Thermal stability of alkali and alkaline-earth substituted LAMOX oxide-ion conductors

Abstract

The high temperature demixing/recombination phenomenon previously observed in Ca-substituted La(2)Mo(2)O(9) oxide ion conductors [A. Selmi et al., Solid State Ionics, 2006, 177, 3051; Eur. J. Inorg. Chem., 2008, 1813] has been visualised using scanning electron microscopy and EDX analysis. The demixed state appears as CaMoO(4) straight solid streams erupted from pores within LAMOX grains. The thermal stability study is extended to other alkali and alkaline-earth substituted LAMOX compounds, all of which are shown, in temperature-controlled X-ray diffractograms, to present similar demixing/recombination processes. The most spectacular effect is observed in La(1.88)K(0.12)Mo(0.6)W(1.4)O(8.88) where demixing takes the form of a total decomposition, before full recombination at a higher temperature. Such a phenomenon is interpreted as originating from temperature-dependent solid solution limits with higher substitutional ranges at higher temperatures. It results in the metastabilisation of pure phases by quenching (or rapid cooling), whereas the stable state is demixed, as shown on slowly cooled samples.

“Free” Volume Expansion and Formation Enthalpy of Defects as Key Parameters Tuning the Oxide Ionic Conductivity in Derivatives of β-La2Mo2O9

“…3.58 (8) 3.67 (6) 3.78 73.93 73.78 73.90 74.06 74.11 74.47 (9) 4.65(9) 0.3137 (7) 0.3150 (7) 0.3171 70.3179 70.3188 (6) 0.3193 (6) 0.3171 (7) 0.3169 (8) 0.3185(8) could have changed the overall oxygen vacancy (intrinsic+extrinsic) distribution over the three crystallographic oxygen sites of the structure. Therefore, a second structural model (model 2) was tested, in which a slight 5% depletion of the O1 site to the benefit of O2 or O3 sites was introduced.…”

“…Whatever the substitute considered, the conductivity decreases nonlinearly as the substitution rate increases. Our previous investigations 7 showed that the β-form of the highest K + -and Ba 2+ -substituted LAMOX exhibit an exsolution of β-KLaMo 2 O 8 and BaMoO 4 scheelite-type phases above a temperature decreasing as the substitute content increases: La 1 is the only composition combining both the "highest" mean La-site cation radius (⟨r⟩ = 1.289 Å) and a stability of the β-form in a temperature range (up to 900°C) as large as the range explored in our previous structural analysis performed on β-La 1.7 Bi 0.3 Mo 2 O 9 derivative, thus allowing a direct comparison. The objective of the current structural analysis on β-La 1.85 Ba 0.…”

Sinterability, reducibility, and electrical conductivity of fast oxide-ion conductors La1.8R0.2MoWO9 (R=Pr, Nd, Gd and Y)

“…Unfortunately, La 2 Mo 2 O 9 presents main drawbacks of poor ionic conductivity at low temperature and degradation in reducing environments. In the last decade, researchers have examined various types of cationic substitutions on La or Mo sites to improve the performance of LAMOX electrolytes [6][7][8][9][10][11][12][13][14]. Several types of substitutions can stabilize the highly conductive β-phase at room temperature, but only substitution of W on the Mo site can effectively improve the phase stability of the electrolyte at low oxygen partial pressures [15][16][17][18][19][20][21][22].…”

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

“…La 2 Mo 2 O 9 undergoes a reversible phase transition at 580 °C from a monoclinic α phase at room temperature to a cubic β phase at high temperature, which causes a 0.53 % abrupt cell volume increase. However, the high temperature cubic β form of La 2 Mo 2 O 9 , which has a higher oxide ion conductivity (6×10 −2 S cm −1 at 800 °C), can be stabilised by various substitutions either on the La site, the Mo site or both . 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) .…”