Specific features of phase transitions and the conduction of La 2 Mo 2 O 9 oxide-ion conducting compound doped with vana
- PDF / 271,685 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 98 Downloads / 240 Views
AL PROPERTIES OF CRYSTALS
Specific Features of Phase Transitions and the Conduction of La2Mo2O9 OxideIon Conducting Compound Doped with Vanadium V. I. Voronkova, E. P. Kharitonova, and A. E. Krasil’nikova Faculty of Physics, Moscow State University, Moscow, 119991 Russia email: [email protected] Received May 27, 2009; in final form, June 25, 2009
Abstract—The Xray powder analysis, calorimetric studies, and conductivity measurements of a series of ceramic La2Mo2–xVxOy specimens with different vanadium content are performed with the aim of following the dynamics of phase formation of the lowtemperature α, hightemperature β, and metastable βms phases. At x ≥ 0.06, the cubic phase becomes stable and the monoclinic phase vanishes; therefore, the main α → β transition is suppressed. According to the data of differential thermal analyses, a weak thermal anomaly is observed in the range 450–470°C at x ≥ 0.06. This anomaly is indicative of the βms → β transition due to the conversion of the cubic phase with statically disordered oxygen atoms into the cubic phase with dynamic dis order. The conductivity of the hightemperature β phase obeys the Vogel–Tammann–Fulcher law. DOI: 10.1134/S1063774510020203
INTRODUCTION Oxideion conductors attract the attention of researchers because they can be used as solid electro lytes in fuel cells and other devices. Until recently, four large structural families of oxide conductors were known, namely, fluorites, perovskites, Aurivillius phases, and pyrochlores. In 2000 Lacorre et al. [1] reported a new LAMOX oxideion family based on the doped La2Mo2O9 (LM) compound, whose conductiv ity of approximately 0.06 S/cm at 800°С exceeds that of the ZrO2 oxide stabilized by yttrium or calcium at the same temperature. Since the discovery of these excellent conducting properties, other properties of the members of the LAMOX family, in particular, phase transitions, have also received intensive studies. LM undergoes a phase transition from the low temperature monoclinic α phase (P21) to the high temperature cubic β phase (P213) near 580°С with an abrupt increase of conductivity by almost two orders of magnitude. The structure of the hightemperature cubic phase was studied in [2]. The unit cell parameter а is equal to 7.20 Å at 617°С. One specific feature of the LM structure is the presence of own oxygen vacan cies. Only one of the three oxygen positions, О1, is completely occupied. The remaining two oxygen posi tions, О2 and О3, contain vacancies which are respon sible for the high conductivity of LM. The lowtem perature α phase has a complex structure [3] charac terized by a large number of atoms (48 La, 48 Mo, and 216 O), the 2а × 3а × 4а superstructure, pronounced pseudocubicity, and unit cell parameter а = 7.15 Å at room temperature. Because of the complexity of the
LM structure, the ordering of oxygen in the mono clinic phase, which exists at room temperature, is complicated. For this reason the metastable cubic βms phase is obtained at room temperature upon quenching.
Data Loading...
