摘要:

Both experiment and theory show that the conductivity of a degenerate electron gas in a ‘dirty metal’ tends continuously to zero in the limit oflow temperatures, as the Fermi energy approaches a mobility edge. A derivation is given in terms of the Kubo-Greenwood formula. This is not the case when the metal-insulator transition is induced by a magnetic field, and then a minimum metallic conductivity σmincan be observed. Doped crystalline semiconductors and amorphous and liquid semiconductors are discussed in this context. A brief account is given of two-dimensional systems.

ISSN:0144-235X    

DOI:10.1080/01442358509353351

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Several complexes of iron and cobalt transform gradually or abruptly from a low-spin state to a high-spin state with increase in temperature and such transitions have been investigated widely in the past few years. Spin-state transitions in complexes are often accompanied by changes in enthalpy and crystal structure and many of them exhibit characteristics of first-order phase transitions. Transitions in certain complexes are affected by pressure, dopant metal ions and grinding and not infrequently show a plateau in the magnetic susceptibility-temperature plots. Seemingly unknown to coordination chemists, there is a fair body of information on the spin-state transitions in transition metal oxides and other extended solids. The transitions in rare earth cobaltates exhibit a plateau or a maximum in the inverse susceptibility-temperature plots, changes in enthalpy and crystal structure and other characteristics, not unlike those of the complexes. An attempt is made in this article, to bring together the essential features of spin-state transitions in metal complexes and oxides, particularly with respect to the nature of the phase transitions associated with spin cross-over. Models for spin-state transitions are examined and scope for further research indicated.

ISSN:0144-235X    

DOI:10.1080/01442358509353352

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Empirical force field calculations (e.f.f.) provide reliable predictions of the geometrical structure of organic molecules, particularly hydrocarbons. When intermolecular interactions are combined with the intramolecular field, the influence of a condensed phase on molecular conformation can be investigated. The method can be applied rather generally to the solution and/or refinement of crystal structures and, more recently, to defining the most probable arrangements of organic molecules chemisorbed on metal surfaces. Finally, a summary is provided of how the e.f.f. technique can be used, if supplemented by calculations of lattice dynamics, in the study of phase transitions in molecular crystals.

ISSN:0144-235X    

DOI:10.1080/01442358509353353

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

A review of the regularities in various lanthanide and actinide properties considered as functions of atomic number is given. The regularities, which have been called the tetrad or the double-double effect, are due to symmetry of the ∫-shell and can be conveniently depicted with the use of the expression for the ground energy term in the Racah representation. The tetrad effect has its lower and upper boundaries, imposing certain restrictions on the relative changes in the Racah parameters. Since the tetrad effect in stability constants depends on the magnitude of the nephelauxetic effect, the region of the possible relative nephelauxetic changes of Racah parameters is also presented. The regularities serve as a basis for a semiempirical approach for determination of ionic radii, ∫q→∫q-1dtransition energies, oxidation potentials and other properties of the lanthanides and actinides. In connection with the regularities in ∫q→∫q-1dtransition energies and oxidation potentials, the relative stabilities of various valence states of the ∫-elements are considered.

ISSN:0144-235X    

DOI:10.1080/01442358509353354

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Specialist Periodical Reports—Nuclear Magnetic Resonance, Vol. 12. Edited by G. A. WEBB. (London: The Royal Society of Chemistry, Burlington House, 1983.) [Pp.338.] £6800; $122-00. ISBN 085186 3523.

ISSN:0144-235X    

DOI:10.1080/01442358509353355

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor