Alkaline Earth Calcium Research Articles

Carbon nano-cages created as cubes

We have produced a new form of graphitic nano-cages, rectangular parallelepipeds or cubes, by arc evaporation of carbon with the alkaline-earth metals calcium or strontium. The cubes contain 5 to 20 layers of multiwalled graphitic carbon and have edges ranging in length from 20 to 100 nanometres. Their shape can be controlled by the type of metal catalyst selected.

Volatile Metal Alkoxides according to the Concept of Donor Functionalization

AbstractSince the fundamental works of D. C. Bradley and R. C. Mehrotra, metal alkoxides have attracted attention because of the diversity of their low‐ and high‐molecular‐weight structures; they are also generating increasing interest as precursor compounds for solving technical materials problems. The understanding of the hydrolytic nucleation behavior is a prerequisite for the optimization of materials from sol–gel processes. For metal alkoxides to be precursors in chemical vapor deposition (CVD) processes in the preparation of inorganic oxidic materials, they should be sufficiently volatile, and sublimation should occur without decomposition at as low a temperature as possible (< 150 °C). Only recently, using the “donor functionalization” concept, was a ligand type systematically developed that unifies the advantages of both steric demands and σ‐donor stabilization and so stabilizes low‐molecularweight metal alkoxides. Even large metal ions of low charge (for example Ba2+) can thus form volatile alkoxides. O‐ and N‐donor functions in bidentate and multidentate alkoxo ligands are particularly advantageous; hence, for example, the vanadium derivative [V(OCMe2CH2OMe)3] is one of the most volatile metal alkoxides known to date. The first alkoxides of the alkaline earth metals calcium, strontium, and barium, which sublime without decomposition, have the formula [M2{OC(CH2OiPr)2tBu}4]. This article presents a critical inventory of the metal alkoxides with particular regard to the aspect of volatility. It also describes successes of the donor functionalization concept and shows—in prespective—how alkoxo ligands can be “tailor made” for metals according to their charge‐to‐radius ratio by further development of the concept.

Superconductivity in barium fulleride

INTERCALATING solid C60 with dopant atoms yields materials with a remarkable range of properties1. Alkali metal atoms, for example, readily form charge-transfer compounds2,3, AxC60 (where A is an alkali metal), which can be metallic, superconducting or insulating depending on the dopant concentration4–9. In all cases, the superconducting phase has a face-centred cubic (f.c.c.) structure and stoichiometry A3C60, which suggests a common mechanism for superconductivity dependent, at least in part, on the external coordination number of the C60 molecules. More recently, it has been shown10 that the alkaline earth metal calcium can also be intercalated with fulleride to form a superconducting phase, again with a f.c.c.-derived structure, near a Ca:C60 ratio of 5:1. Here we report the intercalation of fulleride with barium, in which a pure body-centred cubic phase with a lattice constant of 11.171 A is realized near a stoichiometry of Ba6C60. This phase is also superconducting (with a transition temperature of 7 K), suggesting that the mechanism of superconductivity is related to an intrinsic property of the C60 molecules, rather than the external coordination number.

Preparation and characterization of high purity calcium, strontium and barium

The alkaline earth metals calcium, strontium and barium with the highest purities reported hitherto were prepared by metallothermic reduction of their high purity oxides with high purity aluminium in ultrahigh vacuum. The vapour pressure of aluminium at the reduction temperature is considerable (approximately 0.01 mbar), resulting in contamination of the alkaline earth metal condensates. By subsequent ultrahigh vacuum distillation (calcium, 880 °C; barium, 880 °C; strontium, 760 °C) the content of the aluminium impurity was markedly decreased. The high purity metals calcium, strontium and barium were characterized by trace analytical techniques and by the residual resistivity ratio. The trace analytical techniques were standardized chemically.

Determination of dissociation energies for some alkaline earth (hydro-) oxides in CO/N2O flames

The formation of mono-oxides and mono-hydroxides of the alkaline earth metals calcium, strontium, and barium is investigated by a flame spectrometric method. By comparison of the atomic resonance line intensities of sodium and of the alkaline earth metals mentioned, the ratio of the total molecular compound concentration to the free atomic metal concentration is measured in two CO/N2O flames with varying water input. Dihydroxide molecules appear to be of minor importance in our flames based on published dissociation energies for these species, and the atom to total molecule ratio is corrected for their presence by calculation. From the resulting relative oxide plus monohydroxide concentrations, dissociation energies for these compounds are determined with a trial-and-error method by third-law calculations. The resulting energies are compared with literature values, and ``best values'' are given. These are Do(CaO) = 3.75 ± 0.20 eV, Do(SrO) = 4.06 ± 0.10 eV, Do(BaO) = 5.30 ± 0.10 eV, Do(CaOH) = 4.44 ± 0.10 eV, Do(SrOH) = 4.38 ± 0.10 eV, and Do(BaOH) = 4.88 ± 0.06 eV. The electronic partition functions for the oxide molecules are assumed to be 3, 3, and 6, respectively.

LocalizeddStates for Pseudopotential Calculations: Application to the Alkaline-Earth Metals

A procedure for constructing very localized $d$ basis states for generalized pseudopotential calculations is suggested and applied to the alkaline-earth metals calcium, strontium, and barium. Unlike the $d$ states of the free ion or atom, these localized $d$ states do not significantly overlap their neighbors in the metal. They also appear to lead to more accurate estimates of the $s\ensuremath{-}d$ hybridization. Form factors and energy-wave-number characteristics are calculated and used to study the effects of hybridization on representative physical properties of the alkaline earths. In general, hybridization is found to make important contributions to the liquid-metal resistivity and the low-temperature-phase stability, but not to the binding energy nor to the phonon frequencies. A preliminary calculation also suggests that the fcc-bcc phase transitions in calcium and strontium can be understood in terms of the generalized pseudopotential theory.

Trennungsfreie Bestimmung der erdalkalimetalle—calcium, strontium, barium—mit Hflfe der AAS

The alkaline earth metals calcium, strontium and barium can be determined rapidly by AAS without prior separation when mutual interferences are suppressed by the addition of 1 mg ml of potassium to all solutions. Sensitivities for 1% absorption are 0.03 μg Ca, 0.1 μg Sr and 0.5 μg Ba.