Thermodynamic stability, structural and electrical characterization of mixed ionic and electronic conductor La2Mo2O8.96

Abstract

Thermogravimetric analysis (TGA) technique in controlled oxygen partial pressure (pO(2)) atmospheres has been used to obtain equilibrium oxygen content data as a function of pO(2) on the La(2)Mo(2)O(9-δ) system resulting from the partial reduction of fast oxide-ion conductor La(2)Mo(2)O(9) (LM). Thermodynamic conditions for stabilization of crystalline La(7)Mo(7)O(30) and amorphous La(2)Mo(2)O(7-y) at 718 °C have been determined and discussed. At 608 °C, the compound reported for the first time La(2)Mo(2)O(8.96) (LM896) has been found. The crystalline form and transition temperature in LM896 have been identified by X-ray diffraction at room temperature (XRD) and at controlled temperature. Conductivity curves obtained by electrochemical impedance spectroscopy (EIS) as a function of temperature for both LM and LM896 have been compared. The results indicate that LM896 is a mixed ionic and electronic conductor (MIEC).

Cationic Intermixing and Reactivity at the La2Mo2O9/La0.8Sr0.2MnO3−δ Solid Oxide Fuel Cell Electrolyte–Cathode Interface

“…The resulting lanthanum molybdate (LAMOX) family of materials derived through these chemical substitutions of the parent compound La 2 Mo 2 O 9 were considered, at their discovery, as potential electrolyte materials for SOFC devices because of their higher ionic conductivity at intermediate temperature than that of conventional electrolyte yttrium‐stabilised zirconia (YSZ) . The reducibility of molybdates in a low oxygen pressure make LAMOX compounds inappropriate as the electrolyte in conventional double‐chamber SOFC devices unless an appropriate buffer layer is used at the anodic side to prevent reduction by hydrogen. Under intermediate or high oxygen partial pressures, however, as in a single chamber or in the cathodic atmosphere of the double‐chamber devices, LAMOX electrolytes are stable and could potentially be used in contact with conventional electrode materials provided that no reaction takes place between the phases.…”

Thermodynamic stability of La2Mo2−yWyO9, La2Mo2−yWyO8.96+0.02yand La7Mo7(2−y)/2W7y/2O30(y = 0, 0.5 and 1.0)

“…In order to collect XRD patterns on La 2 Mo 1.5 W 0.5 O 8.97 and La 2 MoWO 8.98 compounds, approximately 500 mg of La 2 Mo 1.5 W 0.5 O 9 and La 2 MoWO 9 raw powders were annealed inside the thermobalance at 608°C under atmospheres with a set pO 2 of 10 −24 atm. The TGA signal reached the steady state at oxygen contents of 8.97 and 8.98 after 280 and 320 min, 22 If one assumes that only hexavalent molybdenum is reduced, the ionic charge balance for the chemical formulas Fig. 8(a) shows the cell parameters of cubic samples of La 2 Mo 2−y W y O 9 (black line) and La 2 Mo 2−y -W y O 8.96+0.02y (gray line) with y = 0, 0.5 and 1.0 at 608°C as a function of y.…”

Reduction Kinetics of La2Mo2O9 and Phase Evolution during Reduction and Reoxidation

“…potentially interesting as anode material in conventional doublechamber SOFC. 10 Further reduction produces an amorphous compound hereafter called La 2 Mo 2 O 7−y . The amorphous reduced phase was successfully tested as a sulfur-tolerant anode material for SOFC.…”

“…8 However, above 900°C, the amorphous phase undergoes a slight crystallization, and at 1000°C under a partial oxygen pressure (pO 2 ) below 10 −10 Pa, the reduction leads to mixtures of crystallized molybdates. The same authors investigated the thermodynamics of the reduction process by studying the phase stability at different temperatures in a wide range of partial oxygen pressures 8,10 (see Figure 1). Thus, at 718°C , amorphous La 2 Mo 2 O 7−y phase appears below pO 2 = 10 −17 Pa. At 608°C, the slightly reduced and crystallized MIEC phase La 2 Mo 2 O 8.96 is formed in the range 10 −20 < pO 2 < 10 −18 Pa, whereas below 10 −20 Pa, the amorphous La 2 Mo 2 O 7−y form is obtained.…”