Masayuki Takashima

Reaction between rare earth oxides and elemental fluorine. II. Kinetics of the fluorination of cerium dioxide

Abstract The reaction between cerium dioxide and fluorine gas has been studied by means of thermogravimetric, X-ray and magnetic measurements. CeO 2 reacts with F 2 gas from 130 °C and converts into CeF 3 with a characteristic weight decrease above 270 °C. The reaction rate was found to be determined by fluorine gas diffusion described by a modified Jander equation, and the activation energy is estimated to be 80 to 90 kJ/mol. Fluorine is incorporated into the crystal lattice of CeO 2 without change of structure or oxidation state of Ce(IV) below 270 °C. To convert into CeF 3 , the valency Ce(IV) drastically changes to Ce(III) with evolution of oxygen at 270 to 290 °C.

Electric conductivity and crystal structure of neodymium-containing binary rare-earth metal oxide fluorides

Abstract Nd 2 Ln 2 O 3 F 6 compounds (Ln; Y-Lu) are prepared by firing the mixture of 1-mol Nd 2 O 3 and 2-mol LnF 3 at 1373 K for 2 h in argon. The crystal system of Nd 2 Ln 2 O 3 F 6 can be assigned into pseudo-tetragonal (monoclinic). Nd 2 Ln 2 O 3 F 6 compounds, except Ln: Nd, Yb and Lu, gave a high oxide ion conductivity of more than 1.0 S m −1 at 923 k. Among them, Nd 2 Eu 2 O 3 F 6 gave the highest conductivity of 5.0 S m −1 under an oxygen partial pressure of 0.4 Pa. The oxide ion transport number for Nd 2 Ln 2 O 3 F 6 (Ln: Y, Ce, Sm, Gd) decreases as the temperature falls. On the other hand, Nd 2 Eu 2 O 3 F 6 holds a number of more than 0.9 even at 723 K. As the result of X-ray diffraction Rietveld analysis, the crystal structure of Nd 2 Eu 2 O 3 F 6 was found to be expressed by the monoclinic based on the defect double layered fluorite structure (monoclinic-D″LF). The lattice constants of the monoclinic D″LF structure were calculated to be a 0 = 0.3959 nm, b 0 = 1.132 nm, c 0 = 0.5615 nm, β = 134.69 ° and the lattice volume was 0.180 nm 3 . The molecular number, Z, was calculated to be 1 from the density data (measured; 6.98 kg m −3 , calculated; 6.96 kg m −3 ). One molecule of Nd 2 Eu 2 O 3 F 6 is in this unit cell. The lattice plane at y = 0.25 crossing vertically with b -axis in the monoclinic-D″LF structure consists of oxide ions and vacant sites formed by defecting of fluoride ion. The oxide ion conduction in Nd 2 Eu 2 O 3 F 6 was supposed to be arising from migration of oxide ion through the defects in the y = 0.25 plane, because the space of the vacant site of fluoride ion is similar to the size of oxide ion.

Effect of LiF addition at preparation of LiCoO2 on its properties as an active material of lithium secondary battery

Abstract By adding LiF at the preparation of LiCoO 2 From Co(OH) 2 and Li 2 CO 3 at 900 °C, the crystallinity, particle shape and particle size of LiCoO 2 were varied along with the amount of added LiF. The charge/discharge capacities of LiCoO 2 prepared with 1, 3, 5 wt.% of LiF were larger than that of LiCoO 2 without LiF addition. The coulombic efficiencies of charge/discharge process for LiCoO 2 prepared with 1,3,5, 10 wt.% of LiF were also higher than that of LiCoO 2 without LiF addition. The results indicate that the electrochemical properties of LiCoO 2 are improved by LiF addition in the preparation process.

Preparation and electrical conductivity of neodymium–europium oxide fluorides

Neodymium–europium oxide fluoride, Nd-EuOxFy, was prepared by heating appropriate mixtures of Nd2O3 and EuF3 at temperatures ranging from 1273 to 1673 K for 2 h in argon. Two monophases, rhombohedral and tetragonal were identified. The rhombohedral phase was obtained at an EuF3 composition of 50–55 mol%, and the tetragonal one was obtained in the composition range between 65 and 75 mol%. Nd2Eu2O3F6 gave a high electrical conductivity of 5 × 10–2 S cm–1 at 923 K under an oxygen partial pressure of 0.4 Pa. The charge carrier was determined to be mainly the oxide ion. The transport numbers of the oxide ion and the electron were measured to be ca. 0.9 and < 0.05, respectively, at temperatures between 723 and 923 K. Resulting from XPS measurements, it was found that: (1) the valence of Nd in Nd2Eu2O3F6 was higher than in Nd2O3 and that of Eu was lower than in EuF3, i.e. the variance of the valency state of Nd (+3-+ 4) and Eu(+2-+ 3) affects the oxide ion conducting structure; (2) the covalency in the bonds between metal ions and oxide ions in Nd2Eu2O3F6 was weaker than that in Eu2O3 or Nd2O3; (3) the bond between the fluoride ion and the rare-earth-metal ion in Nd2Ln2O3F6 was stronger than in the individual rare-earth-metal fluorides. These might result in increased oxide ion mobility.

Pyrohydrolysis of rare-earth trifluorides in moist air

Abstract The kinetics of pyrohydrolysis of rare-earth trifluorides, LnF3 (Ln = Y and La–Lu) have been investigated by means of thermogravimetry. There were two definite breaks on the TG curves that correspond to the conversion of LnF3 into LnOF and from LnOF into Ln2O3, except for CeF3. The beginning temperature of pyrohydrolysis decreased gradually and the reaction rate increased with increasing the acidity of Ln3+ due to decrease of the ionic radius. The rate equation of conversion from LnF3 to LnOF was expressed by [1−(1−x)1/3]=kt, where x is defined as the reaction ratio, with x=1 when LnF3 is completely transformed into LnOF. This rate equation is derived as the tarnishing reaction (LnF3 particle is covered with LnOF layer) in which the rate-determining step is the surface reaction. It was assumed that cracks as gas paths were formed in the LnOF layer produced from the surface of sample particles, because of the smaller molar volume of LnOF compared with LnF3. On the reaction from LnF3 to LnOF, Ln4O3F6 was detected in the XRD pattern. From the results of the kinetic study, the activation energy of pyrohydrolysis of LnF3 decreased with increasing Ln3+ atomic number by the same reason as the decrease of starting temperature of pyrohydrolysis.

Electrochemical Properties of Limn 2 O 4 Coated with Nano-Thickness Carbonand Fluorine

Surface modification of LiMn 2 O 4 by coating with nano-thickness carbon and fluorine were carried out by an arc discharge with carbon electrode (nC-LiMn 2 O 4 , n=number of times of arc discharge for 0.1 s) and a fluorination with NF 3 at 1.3 kPa for 1 hour at 100°C (F-LiMn 2 O 4 ). Cyclic voltammograms revealed that the reversibility of the electrochemical process was improved by coating with nano-thickness carbon. The discharge capacities of nC-LiMn 2 O 4 and F-LiMn 2 O 4 were improved by 5∼10%. 120C-LiMn 2 O 4 exhibits the largest discharge capacity.

Synthesis and oxide ion conductivity of lanthanum–europium oxide fluoride, La2Eu2O3F6

Abstract La 2 Eu 2 O 3 F 6 was prepared by solid–solid reaction between 1-mol La 2 O 3 and 2-mol EuF 3 at 1473 K. The crystal structure of La 2 Eu 2 O 3 F 6 was analyzed to be the monoclinic structure ( a 0 =0.403 nm, b 0 =1.14 nm, c 0 =0.572 nm, β =135.2°) derived from the fluorite structure. The ionic arrangement was suggested to be somewhat disordered in the La 2 Eu 2 O 3 F 6 in contrast to that in the Nd 2 Eu 2 O 3 F 6 with highly ordered ionic arrangement. It was assumed that the disorder in La 2 Eu 2 O 3 F 6 resulted in the lower oxide conductivity, La 2 Eu 2 O 3 F 6 ( σ =0.8 S m −1 , τ O 2− =0.7 at 773 K), than that of Nd 2 Eu 2 O 3 F 6 ( σ =2.0 S m −1 , τ O 2− =0.9 at 773 K). La 2 Eu 2 O 3 F 6 was stable up to ca. 970 K in air, and converted into LaEuO 3 (monoclinic) at 1623 K through the metastable state of La 2 EuO 2 F 2 (rhombohedral) at ca. 1190 K by the pyrohydrolysis. The electrical conductivity declined due to pyrohydrolysis; LaEuO 2 F 2 , σ ≈2.0×10 −2 S m −1 , LaEuO 3 , σ ≈7.0×10 −5 S m −1 at 873 K.

Oxide ion conductive structure and chemical stability of Nd2Gd2O3F6

Abstract Nd2Gd2O3F6 was prepared by solid-solid reaction between 1 mol Nd2O3 and 2 mol GdF3 at 1100°C for 3 h in an argon flow. X-ray powder diffraction-Rietveld analysis revealed that the crystal system of Nd2Gd2O3F6 was assigned to the monoclinic structure with the cell parameters; a0=0.3973 nm, b0=1.123 nm, c0=0.5595 nm and β=134.75°. The ionic arrangement was suggested to be slightly disordered in the Nd2Gd2O3F6 crystal lattice in contrast to in the Nd2Eu2O3F6 with the high ordered ionic arrangement. It was assumed that the slightly disordered ionic arrangement in Nd2Gd2O3F6 resulted in the smaller electrical conductivity of Nd2Gd2O3F6 (0.2 S m−1 at 600°C) than that of Nd2Eu2O3F6 (5.4 S m−1 at 600°C). Nd2Gd2O3F6 was confirmed to be stable up to ca. 650°C in air, and converted into NdGdO3 (monoclinic) at ca. 1350°C through the metastable state of NdGdO2F2 (rhombohedral) at ca. 850°C by the pyro-hydrolysis. The rate of pyro-hydrolysis was analyzed as the tarnishing reaction represented by the surface reaction control. The electrical conductivity largely declined with decreasing fluorine content due to the pyro-hydrolysis. This cause was considered due to disappearance of the vacant sites for oxide ion migration according to change in the crystal structure.

The reaction between chlorine trifluoride and oxygen-deficient YBa2Cu3O7−δ

Abstract On annealing in chlorine trifluoride gas (ClF 3 ), the Iow- T c phase YBa 2 Cu 3 O x ( x ~ 6.6, T c ~ 60 K) is readily converted into a high- T c phase exhibiting 90 K superconductivity. YBa 2 Cu 3 O 6.60 ( T onset = 63 K) began to react with ClF 3 at 423 K. The initial reaction rate was proportional to the ClF 3 pressure and the activation energy was calculated to be 17.1 kJ mol −1 . The reaction process was represented by the rate equation, [1 − (1 − x )1/2] =Kt, derived from Janders model for a gas/solid reaction, the reaction proceeding preferentially in a direction parallel to the CuO 2 network in the YBa 2 Cu 3 O x structure. Optimum annealing conditions required to obtain the high- T c single-phase structure were ClF 3 pressure = 2.7 kPa, temperature = 513 K, reaction time = 60 min. Under these conditions the crystal structure changed from pseudo-orthorhombic YBa 2 Cu 3 O 6.60 to the orthorhombic single phase of chemical composition YBa 2 Cu 3 O 6.17 F 0.78 Cl 0.52 . The T onset value of this product was 94.5 K.

Improvement of superconductivity of oxygen deficient YBa2Cu3O7−δ by ClF3 treatment

Abstract Annealing in ClF3 gas is first found to be effective for the incorporation of fluorine and chlorine into a partially reduced YBa2Cu3O7−δ (YBCOx) of which superconducting transition temperature (Tc is less than 90 K and for the remarkable increase in Tc. ClF3 reacts effectively with YBCOx at a temperature ranging from 250 to 300°C. As a result of the ClF3 treatment of YBCO6.61, the pseudo-orthorhombic structure changed into the single phase of orthorhombic-I with the chemical composition of YBa2Cu3O6.17F0.78Cl0.52. Tc (onset) was enhanced from 62 to 94 K, while the diamagnetic susceptibility is still half of the YBCO6.88.