Thermodynamic stability of La₂Mo_2−yW_yO₉, La₂Mo_2−yW_yO_8.96+0.02yand La₇Mo_7(2−y)/2W_7y/2O₃₀(y = 0, 0.5 and 1.0)

Abstract

The role of W content on the limit oxygen partial pressure (pO2) for stability of fast oxygen-ion conductors La2Mo2-yWyO9 with y = 0, 0.5 and 1.0 has been studied by means of thermogravimetric analysis (TGA) under controlled atmospheres. At 718 °C, below the pO2 stability limit of La2Mo2-yWyO9, the perovskite related compounds La7Mo7(2-y)/2W7y/2O30 were stabilized even for y = 1.0. At 608 °C, the first stage of reduction of β-La2Mo2-yWyO9 leads to the formation of the crystallized oxygen deficient La2Mo2-yWyO8.6+0.02y phase. X-ray powder diffraction shows that the stabilization of the high temperature β-form through tungsten substitution observed in fully oxidized La2Mo2-yWyO9 samples is preserved upon slight reduction. The n-type conductivity arising from the mixed valence state of molybdenum becomes less and less predominant as the W content increases. Further reduction causes amorphization. At both temperatures, W substitution does not enhance the thermodynamic stability of the La2Mo2-yWyO9 ion conductor under a reducing atmosphere but only slows down the kinetics of reduction.

Influence of Isovalent ‘W’ Substitutions on the Structure and Electrical Properties of La2Mo2O9 Electrolyte for Intermediate-Temperature Solid Oxide Fuel Cells

“…In spite of its high chemical and thermal stability, the oxide ion conductivity is insufficient below 800 • C for use in intermediate-temperature solid oxide fuel cells. Among alternative oxide materials, La 2 Mo 2 O 9 [1][2][3][4] provides high ionic conductivity in comparison with LaGaO 3 -based perovskites, CeO 2 -based fluorite type structure, and apatites from La 10−x Si 6 O 26±δ . Indeed, in the case of La 2 Mo 2 O 9, high oxide ion conductivity is observed after a structural phase transition from room-temperature monoclinic (α) to high-temperature cubic (β) at 580 • C [5].…”

“…In addition to stabilizing the cubic phase, the substitution of Mo +6 by W +6 extends the material's redox stability range with a high ion transport number at an oxygen partial pressure of 10 −15 atm [5,18,19]. Fortunately, W-substituted La 2 Mo 2 O 9 could be used as a solid oxide fuel cell (SOFC) electrolyte in a reducing atmosphere and at intermediate temperatures of operation [2]. The effect of W substitution shows a cubic phase with an abrupt decrease in cell parameter above W:Mo = 1 that has been reported and attributed to a possible change in the coordination number of the hexavalent ions [20].…”

“…Interestingly, the presence of W stabilizes the reduction of Mo +6 [23,24]. Apart from oxide ion conductivity, n-type electronic conductivity arises due to the mixed-valence states of Mo, which is reduced as the W content increases [2].…”

On the local order of amorphous La2Mo2O6.7

“…26) and at 718°C. 23,24 In these studies this crystalline reduced molybdate appears as an intermediate state prior amorphization. Although at 608°C, as shown in Fig.…”

“…La 7 Mo 7 O 30 was prepared by partial reduction of La 2 Mo 2 O 9 powders in the Thermogravimetric Analysis (TGA) apparatus at 718°C in an Ar-H 2 -H 2 O mixture as described elsewhere. 23,24 The amorphous sample studied by EPR was also prepared in a TGA balance at 700°C for 9 h in Ar-(10%)H 2 flowing at 4.8 L h −1 , leading to the composition La 2 Mo 2 O 6.7 .…”