Henry R. Hoekstra

The uranium-oxygen system: U3O8UO3

Abstract Preparative methods for five crystalline modifications and an amorphous form of uranium trioxide are described. Some properties of the UO 3 phases are discussed, including the density, crystal structure, heat of solution, thermal stability and infra-red spectrum. Data on the UO 2·9 phase are also given.

HIGH-PRESSURE B-TYPE POLYMORPHS OF SOME RARE-EARTH SESQUIOXIDES

New high pressure forms of the sesquioxides of holminum, erbium, thulium, ytterbium, lutetium, and yttrium have been obtained by exposure of the C-type sesquioxides to pressures between 25 and 40 kilobars and temperatures between 900� and 1020�C. X-ray diffraction analysis shows that these new high-pressure phases are of the monoclinic B-type which is known for several of the lighter rare-earth sesquioxides (samarium through dysprosium).

The low temperature oxidation of UO2 and U4O9

The low temperature oxidation of UO2 to U3O7 is shown to be a diffusion-controlled diphasic reaction. Two tetragonal phases of U3O7 have been identified: α-U3O7, with a = 5·46 A, c = 5·40 A and β-U3O7 with a = 5·38 A, c = 5·55 A. Annealing experiments with β-U3O7 show the existence of a third tetragonal phase with a composition approximately UO2·3, which is stable to 500°C. Its parameters are a = 5·41 A, c = 5·49 A. The oxidation of U4O9 is also controlled by diffusion of oxygen through a second phase, UO2·3 which is in turn converted to β-U3O7. Evidence is given to show that the oxidized surface layer which forms on UO2 at 25°C is amorphous UO3.

INFRA-RED SPECTRA OF SOME ALKALI METAL URANATES

Abstract Infra-red spectra (900-240 cm−1) of twelve alkali metal uranates are presented. Uranyl stretching vibrations of the monouranates occur between 700 and 800 cm−1, and of the diuranates between 750 and 850 cm−1. The infra-red results are correlated with X-ray diffraction data, and uranyl bond lengths of the compounds have been calculated.

The uranium trioxide-water system☆

The uranium trioxide-water system has been investigated in detail. The compounds prepared are UO3. 2H2O, UO3. 0·8H2O, α,β and γ UO2(OH)2 and U3O8(OH)2. The deuterated forms of these phases were also studied. Each of the compounds was characterized by X-ray diffraction, i.r., Raman and thermoanalytical methods.

The uranium-oxygen system at high pressure☆

Abstract Phase relationships existing in the uranium-oxygen system at temperatures to 1600°C and pressures to 60 kb are reported. The compounds identified in the system include UO 2 , U 4 O 9 , U 16 O 37 , U 8 O 19 , U 2 O 5 and UO 3 . Properties of new phases have been studied by X-ray diffraction, thermal and i.r. analysis. Temperature and pressure stability ranges for three crystal forms of U 2 O 5 are outlined. The crystal symmetry of α-U 2 O 5 is unknown, β-U 2 O 5 is hexagonal and γ-U 2 O 5 is monoclinic. Marked changes in the relative stability of uranium oxides are shown to occur at high pressure, and are correlated with the respective oxide structure types.

Structural studies on Li4UO5 and Na4UO5

Abstract A complete crystal structure determination of Li4UO5 and Na4UO5, based on X-ray and neutron diffraction data, is given. The compounds are body-centered tetragonal, space group I4/m-C4h5, with two molecules in the unit cell. The cell parameters are a = 6·736, c = 4·457 A for Li4UO5 and a = 7·576, c = 4·641 A for Na4UO5. The infrared spectra of these compounds are shown to be in accord with the structures.

Thermochemistry of molybdates I. Standard enthalpy of formation of cesium molybdate (Cs2MoO4)

The standard enthalpy of formation of cesium molybdate, Δ°f(Cs2MoO4, c, 298.15K), is reported. The result obtained, −(362.00±0.11)kcalthmol−1, is based on calorimetric measurements of the enthalpy of reaction of molybdenum trioxide with aqueous cesium hydroxide and of the enthalpy of solution of cesium molybdate in aqueous cesium hydroxide. Thermodynamic data are given for the formation of Cs2MoO4 both from its elements and from the oxides Cs2O and MoO3 at 298.15 K.

The crystal structure of beta-platinum dioxide

Abstract β-PtO 2 crystallizes as the orthorhombic CaCl 2 -type structure with a = 4·488(3) A , b = 4·533(3) A , and c = 3·138(2) A . The oxygen coordinates are found to be: x = 0·281 and y = 0·348. This leads to two long PtO bonds of 2·02 A, four short bonds of 1·98 A, and a mininum oxygen-oxygen approach of 2·40 A. The Pt(IV) radius is 0·63 A. A comparison of the PtO 2 structure with the rutile form of TiO 2 is presented.

Thermochemistry of molybdates IV. Standard enthalpy of formation of lithium molybdate, thermodynamic properties of the aqueous molybdate ion, and thermodynamic stabilities of the alkali-metal molybdates

Abstract The enthalpy changes of the processes: Moo 3 +2 LiOH(aq , 0.2 mol dm −3 = Li 2 MoO 4 ( AQ )+ H 2 O(L) ; δH=−(18.54±0.04) kcal th mol −1 and Li 2 MoO 4 ( c )= Li 2 MoO 4 ( aq ); δH=−(7.61±0.01) kcal th mol −1 , have been determined in a solution calorimeter. These results, when combined with auxiliary thermochemical values, yielded the standard enthalpy of formation, ΔH f o (Li 2 MoO 4 , c, 298.15 K) = −(363.26 ± 0.12) kcal th mol −1 . Based on thermodynamic and solubility data from the literature for sparingly soluble metal molybdates, ΔG f o (MoO 4 2− , aq, 298.15 K) was deduced to be −(200.0 ± 0.3) kcal th mol −1 . This paper also includes estimates of ΔH f o for solid Rb 2 WO 4 and Cs 2 WO 4 , and of the enthalpy of sublimation for Cs 2 WO 4 . It is also pointed out that ΔH f o (PbMoO 4 ) may be in error, and a value of −245 kcal th mol −1 is suggested.