Heavy Alkali Metals Research Articles

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The blue compound KC 8 H 2 3 , obtained by action of hydrogen on the first stage compound KC 8, belongs really to the second stage. A good proof of this could be to intercalate a new quantity of metal in the free spaces. Heavy alkali metals (K, Rb, Cs) A calculated amount of alkali metal is heated with KC 8 H 2 3 just over the melting point of the metal. The blue compound becomes copper colored. X rays indicate that each space is occupied in this new compound. The free spaces of KC 8 H 2 3 are now occupied by the new metal. The increase of I c is the same as that from pure graphite. A study of hydrogen physisorption on these new compounds shows that the new layer is as compact as in KC 8 compounds and the formula is K 2 MC 16 H 4 3 or K 2 C 8 H 4 3 , MC 8 . Sodium With the same experimental methods, we obtained no reaction, only a mixture of sodium and KC 8 H 2 3 without change in colour. The behaviour of sodium with the compound KC 8 H 2 3 is about the same as with pure graphite. Lithium We used a different technique because of the very high melting point of Li. The calculated mixture of KC 8 H 2 3 and Li powders is compressed at about 20 kbar at room temperature. The colour becomes blue and yellow. X rays show that this is a mixture of LiH, KC 8 and non-reacted KC 8 H 2 3 . The behaviour of lithium with this compound is very different with respect to its behaviour with graphite and other second stages of heavy alkali metals like KC 24. These results are a proof that the compound KC 8 H 2 3 belongs to the second stage. We prepared new compounds which are more concentrated in metal than the first stage compounds (M/C ratio from 1/8 to 3/16). The behaviour of the alkali metals with KC 8 H 2 3 is very different from Cs to Li, and is almost the same as with graphite.

Electronic structure of metals. V. Density of states effective masses of the alkali metals

For pt.IV see ibid., vol.7, p.1439 (1977). The authors have applied the previously published results of an energy independent model pseudopotential calculation to evaluate the density of states effective masses, m*, at the Fermi level for the alkali metals in both solid and liquid phases. The results thus obtained are used to reduce the range of the uncertainties (i) in the existing theoretical values for m* and the electron-phonon coupling parameters and (ii) in the existing experimental values for the electronic specific-heat coefficients and the paramagnetic spin susceptibilities for the heavy solid alkali metals. Also, these results show that the change in the Fermi level density of states effective mass on melting is larger for the heavy alkali metals than for the lighter alkali metals.

Potential energy surfaces for simple chemical reactions:. Application of valence-bond techniques to the Li + HF ? LiF + H reaction

Ab initio multi-structure valence-bond calculations have been performed to determine the potential energy surface governing the reaction Li + HF → LiF + H. Results for both linear and non-linear nuclear geometries are presented. The system is a prototype for many heavier alkali metal plus hydrogen halide reactions which have been studied using the crossed molecular beams technique. The ab initio valence-bond results are improved by applying corrections, within the framework of the orthogonalized Moffitt (OM) method, for the atomic errors present. The orbital basis set used was of double zeta quality, and was augmented by some extra orbitals. Preliminary calculations on the neutral and ionic diatomic species were performed to ensure the adequancy of the valence-bond structure basis sets used and care was taken to ensure that the basis sets provided an adequate description of F–, HF– and LiF–. The endoergicity of the reaction, ignoring the zero-point vibrational energies, was predicted by the ab initio and OM methods to be 5.8 and 2.5 kcal mol–1 respectively, as compared with the experimental value of 2.6 kcal mol–1. Besides the ground state potential energy surface, several surfaces for excited electronic states have been calculated and are presented. The relationship of the ground state potential energy surface to the reactive cross section, and its variation with energy, is discussed. The ground and excited state potential energy surfaces are compared with previously proposed models and a mechanism for the production of alkali metal ions in hyperthermal alkali metal atom–hydrogen halide collisions is proposed.

Differential spin exchange in the elastic scattering of low-energy electrons by rubidium

The atomic-beam recoil technique has been used to obtain the ratio of spin-flip to full differential cross sections for the elastic scattering of electrons by rubidium at 0.80, 1.10, and 1.40 eV. Data are presented for a range of angles between 30degree and 180degree at each of these energies. These measurements were partly motivated by the expectation that relativistic effects, particularly the spin-orbit interaction, will be significant in electron scattering by the heavier alkali metals, even at low energies. In this context, we show how our measurements are related to the relativistic scattering amplitudes recently discussed by Burke and Mitchell for one-electron atoms. These experiments were performed at intermediate magnetic fields, where the nuclear and valence electron magnetic moments are not fully decoupled. The necessary corrections are discussed and taken into account. (AIP)

Lifetime measurements in the excited S states of K, Rb, and Cs by the cascade Hanle effect*

The cascade Hanle-effect technique has been described in detail. It has been used to measure the radiative lifetimes of several excited S states in heavy alkali metal atoms. We obtained the following measurements (in nanoseconds): τ(K, 6 2S1/2) = 68 ± 9, τ(Rb, 7 2S1/2) = 91 ± 11, τ(Cs, 8 2S1/2) = 96 ± 14, and τ(Cs, 9 2S1/2) = 231 ± 35.

Metallo esters : VI. Esters of isobutyric acid substituted with a heavy alkali metal

Esters of isobutyric acid formally substituted in the α-position with a heavy alkali metal were prepared in a good yield. Two methods were used for the preparation: (A) Exchange reaction between lithio ester and alkoxide of a heavy alkali metal, and (B) direct metalation of the ester with hexamethyldisilazylsodium or -potassium. Both methods gave identical products. The rate of autocondensation of these metallo esters in tetrahydrofuran solution was determined. A coordination complex formed by the ester and hexamethyldisalazylsodium was proved to be the intermediate in the direct metalation.

Intercalation of lithium into graphite and other carbons

Lamellar graphite-lithium compounds of the first, second, third and fourth stages which are respectively brass-yellow, steel-blue, dark-blue and black had been prepared by the two following methods: 1. (a) by heating graphite and lithium at 400°C in a copper or stainless-steel tube; 2. (b) by compressing lithium powder with crushed natural graphite under an argon atmosphere in a glove-box. The reaction begins at room temperature. An homogeneous product is obtained by annealing the pellet at 200°C in vacuum or in argon. Whereas the first method leads only to the first stage compound LiC 6 which is stoichiometric, pure phases of any stage may be obtained according to the composition of the initial mixture by the second method. The analysis of pure phases shows that the compounds of stages n > 1 present large variations of the stoichiometry. The unit cells of first stage LiC 6 and second stage LiC 12 compounds belong to P6/mmm space group with respective parameters a = 4.305 ± 0.001 Å, c = 3.706 ± 0.01 Å and a = 4.288 ± 0.002 Å, c = 7.065 ± 0.02 Å. Lithium intercalates also into soft and hard carbons, including fibers, like the heavy alkali metals.

Ultraviolet optical properties of Li

The reflectances of evaporated films of Li were measured as functions of angle of incidence for photon energies between 3 and 10.7 eV, and were analyzed, by use of Fresnel’s equations, to determine the optical constants n and k for each energy. Above 6 eV, measurements with photons incident on both vacuum and substrate surfaces gave the same values of n and k within experimental limits of about ±5%. Below 6 eV, values of n and k obtained from the analysis were not reproducible; they depend in a complex way on film thickness, light polarization, and surface structure. Failure of the analysis below 6 eV is attributed to surface-roughness effects on the R (θ) curves, which are not accounted for in Fresnel’s equations. These effects are believed to be small above 6 eV. The dielectric function ∊1 and ∊2 and the energy-loss function Im(− 1/∊) were determined from n and k for photon energies above 6 eV. The plasma energy of Li, as determined from the condition ∊1 (ω) = 0, is 6.7 eV; and the plasma loss peak in Im(−1/∊) is centered at 7.1 eV. In contrast to results for the heavy alkali metals K, Rb, and Cs, no evidence was found for an optical-absorption peak in ∊2 in the spectral region above the plasma frequency.

Effect of Pressure on the Electron Transport Properties of Liquid Alkali Metals

The pressure effect on the resistivity and the thermopower of liquid alkali metals (Cs, Rb, K, Cs-Na alloys and Cs-K alloys) has been studied up to around 30 kbar. It has been found that the pressure gives a considerable effect on the transport properties of liquid Cs, Rb and their alloys. The thermopower of liquid Cs changes remarkably with pressure. It exhibits the maximum at pressure of 16 kbar and then decreases with increasing pressure. The importance of d -band in heavy alkali metals is discussed.

Effects of Fermi-surface anisotropy on cyclotron waves in metals

We have shown that the spread in cyclotron frequencies of electrons on different orbits of a nonspherical closed Fermi surface leads to a collisionless damping of cyclotron waves for a range of wave numbers that depends on the geometry of the surface, and also to a shift, from the isotropic limit, in the onset of the waves. Explicit calculation of the dispersion relation, in the long-wavelength limit and in the absence of interactions, is performed for a metal whose Fermi surface has a small cubic distortion (as, for example, the heavy alkali metals, Cs and Rb). We find that Fermi-surface anisotropy produces effects similar in both magnitude and quality to those caused by Fermi-liquid interactions in a metal with an essentially spherical Fermi surface (as, for example, Na and K).

A study of the precipitation of the alkali metals with hexafluorophosphate and hexafluoroarsenate

The separation of the heavier alkali metals from the lighter ones by precipitation with hexafluorophosphate (PF6 −) and hexafluoroarsenate (AsF6 −) has been attempted. It was not possible to obtain a satisfactory separation of potassium from the two heavier alkali metals. The alkali metal salts of hexafluorophosphoric acid have been prepared and characterised, and their solubilities have been determined under comparable conditions, by means of atomic-absorption spectrophotometry. Although the values of these solubilities gave some hope for useful analytical purposes, it was not possible to obtain a satisfactory separation of potassium from rubidium and caesium because of solubility and co-precipitation effects.

The second and third order elastic constants of alkali metals

The second and third order elastic constants of the alkali metals have been calculated on the long wave method using the Heine-Abarenkov lacal model potential with different exchange-correlation corrections. It is found that the use of an exchange correlation correction which satisfies the compressibility sum rule leads to a good agreement between the calculated and measured second order elastic constants of the alkali metals Na, K, Rb and Cs. The shear elastic constants however come out correct even if the compressibility sum rule is violated by the exchange-correlation correction. The third order elastic constants and the pressure derivatives of the second order elastic constants and the pressure derivatives of the second order elastic constants calculated on the HA local potential are lower than the experimental values at room temperature. The discrepancy is pronounced for the heavier alkali metals. Similar calculations using the Wallace potential for Li, Na and K and the Schneider-Stoll potential for Rb give the pressure derivative in good agreement with experiment. In view of the important role by the exchange correlation correction, Suzuki's results calculated without taking this correction into account can only be accepted with some reservation.

Matrix reactions of Na, K, Rb, and Cs atoms with N2O: Infrared spectra and geometries of K2O, Rb2O, and Cs2O

Reactions of the heavy alkali metal atoms with N2O have been studied using the matrix reaction technique. No sodium oxides were produced; however, the metals K, Rb, and Cs yielded products with infrared absorptions appropriate for assignment to ν3 of the M–O–M high-temperature molecules. The identification of these molecules is based on oxygen isotopic shifts and on experiments codepositing two different alkali metals. Apex angle calculations indicate that K2O and Rb2O have valence angles in the 160°–180° range and that the Cs2O angle is approximately 130°–140°. This trend is rationalized on the basis of increasing metal-metal bonding with the more polarizable alkali cation. The KO and CsO fundamentals were observed at 384 and 314 cm−1 in solid nitrogen.

Activation par les metaux alcalins du processus de carbonisation des hydrocarbures aromatiques polycondensés

Heavy alkali metals (K, Rb, Cs) react with polycondensed aromatic hydrocarbons at relatively low temperatures with evolution of hydrogen. The compounds thus obtained contain alkali metal atoms and, depending on previous experimental conditions, their hydrolysis yields either the original hydrocarbon or a mixture of dimers and ‘polymers’. These results can be interpreted in terms of an initial substitution reaction followed by duplication brought about by heating. High reactivity would seem to be especially associated with the presence of three aromatic rings in the phenanthrene configuration.

Measurement of the Spin-Orbit Perturbation in theP-State Continuum of Heavy Alkali-Metal Atoms: K, Rb, and Cs

The spin-orbit interaction for the $P$-state continuum of heavy alkali metals was investigated in a photoionization experiment using spin-polarized alkali atoms and circularly polarized light. From the asymmetry in ion-counting rates corresponding to the two photon helicities, Fano's spin-orbit perturbation parameter $x$ was determined over a range of several hundred angstroms for K, Rb, and Cs. The spin-orbit perturbation was found to increase from K to Rb to Cs as expected, and the nonlinear behavior of $x$ as a function of the photon energy $E$ was demonstrated for K. Knowledge of $x(E)$ was used to establish accurate values for the position of the Cooper minimum and to estimate the magnitude of the cross section at the minimum. In addition, the $x(E)$ data for Cs were used to gain information about the spin polarization of photoelectrons in a Fano-type polarized electron source. Finally, extrapolation of $x(E)$ for cesium into the discrete spectrum indicated the existence of a pole in the function $\ensuremath{\rho}(E)$ which corresponds to the doublet line-strength ratio $\ensuremath{\rho}({E}_{\mathrm{nP}})=\frac{S(n{P}_{\frac{3}{2}})}{S(n{P}_{\frac{1}{2}})}$ at the discrete energies ${E}_{\mathrm{nP}}$. According to our extrapolation, the pole lies in the region of $n=10 \mathrm{to} 15$, in agreement with the early spectroscopic work of Sambursky (1928) and Beutell (1939), whose measurements were discounted by later investigators.

Theory of Knight Shifts and Relaxation Times in Alkali Metals-Role of Exchange Core Polarization and Exchange-Enhancement Effects

The current situation with respect to theory as compared to experiment for Knight shifts and relaxation times in alkali metals is discussed. The roles of exchange core polarization effect and exchange enhancement of the susceptibility due to electron-electron interaction are discussed in detail. It is shown that through a consideration of these effects, combined with the relativistic corrections to the spin density for heavier alkali metals, one is able to obtain an over-all agreement with experiment. However, quantitative agreement with experiment is still lacking, and several additional contributing mechanisms are briefly discussed. The need for better treatments of exchange-enhancement effects on the susceptibility for Bloch electrons and more accurate wave functions, particularly for the lighter metals lithium, sodium, and potassium, is pointed out.

The energy dependence of the scattering amplitudes in alkali-alkali alloys

The energy dependence of the form factors in dilute alloys is investigated and a chice of energy which takes alloying into account is proposed. The results that we obtain give satisfactory agreement with the experimental values of the residual resistivity. The importance of considering the effects due to the d-excited bands in heavy alkali metals is also shown.

Ond-Resonant Levels in Heavy Alkali Metals

An electron-ion scattering amplitude that takes into account the effects due to the $d$ excited bands in heavy alkali metals is proposed. The $d$ bands are treated as resonant levels, and the scattering amplitude is constructed via an effective-range approximation. The cross section is given in terms of two resonance parameters and of the Born approximation for the pseudopotential. We use a modified form of the nonlocal and energy-dependent Heine-Abarenkov model potential, where the high-angular-momentum components of the potential are properly treated. We test our model by calculating the resistivity and thermoelectric power for the solid phase of K, Rb, Cs. The results give satisfactory agreement with experiment. In particular, for $K$ we find that the strong $d$ nonlocality obtained by Lee and Falicov is equivalent to a $d$ resonance built into the electron-ion scattering amplitude.

The solvent extraction of alkali metal tetraphenylborates

The extraction of sodium and small amounts of potassium, rubidium and cesium from aqueous solutions containing sodium tetraphenylborate (TPB), sodium perchlorate and perchloric acid has been studied for nitromethane, nitroethane, nitrobenzene, methyl isobutyl ketone and tributyl phosphate. Nitromethane gives the highest distribution ratio (org/aq) for the perchlorates, and nitrobenzene gives the highest distribution ratio and separation factor for the tetraphenylborates. Most distribution data could be explained by assuming that the tetraphenylborates were fully dissociated in both phases. The equilibrium constants and distribution ratios increase in the order sodium<potassium<rubidium<cesium. The distribution ratio of small amounts of the heavy alkali metal ions will thus not be appreciably influenced by the concentration of sodium tetraphenylborate, but will decrease on the addition of sodium chloride or perchlorate. The effect of perchloric acid can be explained by irreversible acid hydrolysis of the tetraphenylborate ion.

Recent developments in the chemistry of nitrogen fixation.

This discussion is concerned mainly with the biological fixation of nitrogen, and I am the only speaker to discuss its purely chemical aspects. I shall consider first the pure chemistry, and outline recent developments which may be relevant to the biological process. Until three years ago chemistry was concerned only with the mapping of possible routes from nitrogen to simple inorganic compounds, such as ammonia or nitric acid, under mild conditions. Intermediates such as di-imine, hydrazine, hydroxylamine, or hyponitrous acid were postulated. These routes were purely speculative with nothing more to commend them than that they were thermodynamically possible. None had been realized experimentally under mild aqueous conditions and no intermediate between nitrogen and ammonia, the primary product of the biological process, has been detected in the natural system. The nitrogen molecule is a very stable entity with a dissociation energy of 224.5 kcal and an ionization potential as high as 15.58 eV. Its unique electronic structure in relation to nitrogen fixation has already been outlined (Chatt &amp; Leigh 1968). The high dissociation energy and low ionization potential precludes any routes involving either dissociation, or oxidation by stable molecules such as oxygen or even fluorine in their ground states at room temperature. Reduction is the third major reaction type; it depends upon the possibility of adding electrons to the nitrogen molecule, which for this reaction must have vacant orbitals of low energy to receive them. The energetically lowest vacant molecular orbitals are two degenerate antibonding π -orbitals at — 7 eV. These are too high in energy to be attacked by any but the strongest reducing agents, such as the more electropositive metals, and indeed very electropositive metals do attack nitrogen at room temperature. This is part of the explanation of the rapid nitriding of lithium wire observed during the formation of lithium reagents from lithium and alkyl halide in pentane under nitrogen. This reaction can be induced to yield considerable quantities of Li 3 N (J. Chatt &amp; G. J. Leigh unpublished). Indeed a clean film of any metal to the left of Group VII in the long form of the Periodic Table, except the heavier alkali metals, is rapidly nitrided on the surface by gaseous nitrogen at room temperature. All of these are very electropositive metals and very strong reducing agents, which equally rapidly react with water or oxygen under the same conditions. It seems unlikely that nitriding in this manner could form the basis of the activation of nitrogen in the aqueous environment of the biological system, although such a possibility certainly cannot be ignored. I personally do not favour such a nitriding route. However, it is receiving attention at the present time, and I shall outline briefly the direction it is taking.