Showing papers on "Valency published in 1979"

Conductance of symmetrical, unsymmetrical and mixed electrolytes. Part 3.—Examination of new model and analysis of data for symmetrical electrolytes

Abstract: In the first two papers of this series a conductance equation, based on a new model for ions in solution, was derived. The equation described charge transport in solutions containing any number of ionic species of any valency type.In this paper the model upon which the new equation is based is discussed in greater detail and the equation is used to analyse conductance data for a number of symmetrical electrolyte solutions. Excellent agreement is found between experimental results and theoretical predictions and the distance parameter values obtained show clearly the existence of solvent-separated ion-pairs (SSIP's) in many systems.

The problem of intermediate valency

Abstract: The situation is found in a number of rare-earth compounds in which the narrow f level lies near the Fermi level, and configurations of the rare-earth ions differing in the number of f electrons (different valency) have close-lying energies. In this situation many properties of the corresponding substances, both thermodynamic (heat capacity, magnetic susceptibility, compressibility) and kinetic (optical properties, electric conductivity, etc.) are anomalous. Isostructural electronic phase transitions occur in these systems upon changing the external conditions. These are often simultaneously dielectric-metal and magnetic-nonmagnetic transitions. A phase having a narrow resonance level near EF has been termed an intermediate-valence (IV) phase. This article reviews generally the problem of intermediate valency. The fundamental experimental facts are summarized. The conditions for appearance and the qualitative picture of IV states are discussed. A table is given of the currently know IV substances. The fundamental theoretical approaches to describing their properties and the electronic phase transitions in these systems are treated. The connection is established with other problems (the problem of dielectric-metal transitions, the Kondo effect, etc.). Major attention is paid to the fundamental unsolved problems.

Change in valence of Ce ions induced by hydrogen absorption : CeRu2

Abstract: 2014 Ternary hydrides of superconducting cubic Laves-phases CeRu2 and LaRu2 have been prepared. The cubic structure is preserved and a considerable lattice expansion is observed. Magnetic susceptibility measurements show the disappearance of the superconducting transition for both hydrides, and in the case of CeRu2, the appearance of a susceptibility peak at T ~ 1.4 K. For CeRu2 hydride, the hydrogen could be removed and superconductivity reestablished. Magnetization measurements of CeRu2 hydride and comparison of its lattice parameter with that of LaRu2 hydride indicate that, with hydrogenation, the valence of Ce has changed from 4 to 3. LE JOURNAL DL PHYSIQUE LETTRES TOME 40, 1 er MARS 1979, Classification Physics Abstracts 74. 70L 75 . 30K 75 . 50C Rare-earth intermetallic compounds are well-known to absorb large quantities of hydrogen. Such hydrides are of great interest for technological reasons (for example energy storage). From a more fundamental point of view, controlled introduction of hydrogen into these compounds is a good tool for modification of the structure of the material and hence, some physical properties like superconductivity and magnetism. We have studied the cubic Laves-phase compounds CeRu2 and LaRu2. From a comparison of its lattice parameters with those of the trivalent RE-Ru2 it appears that cerium is tetravalent in CeRu2 [1]. Both Ce and La have no localized 4f electrons in these compounds. Furthermore, these two compounds are superconducting and are reported to absorb large quantities of hydrogen [2]. The samples were prepared by induction melting. The CeRu2 was sealed in a tantalum container and annealed at 1 100 ~C for 10 days, and at 1 300 OC I for few hours. The LaRu2 was annealed at 1 150 °C for 3 hours in the induction furnace. Different values of the superconducting transition temperature (7~) have been published for these compounds. The 7~ varies between 4.9 K and 6.2 K for CeRu2 and between 3.08 K and 4.4 K for. LaRu2 [3-4]. We have obtained a Tc of 5.8 K for CeRu2 and 3.6 K for LaRu2. A careful investigation in the case of CeRu2 has revealed that this lower value is due to contamination by tantalum. We are uncertain as to the reason for LaRu2. , For hydrogenation small pieces of the sample were placed in a stainless steel reactor connected to a vacuum line. After each hydrogenation the structure of the compounds was checked by X-ray analysis. Attempts to hydrogenate from 200 to 700 ~C with pressures above 1 atm., have all led to the dissociation of the starting materials. Lines associated with Ru could be identified in the X-ray patterns of both compounds. However, hydrogenation at room temperature has been conducted successfully. The sample is Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:01979004005010500 L-106 JOURNAL DE PHYSIQUE LETTRES activated by going several times through pressure (10 bars)-vacuum cycles. Then hydrogen pressure (10 bars) is introduced and the sample is left for four days. A second method which consists in using higher pressure (110 bars) and letting the sample absorb for one day has led to the same results concerning the data presented in this paper. The samples are broken into tiny particles by hydrogenation. X-ray diffraction measurements have been carried out at room temperature, in a vacuum chamber. For both hydrogenated compounds, the X-ray diagrams show that the cubic Laves-phase structure is preserved upon the formation of the ternary hydrides CeRu2Hx and LaRu2Hx, where x ~ 4, as found by preliminary volumetric measurements. A lattice expansion and a significant line broadening is observed. As can be seen from table I the relative increase of the lattice parameter Aa/a of CeRu2Hx is larger than that of the hydride LaRu2Hx. It is important to notice that the lattice parameters for the two hydrides are much nearer the same value than those of the starting materials. The present experiments do not allow us to determine the location of the hydrogens.

Holographic optical memory element - based on lithium niobate or tantalate crystal doped with chromium and metal with two valency states

Abstract: Holographic optical memory element for reversible storage of volume phase holograms by 2 photon absorption consists of a Cr3+ -ion-contg. LiNbO3 or LiTaO3 crystal, which is also doped with another substance having ions with 2 valency states, the ground state of the ions filled with one electron being min. 2.5 eV below the conduction band. Fe, Cu, Ni, Co or Mn ions are pref. Extremely high storage densities are possible. When not exposed, the material gives a very long storage time at ca. 20 degrees C and the hologram is not erased on reconstruction.

Classical Gibbs distribution for a carbon chain

Abstract: A simplified mechanical model of a carbon chain is examined: lengths of valence bonds and valency angles are determined. The Gibbs distribution is used for this system. A further factor Ω (φ) appears, which is significant. A formula is derived for Ω (φ) in the form of a determinant, elements of which are in a clear form. The effect of Ω (φ) is examined using low n values (n is the number of carbon atoms). The existence of a correlation between angles is emphasized even in the absence of potential interaction.