R. J. Ackermann

The thermodynamics of ionization of gaseous oxides; the first ionization potentials of the lanthanide metals and monoxides

The first ionization potentials of the gaseous lanthanide metals and monoxides have been determined by electron impact from the appearance potentials of ionization efficiency curves. A method of simultaneous and intercomparative measurements with known standards was used and the results for the lanthanide metals are in excellent agreement with values obtained previously from spectroscopic and surface ionization studies. In the early part of the lanthanide sequence the ionization potentials of LnO(g) are less than those of Ln(g), whereas the converse is true in the latter part. The differences in the ionization potentials of LnO(g) and Ln(g) are simply related to the differences in the dissociation energies of LnO(g) and LnO+(g). Values of D0(LnO+) are derived. The nature of the chemical bonding in LnO(g) and LnO+(g) is examined for the lanthanide sequence by means of an electrostatic point‐charge model. The assumption of monotonic variation of the interatomic distance and the electrostatic repulsion param...

First ionization potentials of some refractory oxide vapors

The ionization potentials of the gaseous atoms, monoxides, and dioxides over the refractory oxides of Ti, Zr, Hf, Th, U, Y, and La have been obtained from ionization efficiency curves and appearance potentials by electron impact. A method of simultaneous and intercomparative measurements with known standards was used. The measured values for the gaseous metal atoms are in agreement with the spectroscopic values except for Zr and Hf which are about 0.4 eV lower. These results and some of previous studies show that the values for the monoxides are somewhat less than those for the metals, and the dioxides of those metals in their highest oxidation state are generally larger by 3–4 eV.

Thermal expansion and phase transformations of the U3O8−z phase in air☆

Abstract Detailed thermal expansion data have been obtained by high temperature X-ray diffractometry of U3O8 and U3O8−z in air between 22 and 1100°C. Stoichiometric U3O8 changes continuously, reversibly and anisotropically above room temperature with expansion along the a-axis and contraction along the b-axis from orthorhombic to hexagonal symmetry at 350 ± 10°C. A small but continuous contraction along the c-axis occurs up to 1100°C. Both parameters of the hexagonal phase expand smoothly and reversibly to approximately 875°C and exhibit no discontinuity as a result of loss of oxygen which begins near 600°C. At 875–925°C the structure changes to lower symmetry by contraction along the a-axis and expansion along the b-axis. This structural change is accompanied by a more extensive loss of oxygen and is usually irreversible unless the crystallite size is sufficiently small, ∼0.05 μm for which both the compositional and structural changes are reversible with temperature. Calculated unit cell volumes undergo slight expansion and contraction up to 350°C, with a broad maximum at approx. 200°C and a smooth expansion at a decreasing rate from 350–875°C. The irreversible transition beginning at 875°C is ostensibly associated with an expansion-contraction anomaly in the temperature range 875–925°C, followed by smooth expansion at a decreasing rate to 1100°C. The thermal expansion characteristics have been interpreted in terms of rotation of octahedral linkages in the orthorhombic and hexagonal structures and the irreversible transition has been tentatively explained in terms of two U3O8−z phases, one orthorhombic and one monoclinic. The apparently contradictory structural analyses for the U3O8 and U3O8−z phases in the literature can be reconciled on the basis of the thermal expansion results which involve both changes in temperature and composition.

Thermodynamic Properties of Gaseous Yttrium Monoxide. Correlation of Bonding in Group III Transition‐Metal Monoxides

The vapor from solid yttrium sesquioxide contained in a Knudsen cell made of tungsten has been investigated by measuring the absolute effusion rate and by studying the effusate in a mass spectrometer. The latter demonstrates that the gaseous species are yttrium monoxide, oxygen, and a small amount of yttrium. Measurements of the appearance potentials for the ions enable one to conclude that the ratio of monoxide‐to‐metal is at least 65 and that above 15 eV significant fragmentation of YO by the ionizing electrons to yield Y+ occurs. From the absolute effusion rates one derives the free energy of formation, ΔFf∘(YO,g)=−21 800–10.96 T, and the dissociation energy, 7.31±0.10 eV, for gaseous yttrium monoxide. A correlation of the dissociation energies and the molecular parameters of the monoxides of Group III transition metals indicates that the unexpected increase in bonding with increasing radii is accompanied by an increase in the electronic entropy of the metal atom. The correlation demonstrates the role ...

Vapor Pressures of Scandium, Yttrium, and Lanthanum

The vapor pressures of scandium, yttrium, and lanthanum were measured by the effusion method with a vacuum balance, and the heats of vaporization were measured with a mass spectrometer. The data are represented by Scandium:logp (atm)=(5.44±0.07)−(17020±90)/T,Yttrium:logp (atm)=(5.59±0.07)−(19950±90)/T,Lanthanum:logp (atm)=(6.08±0.09)−(21940±150)/T. The errors quoted are standard deviations. The heats of vaporization at temperature are 77.9±0.4, 91.3±0.4, and 100.4±0.7 kcal mole—1 for scandium, yttrium, and lanthanum, respectively, and the corresponding entropy changes are 24.9±0.3, 25.6±0.3, and 27.8±0.4 eu.

The first ionization potentials of the transition metals

The appearance potentials of the transition metals yttrium through molybdenum, ruthenium through palladium, and lanthanum through platinum have been determined by electron impact. A method of simultaneous and intercomparative measurements with known standards was used. Corrections to the appearance potentials for thermally excited levels in the atoms and for possible ionization paths to other than the ground states of the ions are discussed and used to deduce the ionization potentials.

The preparation and characterization of the metastable monoxides of thorium and uranium

The solid monoxides of thorium and uranium have been obtained by thermal decomposition of previously reported compounds in which the lower valence states (< + 4) of the metals are stabilized by HCl and H2O. The respective monoxides were characterized by X-ray diffraction and yield lattice constants of 5·302 ± 0·003 A for ThO(s) and 5·00 ± 0·01 A for UO(s). Both monoxides are shown to be thermodynamically unstable via disproportionation to the respective metals and dioxides. The molar volumes of these and higher actinide monoxides lie between those of the respective metals and dioxides and the calculated cationic radii are significantly larger than those in the monosulfides and the dioxides. Volatile chlorides including ThCl4(g) and UCl3(g) were observed mass-spectrometrically to vaporize during thermal decomposition. Ionization potentials of 12·7 ± 0·3, 11·0 ± 0·3, and 10·0 ± 0·5 eV, respectively, were measured.

Thermodynamic properties of ZrO (g) and HfO (g); a critical examination of isomolecular oxygen‐exchange reactions

The thermodynamic properties of ZrO(g) and HfO(g) have been determined by mass‐spectrometric studies of five isomolecular oxygen‐exchange reactions of the type M(g)+M′O(g)=MO(g)+M′(g) involving the gaseous atoms and monoxides of yttrium and thorium. Third law values of the standard free energies of formation over the temperature range 2000–2800 K are adequately (within 2%) expressed by the linear equations ΔG°f(ZrO,g)=13 040−16.04T, ΔG°f(HfO,g)=9930−14.68T. Uncertainties of ±1 kcal mol−1 are estimated. Third law analyses of the data yield the enthalpies of formation at absolute zero, ΔH°0(ZrO,g)=21.4, and ΔH°0(HfO,g) = 18.5 kcal mol−1, and the dissociation energies at absolute zero, D0(ZrO)=180.7, and D0(HfO)=188.9 kcal mol−1, with uncertainties of 1–2 kcal mol−1. The equilibrium constants for the isomolecular reactions in this and in previous studies were obtained from ion currents and relative instrumental sensitivities that were approximated from the mean square radii of the gaseous atoms and the squar...

The evaporation behaviour of neptunium dioxide

Abstract The evaporation behaviour of neptunium dioxide in vacuo has been studied over the temperature range 1850–2475°K. The gaseous oxides NpO 2 and NpO were the only species observed and the temperature dependence of the major species NpO 2 was measured mass-spectrometrically. The total mass effusion rates were determined by collecting known fractions of effusate which were subsequently assayed by alpha counting of 237 Np. The effect on the partial pressures due to the variation of sample size and the fractional amount evaporated indicates that the solid dioxide becomes substoicheiometric and produces a bivariant system. The extent of substoicheiometry, although unknown, grossly affects the partial pressure of NpO(g) but has an insignificant influence on the partial pressure of NpO 2 (g). The experimental results are combined with known and estimated thermodynamic data to yield the standard free energy of formation of NpO 2 (g), ΔG f °(NpO 2 , g) = −114,000 + 3·5 T the dissociation energy of NpO 2 (g), 14·3 ± 0·3 eV, and an estimated value for the dissociation energy of NpO(g), 7·4 ± 0·3 eV.

Effect of Oxygen on the Vapor Pressure of Uranium

The published observations pertaining to the vapor pressure of uranium have been examined for consistencies. Since it was shown previously by Rauh and Thorn that oxygen suppresses the vapor pressure, the experiment by DeMaria, Burns, Drowart, and Inghram involving a mixture of uranium and aluminum oxide contained in a molybdenum effusion cell cannot yield the vapor pressure of pure uranium, but rather yields the individual partial pressures for the equilibrium U(Mo and O in solution)+UO2(s)=2UO(g), and a composition of the vapor which is approximately that of the monoxide. Using all the data one can construct a diagram schematically describing the effect of oxygen on the vapor pressure of uranium.