Akio Nakamura

Thermodynamic study of UO2 +x by solid state emf technique

Abstract A complete set of thermodynamic parameters of UO2 +x — the relative partial molar thermodynamic quantities of oxygen: g(O2), h(O2) and s(O2) as a function of nonstoichiometry x and temperature T — have been determined with sufficient accuracy by the precise emf measurements of the solid state galvanic cell of the type Ni.NiO/Stabilized ZrO2/UO2 + x, at 0.0030 ⩽ x ⩽ 0.23 between 500 and 1100°C. Nonstoichiometry x was controlled and determined by the coulometric titration of oxide ions at 1000°C by using NiO in the Ni/NiO reference mixture as a source of oxygen. UO2 +x samples of two different preparation procedures give almost identical results and show that g(O2) versus T plots at various compositions x are not a linear function of temperature, but curve downwards with temperature in the composition and temperature ranges studied. This tendency becomes more pronounced with decreasing nonstoichiometry x and indicates that both h(O2) and s(O2) of UO2 +x increase with temperature, their temperature dependence becoming stronger with decreasing nonstoichiometry x. The statistical analysis on about forty emf versus T plots at various compositions x confirms that g(O2), h(O2) and s(O2) of UO2 +x at 0.0030 ⩽ x ⩽ 0.23 and 500 ⩽ T ⩽ 1100°C are accurately expressed by the following equations utilizing the polynomial forms of log x for the temperature independent heat capacity-, entropy- and enthalpy-parameters: cp(O2), s0 and h0 g(O2) = h(O2)−Ts(O2), s(O2) =s0 +cp(O2) In T, h(O2) = h0 +cp(O2)T, where temperature T is in Kelvin, and cp(O2), s0 and h0 are given by c(O2) = −43.4642−120.129 log x−60.9395(log x)2− 19.4064(log x)3 (J/mol·K). s0= −93.807 + 281.272 log x + 119.575(log x)2 + 72.651(log x)3 (J/mol·K). h0 = 373.411 + 277.706 log x + 23.4894(log x)2 + 654.207(log x)−1 + 198.212(log x)−2 (kj/mol). The present results are extensively discussed in comparison with the available literature data on these quantities and also in connection with the point defect model of UO2 +x recently proposed by the present authors.

Magnetism and transport of Ln0.5Sr0.5CoO3 (Ln = Pr, Nd, Sm and Eu)

Abstract Magnetism and transport have been investigated for perovskite cobalt oxides Ln 0.5 Sr 0.5 CoO 3 ( Ln = Pr, Nd, Sm and Eu). The crystal structures are monoclinic ( P2 1 /n ) for Pr 0.5 Sr 0.5 CoO 3 , orthorhombic ( Pnma ) for Nd 0.5 Sr 0.5 CoO 3 and Sm 0.5 Sr 0.5 CoO 3 , and cubic ( Pm 3 m ) for Eu 0.5 Sr 0.5 CoO 3. DC magnetization measurements showed ferromagnetic transitions with Curie temperatures ( T C ) between 233 and 155 K. Metallic behavior was observed in resistivity-temperature curves below 300 K. This is an entirely different property from that of Ln 0.5 Ba 0.5 CoO 3 , which may be qualitatively explained in connection with the crystal structures.

Magnetic and neutron diffraction studies on double perovskites A2LnRuO6(A = Sr, Ba; Ln = Tm, Yb)

Magnetic properties of double perovskites A2LnRuO6 (A = Sr, Ba; Ln = Tm, Yb) have been reported. Powder neutron diffraction measurements have been performed at 10 K and higher temperatures (≥100 K) to determine their crystal and magnetic structures. As a result of the Rietveld analysis of the diffraction profiles, it is found that they are monoclinic with space group P21/n (A = Sr) or cubic with space group Fmm (A = Ba). From the magnetic susceptibility and specific heat measurements, a magnetic transition at 36–48 K is observed in each compound. The neutron diffraction data collected at 10 K show that this magnetic transition is due to a long range antiferromagnetic ordering involving both Ru5+ and Ln3+ ions. Each of the magnetic moments of Ru5+ and Ln3+ orders in a type I arrangement.

A defect-thermodynamic approach to PuO2−x and CeO2−x

Abstract The prominent feature of the thermodynamic data of PuO2−x and CeO2−x is the strong sigmoidal variation of h (0 2 ) and s (O 2 ) with x. In this paper, in search for a possible basic mechanism for the onset of such feature of h (O 2 ) and s O 2 ) in these hypostoichiometric fluorite-type oxides, a defect-thermodynamic formulation is put forward based on the local Pu3+(Cs3+)-Vo interaction model in the mutual first nearest neighbor coordination sphere. Under some simplified assumptions on the Vo arrangement on the oxygen sublattice the number of the local defect—defect configurations is reduced to 16, and using the virtual Gibbs formation energies of these 16 local configurations as the basic parameter the explicit analytical expressions are derived for their concentrations and for the thermodynamic quantities of oxygen ( g (O 2 ) , h (O 2 ) and s (O 2 ) ). The numerical evaluations show that such sigmoidal shape of both h (O 2 ) and s (O 2 ) as observed in these systems is interpreted and reproduced well by the present approach as a result of the coupled local defect-defect configurational changes between Pu(Ce) and O sites. Discussions are also made on the validity and the limitation of the present model.

Thermodynamic analysis on point defects of UO2+x at relatively small deviation from stoichiometry between 600 and 1400°C

Abstract Thermodynamic analysis on point defects of UO2 + x at relatively small deviation from stoichiometry, 0 ≤ x ≤ 0.05 between 600 and 1400°C has been made based on the available literature data on the relative partial molar thermodynamic quantities of oxygen in UO2 + x; g (O 2 ) , h (O 2 ) and s (O 2) as a function of nonstoichiometry, x at these temperatures. In the model, as majority defects, Frenkel disorder between oxygen vacancy and oxygen interstitial cluster with predominant electronic disorder was assumed. Numerical evaluation of the theoretical model shows that the model reproduces almost quantitatively the microcalorimetrically measured h (O 2 )−x curve at 1100°C and also gives the theoretical log P ∗ O 2 −x curves which agree with reported data within their experimental scatters. It was also found that both theoretical −h(O2) and −s(O2) exhibit strong temperature dependences at approximately x ≲ 0.02, contrary to the majority of the published experimental data on these quantities.

Correlation of crystal structures with electric field gradients in the fluorite- and pyrochlore-type compounds in the Gd2O3–ZrO2 system

Abstract Correlation of crystal structure with electric field gradient (EFG) in the fluorite- and pyrochlore-type compounds in the Gd2O3–ZrO2 system GdxZr1−xO2−x/2 with 0.18⩽x⩽0.62 were investigated by 155Gd Mossbauer spectroscopy, powder X-ray diffraction and point-charge model (PCM) calculation. An intermediate ordered pyrochlore phase forms for 0.45⩽x⩽0.55, sandwiched with a disordered fluorite phase for 0.18⩽x

Thermodynamic model of UO2 + x

A thermodynamic model of hyperstoichiometric uranium dioxide, UO2 + x is presented. The model takes four kinds of oxygen interstitial defect clusters into account and in addition treats the remaining excess enthalpy term as the molar volume dependent parts of both the defect formation energies and the phenomenological interaction energy between the defects. The model gives a consistent and satisfactory description of the full set of thermodynamic data of oxygen in UO2 + x, g(O2), h(O2) and s(O2) reported by the present authors.

Magnetic and Electrical Properties of UIr

We succeeded in growing a high-quality single crystal of UIr with the monoclinic PbBi-type structure by the Czochralski pulling method in a tetra-arc furnace. The magnetization measurement indicates that UIr is an Ising-like ferromagnet with the Curie temperature TC= 46 K. The magnetic moment is found.to be oriented along the [101] direction in the (010) plane. The saturated moment is 0.5 μB/U. The magnetic property, reflected in the magnetic susceptibility, electrical resistivity, Hall coefficient and thermoelectric power, is highly anisotropic. We also observed the de Haas-van Alphen (dHvA) oscillation. All the detected branches have large cyclotron effective masses ranging from 9.6 to 32 m0. They are consistent with the electronic specific heat coefficient γ=49 mJ/K2·mol.

Oxygen potential measurements of pyrochlore-type Zr2M2O7+x (M= Pu, Ce) by EMF method

Pyrochlore-type (P-type) zirconia has been widely studied as a disposable form of the high-level radioactive nuclear waste. We have developed the EMF (electromotive force) measurement system using the cubic zirconia oxygen sensor, and investigated the relation between oxygen potential (g(O2)), oxygen composition (x) and temperature (T) for oxygen-excess pyrochlore Zr2Pu2O7+x between 798 K and 1078 K over 0.42<x<0.95. Similar experiments were also carried out for P-type Zr2Ce2O7+x. It was found that g(O2) of P-type Zr2Pu2O7+x. are significantly higher than those of oxygen deficient fluorite PuO2-x at the same values of the oxygen to metal molar ratio (O/M) or the molar ratio of Pu(Ce)3+ ions to the total Pu(Ce)3++Pu(Ce)4+ions (y). g(O2) of P-type Zr2Pu2O7+x. are about -150 kJ/mol lower than those of P-type Zr2Ce2O7+x at the same values of O/M or y, though the shape of g(O2) versus x curves resembles each other over the range of x investigated in this study. These results are consistent with the fact that g(O2) of PuO2-x are about -150 kJ/mol lower than those of CeO2-x at the same values of O/M or y, and their g(O2) curves resemble each other.

Nuclear Magnetic Moment of the First Excited State (I= 2 +) of 238U

The 238 U Mossbauer effect of UO 2 has been investigated in the temperature range from 5.4 K to 280 K. Nuclear Zeeman splitting of the 238 U nucleus in antiferromagnetic UO 2 at 5.4 K is 59.1±3.9 mm s -1 and corresponds to the value of 252.3±0.5 T obtained by the 235 U pulsed NMR technique. The nuclear magnetic moment of the first excited state ( I =2 + ) of 238 U has been determined to be 0.254±0.015µ N . The conversion factor of 4.27±0.28 T/mm s -1 enables us to determine the hyperfine magnetic field at the 238 U nucleus using 238 U Mossbauer spectroscopy. The temperature dependence of the recoil-free fraction of the 238 U Mossbauer effect in UO 2 has been discussed.