Heavy Alkali Metals Research Articles

Density-functional calculation for K lattices in condensed phase and quantum-chemical model for the cohesive energy of heavy alkali metals

Following a summary of deductions from experiment on the bonding of Rb and Cs in very different liquid and solid environments, energy calculations based on density-functional theory (DFT) are presented on ordered chains of K as a function of nearest-neighbor distances. At a given bond length, and sufficiently low density, a regime occurs in which modest zig-zag behavior is found to stabilize the original linear chains. As for Rb and Cs, we conclude that K may exhibit low coordination in highly expanded forms. The previous ab initio results on lattices of K atoms for coordination numbers $z=$ 8, 4, and 2 are analyzed by means of a quantum-chemical model in which a nearest-neighbor Heisenberg Hamiltonian is characterized by free-space ${\mathrm{K}}_{2}$ dimer potential energy curves. The satisfactory accord between the two different treatments has prompted us to present results also for Rb and Cs lattices for five different coordination numbers for which DFT calculations are not currently available. The relevance to experiments on expanded fluid Cs and to zig-zag chains of Cs on semiconductor substrates is briefly referred to.

Spectroscopy of alkali atoms (Li, Na, K) attached to large helium clusters

Large liquid helium clusters (Hen, n ≈ 104) produced in a supersonic jet are doped with alkali atoms (Li, Na, K) and characterized by means of laser induced fluorescence. Each cluster contains, on average, less than one dopant atom. Both excitation and emission spectra have been recorded. The observed excitation spectra are analyzed, calculating the transitions within an approach based on the hypothesis that the chromophores are trapped in a dimple on the cluster’s surface as predicted by the theoretical calculations of Ancilotto et al. [9]. The results of the model calculations are in good qualitative agreement with the experimental findings. In spite of the very weak binding energy (a few cm−1), some of the excited atoms remain bound to the surface, provided the excitation occurs at frequencies not too far from the alkali’s gas phase absorptions. These bound-bound excitations produce very broad, red shifted emission spectra. At other, blue shifted frequencies, the excited atoms desorb from the cluster’s surface, giving rise to unshifted, free atom, emission spectra. The heavier alkali metals (Na, K) show, compared to the calculations, an additional broadening which is attributed to surface excitations on the helium droplet.

New synthesis routes for donor-type graphite intercalation compounds

Abstract New original donor-type graphite intercalation compounds have been synthesized by chemical or electrochemical means, using two main routes: in the first one, the reactions take place in a molten alkali metal: the second one consists of using molten ionic compounds like molten alkali metal halides or hydroxides. Reactions between pyrographite and a heavy alkali metal in the presence of a small quantity of oxygen give new quasi-binary graphite intercalation compounds with a high alkali metal content. Each intercalated sheet is a double layer of alkali metal. Reaction between graphite and sodium in the presence of oxygen or another strongly electronegative element gives new alkali metal-rich ternary compounds. The intercalated sheets are polylayered. The intercalation of alkali metal salt or hydroxide leads to new graphite intercalation compounds containing ionic species. These compounds have a high alkali metal content and the intercalated sheets are also polylayered. All these new compounds show a high stability in air and in water. Most of them exhibit a strong electrical resistivity anisotropy.

New graphite intercalation compounds with heavy alkali metal superoxides

Abstract The study of potassium-oxygen and cesium-oxygen graphite intercalation compounds (GICs) first synthesized by the authors is presented. It is shown that these GICs are formed in reactions of graphite with alkali metal/alkali Superoxide mixtures or with alkali suboxide (in the case of cesium). Stage 1 and 2 potassium-oxygen GICs Jmterplanar distance 8.44 A), and stages 1, 2 and 5 cesium-oxygen GICs (interplanar distance 9.84 A) were isolated. X-ray diffraction, XPS, TEM electron diffraction and chemical analysis were used to characterize the content and constitution of the GICs synthesized. Metal-like temperature dependence of c -axis resistivity was obtained in the 1.5–300 K range where the resistivity values at room temperature are 0.07, 0.81, 2.6 and 0.089 Ωcm for KO stagel, KO stage 2, CsO stage 1 and CsO stage 2 compounds, respectively. All the GICs exhibit low spin susceptibility (order of 10 −7 emu/g) which suggests the absence of magnetic species O 2 − .

Na5Rb[(WN3)2] and Na5Cs[(WN3)2], two new sodium nitrido tungstates(VI) with the heavier alkali metals rubidium and cesium

Alkali metal amides react above 600°C with tungsten powder in autoclaves to form nitrido tungstates(VI). Autoclaves are used for this type of reaction in order to avoid rapid decomposition of amides at relatively low temperatures to metal, nitrogen and hydrogen.Starting with an excess of the corresponding mixtures of amides the compounds Na5Rb[(WN3)2] and Na5Cs[(WN3)2] were obtained. After the reaction is carried out the nitrido metallates(VI) are embedded in the alkali metal. Orange-brown (Rb) and orange (Cs) crystals were isolated by extracting the metals with liquid ammonia at room temperature.The structures of the two compounds were determined by X-ray single crystal methods.Space group P21/c (No. 14), Z = 8.Na5Rb[(WN3)2]: a = 9.355(2) Å, b = 10.277(3) Å, c = 19.165(3) Å, β = 90.12(2)°.Na5Cs[(WN3)2]: a = 9.403(3) Å, b = 10.236(2) Å, c = 19.501(4) Å, β = 90.01(2)°.The isotypic compounds contain infinite chains of corner sharing tetrahedra ∞1[WN2N2/23−]. These chains are linked together three dimensionally by sodium and the heavier alkali metal within each compound in a somewhat differing way.

Comparison of the oxidizing ability of graphite and fullerite

This study is devoted to the comparison of the oxidizing ability of graphite and fullerite towards the three heavy alkali metals. These elements are able to intercalate very easily into graphite and fullerite, leading to various binary compounds. In the case of graphite, that is 2D host material, the alkali metal atoms intercalate in the bidimensional van der Waals's gap. These latter can be systematically occupied (stage 1 compound) or not (stages 2, 3…); the alkali metal concentration decreases from the first to the higher stages. On the other hand, C 60 fullerite is OD host material, and the alkali metal atoms intercalate into the octahedral and tetrahedral sites of the fullerite classical FCC structure. According to the cases, these interstitial sites can be more or less occupied, leading to several well defined phases. For all these compounds, a charge transfer occurs between carbon host material and alkali metal, that greatly reduces graphite and fullerite, so that the binary compounds are considered as salts, that is, graphitides and fullendes, respectively. We have shown that fullerite is much more oxidizing than graphite. This latter indeed is strongly (in cesium) or even completely (in rubidium and potassium) striped of its alkali atoms by fullerite. In other words, graphitides are partly or entirely oxidized by fullerite.

Reduction of MoCl5Graphite Intercalation Compounds by Heavy Alkali Metal Vapour

La reduction des composes d’insertion du graphite avec le chlorure de molybdene V par les metaux alcalins lourds (potassium, rubidium et cesium) en phase vapeur conduit a la formation de produits complexes. L’etude de ces produits reduits par Microscopie Electronique a Transmission montre en effet qu’ils contiennent des inclusions de chlorures alcalins mais egalement des phases du metal de transition presentant plusieurs organisations bidimensionnelles et tridimensionnelles selon la nature du reducteur.

Alkali suboxides as weaker binding substrates for helium

Suboxides of heavy alkali metals are known for their particularly low work function. This property should make the surface of those materials less binding for helium than that of pure metals. A semi-quantitative assessment of their wetting properties for liquid helium is proposed. Preparation of such surfaces has been undertaken for rubidium suboxides. The wetting properties obtained for liquid helium will be reported.

Investigation of the effect of oxygen impurities on the wetting properties of alkali metal surfaces by liquid helium

The weak binding properties of heavy alkali metal surfaces for helium have been related to their metallic nature and their particularly small work function. A natural consequence would be that oxidized surfaces should loose their non wetting properties for liquid helium. It is pointed out that the special oxidization process of these metals could unexpectedly lead to an opposite result at some intermediate stage. First experimental results on rubidium shows evidences for such an effect.

Coordination-Dependent Equation of State of Expanded Liquid Alkali Metals and Critical Point Properties

Abstract To take account of the known structure of expanded heavy alkali metals, a coordination number-dependent equation of state is set up. It is then shown that the critical constants Tc and Vc are related by Tc Vc 1/3 = constant, while the compressibility ratio is no longer constant through the alkali series, as required by experiment.

Similarity Relations and Critical Constants of Heavy Fluid Alkali Metals

Abstract Chapman and March demonstrated from experimental data on critical constants for temperature and volume of the fluid alkali metals that TCVc n = constant, where n is about 0.3. It is pointed out here that the similarity relations given by Likalter, though restricted in their validity to the heavy fluid alkali metals, yield n = 1/3.

High-pressure x-ray diffraction on potassium and rubidium up to 50 GPa.

The effect of pressure on structural properties of potassium and rubidium is studied with diamond-anvil cells by energy-dispersive x-ray diffraction with synchrotron radiation at room temperature up to 50 GPa. The K III structure of potassium at pressures above 23 GPa is indexed by the use of a tetragonal-body-centered lattice (tI16) with 16 atoms in the unit cell. A new phase is found for rubidium above 46 GPa. Equation of state data for these heavy alkali metals are presented and compared with earlier data for cesium.

Electronic states in the presence of the inner-shell hole studied by the method of DV-Xα

The molecular orbital theory by the method of DV-Xα is applied to investigate electronic states of electrons in the presence of the inner-shell hole. The hole is assumed to be at the center of the cluster which is composed of nine ions in the bcc structure. Using the present model, the hole potential born by missing an outermost inner-shell hole is found not to be atomic but extended to the cluster size. The result is applied to investigate the anomalous edge of absorption spectra and the related problem of heavy alkali metals with the inner-shell hole in the final state. Phase shifts for l = 2 and l = 3 of wavefunctions of conduction electrons due to the presence of the inner-shell hole in the final state are found to be negative. For Cs, we could not obtain phase shifts that are consistent with both the results of experiments on X-ray absorption and XPS.

Reduction of CuCl2Intercalated Graphite by Heavy Alkali Metal Vapour

Abstract Reduction of CuCl2 intercalated graphite by heavy alkali metal vapour at low temperature gives several intercalated copper phases, depending on the reducing agent. These phases were studied by transmission electron microscopy.

Experimental investigation of wetting properties of alkali metal surfaces by liquid 4He

Heavy alkali metals are weak binding substrates for liquid helium films. A wetting transition exists on cesium surfaces, while rubidium surfaces are found to be wetted by liquid helium, even at low temperatures. A prewetting transition is predicted to exist on rubidium. An experimental investigation of these phenomena is being pursued using the heat conduction technique to monitor the presence of helium films. We report recent results which provide quantitative data on the wetting properties of helium on these substrates.

ChemInform Abstract: Iron Chloride and Cobalt Chloride (FeCl3 and CoCl2) Graphite Intercalation Compounds Reduced by Heavy Alkali Metal Vapor

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Effect of semicore banding on heavy-alkali-metal lattice constants: Corrections to the frozen-core approximation.

Equilibrium lattice constants and bulk moduli of the heavy alkali metals K, Rb, and Cs were calculated using the Troullier-Martins pseudopotentials and plane-wave basis functions. The treatment of the outermost p-shell electrons as Bloch states yielded lattice constants 2\char21{}3 % larger than those obtained within the frozen-core approximation (including the partial core correction of Louie, Froyen, and Cohen [Phys. Rev. B 26, 1738 (1982)]), which narrows a long-standing discrepancy between local-density functional theory and experiment. Predicted bulk moduli are 30\char21{}50 % larger than measured values, within either treatment. The band dispersion of the semicore states (with bandwidths 0.067, 0.14, and 0.25 eV for K, Rb, and Cs) is attributed primarily to core-electron\char21{}conduction-electron hybridization rather than direct core-core overlap. The semicore density of states has a flat line shape, rather than the peaked shape expected for an idealized tight-binding band.

The electronic structure of liquid heavy alkali metals from electron spectroscopy

The first investigation of the liquid alkali metals K, Rb and Cs by photoelectron spectroscopy has been performed, complementing previous studies of liquid Li and Na. The shallow bands of these metals enable in some cases their entire investigation by rather unconventional satellite lines of a noble gas discharge lamp. The trend found for the light alkali metals is continued, i.e., a systematic narrowing of the valence bandwidth with increasing atomic mass. The photoemission bandwidths appear to be significantly reduced compared with both the free electron model (FEM) and calculated photoelectron spectra based on ab initio calculations of the electronic structure. The triangular shape of the valence band spectra is reasonably reproduced by the calculated spectra indicating a distinct binding energy dependence of the partial photoelectric yields.

Synthesis and structures of the heavier alkali metal alkyls; the X-ray structures of [Na(µ-R)]∞and [Rb(µ-R)(pmdeta)]2[R = CH(SiMe3)2, pmdeta =(Me2NCH2CH2)2NMe

The crystalline, hexane-soluble metal alkyls [Na(µ-R)∞1, [KR(pmedta)]m, and [Rb(µ-R)(pmdeta)]22[R = CH(SiMe3)2, pmdeta =(Me2NCH2CH2)2NMe] have been prepared from LiR with equimolar portions of NaOBut, KOBut+ pmdeta, and Rb(OC6H2But2-2,6-Me-4)+ pmdeta, respectively; 1has chains of alternating cations and planar R– anions which are approximately orthogonal to the chains, (Na–C)av 2.555(10)A, (Na–C–H)av 76(3) and (Na–C–Na)av 152(1)°, whereas 2 consists of discrete dimers, (Rb–C)av 3.412(9)A, Rb–C–Rb 75.3(2) and C–Rb–C 104.7(2)°.

Plasmon dispersion constant of the alkali metals

Exchange-correlation and band structure corrections to the plasmon dispersion constants of the alkalis are calculated using local-and nonlocal density functional theory, in order to understand the anomalously low plasmon dispersion for small wavevectors of the heavy alkali metals recently observed in EELS experiments. We find a qualitative improvement of the plasmon dispersion at small wavevector due to the combined effect of exchange-correlation and crystal potential. Possible mechanisms to explain the remaining discrepancies are discussed.