Structural relationships, crystal chemistry and anion substitution processes for M(III)X3 systems of the lanthanides and actinides

An examination of data for lanthanide and actinide phases with UCl3-type and PuBr3-type M(III)X3 structures has shown that these systems are conveniently described by alternating layers of [MX2]n+n and [X]n-n. The relationships between the UCl3- and PuBr3-type structures are described and expanded to include a variety of anion substitution systems, M(III)X3-xYx. The two different types of [MX2]n+n layers observed in these systems are consistent with the existence of a novel structural unit, [M2X4]2+. The effects of radius ratio constraints and layering mechanisms on the phase equilibria and anionic substitution processes, polymorphism and crystal growth in the MX3-xYx systems are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22030/1/0000448.pd

Structural correlations of the lanthanide halides and related compounds. Derivatives of the anti-nickel-arsenide structure

An examination of structural data for lanthanide halides and related compounds has shown that a substantial number of different structure types are conveniently described as layered structures derived from anti-NiAs by removal or shear of cation layers and distortion of the residual layers. The structural correlations of various MX, MX1.5, MX2, MXY, MX3, and MX2Y compositions (M = cation, X and Y = anions) are described by the presentation of a subgroup-supergroup diagram relating their space groups and by comparison of their structural projections. A close relationship between the CsCl- and NiAs-type structures is observed. The occurrence of displacive and order-disorder phase transitions, the formation ternary derivatives by ion accommodation processes and the possible formation of intermediate halides, M2X2n+1, by coherent intergrowth of MX2 and MX3 structures are discussed. The effects of radius ratio and cation coordination number on the stabilities of halide structures and on the formation of complex MX2 layers derived from hexagonal-closest-packed metal arrays are examined.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/21736/1/0000129.pd

The lanthanum hydroxide fluoride carbonate system: The preparation of synthetic bastnaesite

Hydrothermal phase equilibria in the lanthanum + hydroxide + fluoride + carbonate system have been investigated along an isobaric and isothermal section of variable metal to fluoride ratio, x. Quantitative substitution of fluoride into LaOHCO3 proceeds with the formation of a continuous solid solution, La(OH)1-xFxCO3, for 0 0 [les] x [les] 1 and a two-phase region, LaFCO3 + LaF3, for 1 x 3 is an orthorhombic phase (a = 21.891(5), B = 12.639(3) and C = 10.047(2) A) which is not isostructural with LaFCO3. Hydrolysis of the La(OH)1-xFxCO3 phase to the corresponding UCl3-type La(OH)3-xFx compositions has been observed. Thermal decomposition reactions of the hydroxide fluoride carbonates are described, and a geochemical process for the formation of bastnaesite and tysonite is proposed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22168/1/0000599.pd

The phase equilibria, vaporization behavior, and thermodynamic properties of europium tribromide

Europium tribromide has been found to vaporize incongruently according to the reaction:2EuBr3(s)=2EuBr2(s)+Br2(g).Static equilibrium vapor pressures have been measured by a spectrophotometric procedure in the temperature range 502 to 623 K. Equilibrium pressures were not attained in Knudsen effusion experiments. For the vaporization reaction, ΔHo(563 K)/kcalth mol-1 = (22.85 ± 0.18) and ΔSo(563 K)/calth K-1 mol-1 = (34.26 ± 0.32). Heat capacities have been estimated for the condensed bromides, and second- and third-law results are given. For the tribromide, ΔHof(EuBr3, s, 298.15 K)/kcalth mol-1 = -(186.1 ± 3.0), ΔGof(EuBr3, s, 298.15 K)/kcalth mol-1 = -(179.3 ± 3.0) and So(EuBr3, s, 298.15 K)/calth K-1 mol-1 = (50.7 ± 3.0). The preparative chemistry and phase equilibria of the europium + bromine system and the thermochemical data are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33930/1/0000197.pd

Vaporization and thermodynamic properties of samarium dicarbide and sub-stoichiometric disamarium tricarbide

The vaporization reactions of SmC2(s) and Pu2C3-type SmCy(s, 1.36 y SmC2(s) = 2C(s)+Sm(g){2/(2-y)}SmCy(s) = {y/(2-y)}SmC2(s)+Sm(g)The complex reaction for SmCy occurs because the composition of the carbon-rich boundary of the phase decreases as temperature increases. For reaction (i), the equiliblium pressure is described by log10{(p/po)(Sm, g, 1548 K T T/K)-1. The non-linear pressure equation for reaction (ii) is log10{(p/po)(Sm, g, 1.42 y T T/K)-1 + 7018500(T/K)-2 +/- 0.038}. Thermodynamic values for the vaporization and formation reactions of SmC2(s) and SmCy(s) at 298.15 K have been calculated: [Delta]Hto(SmC2, s, 298.15 K) = -(96.2 +/- 7.5) kJ [middle dot] mol-1 and [Delta]Hfo(SmC1.43, s, 298.15 K) = -(89.0 +/- 8.0) kJ [middle dot] mol-1. The thermodynamic results are discussed and compared with values reported for other lanthanide carbides.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24147/1/0000404.pd

Preparation, phase equilibria, and crystal chemistry of lanthanum, praseodymium, and neodymium hydroxide chlorides

The preparation of hydroxide chlorides of lanthanum, praseodymium, and neodymium has been achieved by hydrothermal methods at 550[deg]C and 1530 atm. Three phases (Ln(OH)3, Ln(OH)2.55Cl0.45, and Ln(OH)2Cl) have been characterized by analytical and X-ray methods. The observed compositions are closely defined by the x = 0, 0.5, and 1 members of the homologous anion substitution series, Ln(OH)3-xClx. The previously unreported Ln(OH)2.55Cl0.45 phases are clearly substoichiometric in chloride and are described by the Ln7(OH)18Cl3 composition. X-ray diffraction data for the phases at x = 0.45 show a pronounced UCl3-type substructure and complex superstructure reflections that have been indexed on a hexagonal cell. For the lanthanum phase, a = 17.662 (6) and c = 3.914(1) A. Efforts to obtain single crystals have been unsuccessful. Thermal decomposition processes of the hydroxide chlorides have been investigated, and the characterization of LaO(OH)0.55Cl0.45, a monoclinic YOOH-type intermediate phase, is reported. Structural features and phase equilibria of the hydroxide chlorides are discussed and analogies with the hydroxide nitrate systems are drawn.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/21828/1/0000231.pd

Interactions of Plutonium Dioxide with Water and Oxygen-Hydrogen Mixtures

Pressure-volume-temperature data and mass spectrometric results obtained during exposure of PuO{sub 2} to D{sub 2}O show that the dioxide reacts with water at room temperature to produce a higher oxide (PuO{sub 2+x})and H{sub 2}. Results demonstrate that PuO{sub 2+x} is the thermodynamically stable oxide in air. The absence of O{sub 2} at detectable levels in the gas phase implies that radiolytic decomposition of water to the elements is not a significant reaction. The rate of the PuO{sub 2}+H{sub 2}O reaction is 6{+-}4 nmol H{sub 2}/m{sup 2} day, a value that is independent of the H{sub 2}O concentration on the oxide over a range that extends from fractional monolayer coverage to saturation by liquid water. Evaluation of literature data shows that oxide compositions in excess of PuO{sub 2.25} are attained, but the maximum value of x is unknown. During exposure of PuO{sub 2} to a 2:1 D{sub 2}:O{sub 2} mixture at room temperature, the elements combine by a process consistent with a surface-catalyzed reaction. Water is simultaneously formed by the H{sub 2}+O{sub 2} reaction and consumed by the PuO{sub 2} + H{sub 2}O reaction and accumulates until the opposing rates are equal. Thereafter, PuO{sub 2+x} is formed at a constant rate by the water-catalyzed PuO{sub 2} + O{sub 2} reaction. The failure of earlier attempts to prepare higher oxides of plutonium is discussed and the catalytic cycle that promotes the reaction of PuO{sub 2} with O{sub 2} is described. Implications of the results for extended storage and environmental chemistry of oxide are examined. Moisture-catalyzed oxidation of PuO{sub 2} accounts for observation of both pressure increases and decreases in oxide storage containers with air atmospheres. Application of the experimental rate results indicates that the reaction of a typical oxide with 0.5 mass % of adsorbed water maybe complete after 25 to 50 years at room temperature

Entropy and heat capacity of europium(III) bromide from 5 to 340 K

The heat capacity of a polycrystalline sample of EuBr3 has been measured from 5 to 340 K and found to be without transitions in this region. Values of the thermodynamic functions Cp(T), {So(T) - So(5 K)}, and {Ho(T) - Ho(5 K)}/T are 26.44, 43.70, and 19.66 calth K-1 mol-1 respectively at 298.15 K. A value of So(298.15 K) = 41.42 calth K-1 mol-1 for EuBr3 from which the Schottky contribution has been deleted, is compared with an estimate of the lattice heat capacity by an empirical method.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22189/1/0000620.pd

Hydride-catalyzed corrosion of plutonium by air: Initiation by plutonium monoxide monohydride

Chemistry and kinetics of air reactions with plutonium monoxide monohydride (PuOH) and with mixtures of the oxide hydride and plutonium metal are defined by results of pressure-volume-temperature (PVT) measurements. Test with specimens prepared by total and partial corrosion of plutonium in 0.05 M sodium chloride solution show that reaction of residual water continues to generate H{sub 2} after liquid water is removed by evacuation. Rapid exposure of PuOH to air at room temperature does not produce a detectable reaction, but similar exposure of a partially corroded metal sample containing Pu and PuOH results in hydride (PuH{sub x})-catalyzed corrosion of the residual Pu. Kinetics of he first-order reaction resulting in formation of the PuH{sub x} catalyst and of the indiscriminate reaction of N{sub 2} and O{sub 2} with plutonium metal are defined. The rate of the catalyzed Pu+air reaction is independent of temperature (E{sub a} = 0), varies as the square of air pressure, and equals 0.78 {+-} 0.03 g Pu/cm{sup 2} min in air at one atmosphere. The absence of pyrophoric behavior for PuOH and differences in the reactivities of PuOH and PuOH + Pu mixtures are attributed to kinetic control by gaseous reaction products. Thermodynamic properties of the oxide hydride are estimated, particle size distributions of corrosion products are presented, and potential hazards associated with products formed by aqueous corrosion of plutonium are discussed