Decomposition behavior of Ln 2 Mn 4/3 W 2/3 O 7 (Ln = Eu and Er) in reducing atmosphere and their thermodynamic properti
- PDF / 196,107 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 78 Downloads / 201 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Decomposition behavior of Ln2 Mn4/3 W2/3 O7 (Ln Eu and Er) in reducing atmosphere and their thermodynamic properties Taroh Atsumia) and Naoki Kamegashira Department of Materials Science, Toyohashi University of Technology, Tenpaku-cho, Toyohashi 441, Japan. (Received 1 August 1996; accepted 18 February 1997)
Decomposition behavior of Ln2 Mn4/3 W2/3 O7 (Ln Eu and Er) in reducing atmosphere at 1273 K was determined. These compounds decomposed to LnMnO3 and Ln2 MnWO7 at PO2 1028.46 atm for Ln Eu and 1027.46 atmosphere for Ln Er, respectively. Decomposition product Eu2 MnWO7 decomposed to Eu2 MnWO6 at PO2 10215.67 atm, while Er2 MnWO7 decomposed to Er2 O3 , MnO, and W less than 10217 atm. The standard Gibbs free energy changes of decomposition for Eu2 Mn4/3 W2/3 O7 , Eu2 MnWO7 , and Er2 Mn4/3 W2/3 O7 at 1273 K were determined to be 34.4 kJ ? mol21 , 191.9 kJ ? mol21 , and 44.8 kJ ? mol21 , respectively. The standard Gibbs free energy changes of formation of Er2 Mn4/3 W2/3 O7 and Er2 MnWO7 were also determined to be 22429.5 and 22463.2 kJ ? mol21 at 1273 K, respectively.
I. INTRODUCTION
II. EXPERIMENTAL
Complex oxides Ln2 M22x M0x O7 (Ln rare earth; M, M0 transition metals with different oxidation states) having the pyrochlore or the pyrochlore-related structure have been extensively studied in the literature. Most of these compounds have the pyrochlore structure with a cubic cell1–7 and some of the compounds have the distorted pyrochlore structure with a rhombohedral (or hexagonal) cell.8–10 In the Ln–Mn –W–O system, Ln2 Mn4/3 W2/3 O7 (Ln Sm–Lu and Y) were first synthesized by Bazuev et al.11 They reported that the powder x-ray diffraction patterns of those compounds could be indexed by hexagonal symmetry, and also reported the magnetic susceptibility of Y2 Mn4/3 W2/3 O7 . However, there is no report for their properties other than the magnetic one of Y2 Mn4/3 W2/3 O7 , and there is no report for the synthesis of the compounds except the cation ratio of MnyW 2. The object of this study is to investigate the thermodynamic properties of Ln2 Mn4/3 W2/3 O7 (Ln Eu and Er) compounds. The decomposition behaviors of those compounds at 1273 K were determined by thermogravimetric analysis and powder x-ray diffractometry. The temperature dependences of the equilibrium oxygen partial pressures of decomposition for those compounds were determined by the emf technique using stabilizedzirconia electrolytes, and the standard Gibbs free energy changes of decomposition for those compounds were calculated from their equilibrium oxygen partial pressures.
Complex oxides, Ln2 Mn4/3 W2/3 O7 , were synthesized by the solid-state reaction from Ln2 O3 , Mn2 O3 , and WO3 (Rare Metallic Co., Ltd.; 99.9% purity) as starting oxides. Mixtures of the starting oxides with the required cation ratios were pressed into pellets and heated in air at 1623 K for 48 h. No other compounds except the proposed ones were suggested by powder x-ray diffraction (XRD). XRD measurements were carri
Data Loading...
