A. Glasner

Absorption and Fluorescence Spectra of Eu 2+ in Alkali Halide Crystals

Alkali halide crystals and pressed disks doped with divalent europium were prepared and their absorption and fluorescence spectra measured. There are two broad absorption bands in the ultraviolet, peaking at ~250 and 330 mμ, both due to the transition 4f7→4f65d1, and attributed, after comparison with the absorption spectra of Eu2+ observed by previous workers in various other media, to the splitting of the 5d orbitals by the crystal field.In addition to the many sharp bands riding on the main absorption bands, one peaking at 270 mμ has been observed, in complete analogy with the Gd3+ spectrum. The fluorescence emission spectra are identical, irrespective of the wavelength of irradiation (250 or 330 mμ).

THERMAL DECOMPOSITION OF THE LIGHT RARE EARTH OXALATES

Abstract The thermal decomposition of the light rare earth oxalates was studied in vacuum in the temperature range 340–440°C. The behaviour of the whole group (with the exception of europium oxalate) was analogous to that of lanthanum oxalate. The relative order of stability, as well as that of the intermediate normal and basic carbonates, is: Gd > Sm > Nd > La > Pr > Ce. The carbon monoxide, initially formed by the decomposition of the oxalates, underwent disproportionation. The degree of disproportionation was highest in the case of cerous oxalate (72 per cent), and lowest in the case of gadolinium oxalate (58 per cent), decreasing in reverse of the order of stability, with lanthanum and praseodymium changing places.

The crystallization of KCl from aqueous solutions in the presence of lead ions. I. A calorimetric study

Abstract The temperature of initial crystallization T H and the supersaturation values of KCl solutions crystallizing in the presence of lead ions were determined by recording the cooling curves in the temperature range 60 to 31.5 °C. A simple relation between the supersaturation at the crystallization point S H and the concentration of the Pb 2+ ions in the solutions was established (eq. (3)). The amount of co-precipitated lead was also determined for some of the experiments. The great variety of observed phenomena is explained by assuming the distribution of the supersaturation in stable macro-molecular nuclei formed around a central lead ion.

The crystallization of NaCl in the presence of [Fe(CN)6]4- ions

Abstract Supersaturated solutions of NaCl were produced by controlled rate of evaporation and the supersaturation limits, in the presence of varying amounts of [Fe(CN) 6 ] 4- ions, were determined. The cyanoferrate was coprecipitated with the NaCl crystals giving an “apparent” distribution coefficient = 7.56 between crystals and solution. At high concentrations of the cyanoferrate > 2.8 × 10 4- molal, a fixed amount of cyanoferrate, about 1150 μg per g NaCl, was found in the crystallizate. At the same concentration a change in the crystal-habit from star-like to plate-like crystals occurred. The results are interpreted by a heterogeneous complex mechanism of crystallization, which is further elaborated in the discussion.

Thermal decomposition of europium(III) oxalate

Abstract It is shown that the first step in the thermal decomposition of europium(III) oxalate leads to reduction to the divalent state. In an oxidizing atmosphere the europium is reoxidized and continues decomposing gradually to Eu2O3 at 390°. In an atmosphere of CO2, Eu(II) Ox is stabilized at 320°. In vacuum, Eu(II)CO3 admixed with carbon (originating in disproportionation of CO) is obtained in the temperature range 360–420°. A unified mechanism for the decomposition of oxalates is proposed, in which a trace of carbon monoxide acts as catalytic agent in the reductive decomposition of oxalates of reducible metal ions.

The crystallization of calcite from aqueous solutions and the role of zinc and magnesium ions—I. Precipitation of calcite in the presence of Zn2+ ions

Abstract When calcium chloride and ammonium carbonate solutions in the 10 −1 –10 −2 M range are mixed a definite proportion of the CaCO 3 is spontaneously precipitated as calcite; the rest is held in solution as small aggregates of the “soluble” polymorphs. The latter convert on ageing into calcite nuclei. Zn 2+ ions added to mixture partially coprecipitated. Comparatively large amounts of Zn 2+ ions in the mixture hinder the crystallization of calcite and simultaneously some hydrozincite is precipitated. When (NH 4 ) 2 CO 3 is allowed to diffuse into CaCl 2 solutions containing Zn 2+ ions, however, the Zn 2+ ions are coprecipitated completely with the calcite. The phenomenological results are interpreted by the primary formation of the carbonato-complex [Zn(CO 3 ) 2 ] 2− ions which serve as centres of nucleation for the calcite crystallization.

Crystal habit modifiers: I. The relationship between the structure of the additive and the crystal lattice

Abstract The crystal habits of NaCl and KCl are significantly modified by nitrilotriacetamide (NTAA) and nitrilotripropionamide (NTPA), as shown by scanning electron microscope photomicrographs. The various effects are correlated with the fit between the molecular structures of the additives and the lattice of the crystals.

The thermal effects of nucleation and crystallization of KBr solutions: I. Experimental and mathematics, illustrated

Abstract The rate of cooling of concentrated KBr solutions containing varying amounts of added Pb2+ ions was studied. Internal evolution of heat due to nucleation and to crystallization was observed and measured. This part of the paper includes the mathematical treatment of the continuous temperature measurements, some examples of curves plotted by the computer together with a general interpretation of them, and a discussion of the effect of Pb2+ ions on nucleation and the rate of crystallization.

Growth of potassium chloride crystals

Abstract Growth rates of potassium chloride crystals in cooling solutions were measured. Systems with either linear (0.2°C/min) or non-linear profiles and spontaneous nucleation were investigated. The range of the average overall measured growth rates was 0.6−1.5 × 10 -7 m s -1 . Most crystals were aggregates of several cubes. Crystals 0.5 mm. The decrease of growth rate could be attributed to attrition which is evident in rounding of corners and edges and presumably breakage of ill fitted aggregates. Crystals grown at initial slow rate of cooling (∽0.01°C min -1 ) with subsequent fast cooling, were spherical, built around single central cubes. The existence of an analogy was suggested between this mechanism of growth and the sub-microscopic processes in crystallizers where cycling phenomena occur.

The mechanism of crystallization; a revision of concepts

Abstract It is shown, by a number of examples taken from the literature, that heterogeneous nucleation can be described in terms proposed by Bunn for epitaxial growth. Complex foreign compounds acting as nucleators determine the habit of the crystals and are naturally coprecipitated with the crystallizing material. The supersaturated solution is a solution of subcolloidal particles, a solution of nuclei, which contribute en bloc to the growth of seed crystals.