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.

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The (100) and (101) interplanar spacings of 400 mesh quartz powder were measured by high‐temperature X‐ray diffractometry from 22 to 1400°C. Expansion through the high‐low transformation was reversible and continuous, with evidence of 1–2° hysteresis; expansion from 574 to 1000° is zero, with a net decrease from 1000 to 1400°. Maximum linear coefficients of expansion at the transformation were 400±50 x 10−6 deg−1 across the (100) plane and 140±30 x 10−6 deg−1 across the (101) plane. The maximum volume coefficient of expansion was 100±20 × 10−5deg−1 between 572 and 574°. Differential thermal analyses provided ambiguous data, indicating a latent heat of transformation of 160±40 cal/mole, in apparent contradiction with thermal expansion measurements.

A thermodynamic analysis of the vaporization of the substoichiometric plutonium dioxide phase [~1.6 O/Pu 5$ 2.0] is presented. The results are based on measurements of the vapor pressure of known compositions in tungsten, rhenium, and tantalum effusion cells over the range of temperature 1650 to 2100°K. The vapor pressures are interpretable in terms of Pu02(g), PuO(g), and oxygen as the important vapor species; the partial pressure of the gaseous dioxide is relatively insensitive to the composition of the solid phase whereas the partial pressures of gaseous monoxide and oxygen (both atomic and molecular) show a marked dependence and become predominant near the lower and upper phase boundaries, respectively. Known thermodynamic data for the solid phase are combined with the vapor pressure data to yield equations for the standard free energies of formation of gaseous dioxide and monoxide: A(7f0[(Pu02(g)] = -113,100 + 4.35T and &G°[ (PuO(g) ] = -29,000 -12.1 T. The congruently vaporizing composition is calculated to vary from O/Pu = 1.92 at 1600°K to 1.84 at 2400°K. ently vaporizing composition (minimum vapor pressure) was shown to exist for the Pu02_z phase although(1) Based on work performed under the auspices of the U. S. Atomic Energy Commission.(2) T. E. Phipps, G. W. Sears, and O. O. Simpson, J, Chem. Phys 18, 724 (1950).