The first ionization potentials of neptunium and neptunium monoxide J. Chem. Phys. 62, 1584(1975; 10.1063/1.430578First ionization potentials of some refractory oxide vaporsThe 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 reg). Values of Do(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 parameter and the nonmonotonic variation of the polarization energy derived from the known f-to-d electronic transitions are all mutually consistent and describe rather closely the variation in bonding in LnO(g); a similar scheme without the f-to-d transition energies is shown to describe the variation in the bonding of LnO+ (g).

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.

341to 30" crack-front configuration, K is constant over ='I/eT and nearly equal to the strength-of-materials value. The strength-ofmaterials formula (Eq. (2)) is approximately correct for crack lengths >0.55W and ligament lengths >0.65W. ReferencesI J. A. Kies and A. B. I. Clark; pp. 483-91 in Proceedings of the Second International Conferenceon Fracture, Brighton, April 1969.The melting, eutectic, peritectic, solidus, and liquidus temperatures in the system Zr-0 have been measured directly by a simple optical pyrometric technique requiring only a few hundred milligrams of sample. The saturation solubility of oxygen in a-Zr(s) between 1270" and 1980°C and the lower phase boundary of the ZrO,-, phase between 1900" and 2400°C have been measured by an isopiestic equilibration method. The oxygen solubility limit in a-Zr(s) agrees well with previous low-temperature studies and reaches a maximum solubility of 35*1 at.% 0 at the eutectic temperature, 2065"*5"C. The maximum melting temperature of a-Zr(ss) is 2130'2 10°C and corresponds to a composition of 25k1 at.% 0. Both of these temperatures are approximately 150" higher than previously reported. Liquidus compositions above the eutectic temperature were obtained via mass spectrometry from the kinetic behavior of the liquid solution-ZrO,-,(s) mixture as it approached equilibrium at 2125"*5"C. The lower phase boundary or solidus of the ZrOz-s phase departs appreciably from ideal stoichiometry above 1900°C and smoothly reaches its most reduced composition, 61 at.% (Zr0,.56), near 2300°C. The solidus is retrograde at higher temperatures. The melting temperature of the stoichiometric dioxide is 2710'2 15°C.

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