

The Physics and Chemistry of the Reactions of Positrons and Positronium in Polymers

Annihilation of positrons in ionic crystals

Determination by a positron method of the effective oxygen charge in oxides of alkali-earth metals