Gregory J. McCarthy

Synthesis of nuclear waste monazites, ideal actinide hosts for geologic disposal

Abstract Monazite, an orthophosphate mineral of the lanthanides (Ln) and the actinides (An) U and Th, is a model for an ideal synthetic mineral waste form for geologic disposal of long-lived nuclear waste actinides. Natural monazites are known to have survived many of the conditions that might be inflicted on a nuclear waste repository by geological disruptions. High Th and U monazites with compositions typical of nuclear wastes have been synthesized with a routine calcination-pelletization-crystallization procedure. Charge balance for the Th 4+ → Ln 3+ substitution can be provided by either an equimolar Ca 2+ → Ln 3+ or Si 4+ → P 5+ substitution. For U 4+ → Ln 3+ , only the Ca 2+ → Ln 3+ substitution resulted in a phase-pure monazite. Unit cell parameter data were obtained for each nuclear waste monazite phase.

Structure of memory-switching glasses I. crystallization temperature and its control in GeTe glasses

Abstract Binary Ge-Te glasses have been prepared by splat-cooling and water quenching over the ranges of 10–25 and 15–20 at. % Ge respectively. The temperature at which crystallization occurs on heating has been shown to be a highly reproducible and sensitive structural parameter of such glasses. “Crystallization” and glass transition temperatures for 6 glasses were determined by DTA. The latter were found to decrease smoothly with the increase of Te content from 160°C at 75 at. % Te, whereas the former passed through a maximum (220°C) at about 20 at. % Ge and decreased more than 60°C with a 10 at. % increase of Te content in the splat-cooled glasses. The possibility that diphastic or microheterogeneous structures exist at both ends of the glass-forming region is indicated from the DTA and X-ray diffraction data. Some implications of the present results for memory-switching device materials are described.

Preparation and X-ray characterization of CsAlSiO4

A systematic study of the preparation of CsAlSiO/sub 4/ using various cesium oxide, alumina, and silica sources and a typical set of firing conditions was performed. The object was to determine effective methods of preparing phase-pure CsAlSiO/sub 4/. The reaction of Cs/sub 2/CO/sub 3/ with metakaolin at 600/sup 0/C (decomposition and calcining), 850/sup 0/C (prefiring) and 1050/sup 0/C (crystallization) produced the only phase-pure CsAlSiO/sub 4/ obtained by these methods. None of the eighteen sets of starting materials yielded a phase-pure CsAlSiO/sub 4/ in the 1100/sup 0/C and 1200/sup 0/C firings. CsAlSiO/sub 4/ was determined to be isomorphous with RbAlSiO/sub 4/ as reported by Klaska and Jarchow. CsAlSiO/sub 4/ is orthorhombic with lattice parameters of a/sub 0/ = 8.907(2)A, b/sub 0/ = 9.435(1)A, and c/sub 0/ = 5.435(1)A. The space group in Pna2/sub 1/, with Z = 4. Single crystals of CsAlSiO/sub 4/ were grown hydrothermally from a 1Cs/sub 2/O:1A1/sub 2/O/sub 3/:2SiO/sub 2/ gel in a 3M CsOH solution. The reaction conditions were 770/sup 0/C and 11,700 psi. The crystal habit of CsAlSiO/sub 4/ is needle-like.

Crystal chemistry of catalyst materials. I. Composition and unit cell parameters of “REMnO3” phases prepared in air

Abstract Oxygen stoichiometry and manganese valences have been determined for “REMnO 3 ” phases prepared in air where RE are the rare earths from Nd to Lu including Y but excluding Tb. All of the phases had a slight oxygen deficiency and partial Mn 2+ for Mn 3+ substitution. This contrasts with “LaMnO 3 ” which is known to have extensive Mn 4+ for Mn 3+ substitution when prepared in air.

Divalent europium compounds in the systems EuMoO and EuWO

Abstract A study was made of compound formation in the systems EuMoO and EuWO with the objective of preparing the Eu 2+ analogues of the known Sr 2+ compounds EuMoO 4 , EuWO 4 and EuMoO 3 . The scheelite-type compounds EuMoO 4 and EuWO 4 were easily prepared. No EuMoO 3 compound could be prepared over the temperature range 1200° – 1400°C. In the course of the study three new compounds containing trivalent rare earths were encountered, Eu 2 Mo 2 O 7 , Sm 2 Mo 2 O 7 and Eu 6 WO 12 . The first two have cubic pyrochlore-type structures while Eu 6 WO 12 has a defect cubic fluorite structure. The necessity of considering thermodynamic parameters as well as size, charge and polarizability in systematic crystal chemistry is discussed.

Crystal chemistry of REVO3 phases (RE = La-Lu, Y)

Abstract All REVO 3 phases have been prepared and characterized by x-ray powder diffraction for symmetry and unit cell parameters. LaVO 3 and CeVO 3 have tetragonal variations of the perovskite structure while all remaining REVO 3 phases are orthorhombic and isostructural with GdFeO 3 . The variation of cell parameters with RE 3+ ionic radius shows a steady contraction with decreasing radius for a o , c o and V while b o goes through a maximum.

Preparation and structure of the rare earth titanates

Abstract The titanates of the rare earths from Gd through Lu, and of Y, have been prepared by heating RE 2 O 3 and TiO 2 in an evacuated silica capsule at 1200°C. They all have the GdFeO 3 structure. Cell parameters a o and c o show an essentially steady decrease in value while b o remains roughly constant. Comments are made on the use of silica capsules to prepare highly reduced compounds.

The system EuTiO: Phase relations in a portion of the 1400°C isotherm

Abstract Phase relations in a portion of the system EuTiO have been investigated at 1400°C. The following compounds were located: Eu2Ti2O7, Eu2TiO5, Eu3TiO4, Eu3Ti2O7 and what is believed to be EuTi12O19. The existence of the compound Eu4Ti3O10, was ascertained although it could not be prepared as an equilibrium phase at 1400°C. What was previously thought to be the unique composition EuTiO3 has been shown actually to be an extensive solid solution.

Preparation and X-ray characterization of pollucite (CsAlSi2O6)

Abstract A systematic study of the preparation of synthetic pollucite (CsAlSi2O6) by high temperature solid state reaction was performed. Eighteen different combinations of cesium oxide, alumina and silica sources and three firing temperatures were evaluated. The most effective methods of preparing phase-pure pollucite were identified and the chemical compositions of large batches of pollucite prepared by three of these methods were obtained. Reference X-ray powder diffraction data were obtained for the most nearly stoichiometric synthetic pollucite specimen.

Crystal chemistry and magnetic properties of Eu2TiO4 and Eu3Ti2O7

The materials Eu2TiO4 and Eu3Ti2O7 have been found to be ferromagnets with Curie temperatures of about 9°K and 8.5°K respectively. An attempt was made to understand their magnetic properties by considering the crystal chemical relationships which exist among the compounds EuO, EuTiO3, Eu2TiO4 and Eu3Ti2O7 and the implications of this crystal chemistry for the magnetic interactions.