Structural studies on W⁶⁺ and Nd³⁺ substituted La₂Mo₂O₉ materials

Abstract

The structure of a series of new ionic conductors based in lanthanum molybdate (La₂Mo₂O₉) has been investigated using transmission electron microscopy (TEM), high-resolution X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The superstructure 2a_c×3a_c×4a_c of the low temperature α-polymorph relative to the β-polymorph was confirmed by HRTEM imaging and electron diffraction. Furthermore, the effects of partial cation substitution in the La_2−xNd_xMo₂O₉ and La₂Mo_2−yW_yO₉ series have been also evaluated in the search of new clues to understand the structure and stabilisation of the high temperature and better conductor β-polymorph. The thermal analysis studies show that Nd-substitution does not stabilise completely the β-polymorph at room temperature, although no superstructure ordering was observed by both XRD and HRTEM. On the other hand, W-substitution stabilises the cubic β-polymorph for $y>0.25$, although, electron diffraction indicates a slight distortion from the cubic symmetry for low W-content. This distortion disappears as the W content increases and the Rietveld refinements gradually render better results.

Introduction

Oxide ion conductors are materials with a number of important applications such as oxygen sensors, dense ceramics for oxygen separation and fuel cell components. Fuel cells are electrochemical devices that convert directly chemical energy into electricity and promise important advantages in comparison to the current technologies based on the combustion of fossil fuels, because they are environmentally friendly and render higher efficiencies [1], [2], [3], [4].

Fuel cells require materials exhibiting high ion oxide mobility, which in turn implies very specific structural features only met for a restricted number of solids. The most widely used solid electrolyte nowadays is yttria-stabilised zirconia (YSZ), which is a very good oxide ion conductor at high temperature (typically 0.1 S cm⁻¹ at 1273 K) and stable for prolonged operation times [5]. However, the high operation temperatures (1000–1273 K) limit the choice of stable materials for the other SOFC components, hence increasing the costs of production.

Over the last two decades, other ionic conductors such as rare-earth doped ceria and LaGaO₃-based phases have been studied and developed based upon fluorite or perovskite structures, respectively, showing higher conductivity than YSZ at lower temperatures [6], [7], [8], [9], [10]. Unfortunately, they present other drawbacks such as high cost or residual electronic conductivity that must be overcome. Therefore, an electrolyte with very high conductivity at relatively low temperatures is highly demanded to produce efficient fuel cells operating at intermediate temperatures (IT-SOFCs).

Since the discovery of very high oxide-ion conductivity in La₂Mo₂O₉ by Lacorre et al., much attention has been directed towards molybdate-based materials due to their potential applications as electrolyte [11]. This compound was first synthesised by Fournier et al. in 1970 [12], although the reports on the structural characterisation and the high ionic conductivity are more recent [13], [14]. The very high oxide ion conductivity in La₂Mo₂O₉ has been explained using the lone pair substitution (LPS) concept, which considers that the partial substitution of lone-pair cations by other cations without lone-pair electrons may generate intrinsic-vacancies in the anionic sublattice [15].

La₂Mo₂O₉ undergoes a structural phase transition from the slightly monoclinic α-polymorph to the much better conductor cubic β-polymorph (isostructural to β-SnWO₄) at 833 K. This transition is similar to those reported in other fast ionic conductors such as Bi₂O₃ and Bi₂V₄O₁₁ and is usually associated to a rearrangement in the oxygen sublattice [16], [17].

The applicability of La₂Mo₂O₉ is limited by the $\alpha \leftrightharpoons \beta$ phase transition and by its stability under reducing conditions. The partial substitution of either La³⁺ or Mo⁶⁺ by cations such as Bi³⁺, Ca²⁺, Sr²⁺, Y³⁺ and W⁶⁺ seems to stabilise the cubic polymorph at room temperature, whilst Nd³⁺ has been reported to adopt the monoclinic superstructure [14], [18], [19], [20], [21], [22]. Recent reports suggest that La₂Mo_2−xW_xO₉ phases can be used as SOFC electrolytes, although only at low temperature and under moderate reducing conditions. The introduction of tungsten enhances the stability range and prevents the presence of residual electronic conductivity [23], [24], [25], [26].

In the present work, La₂Mo₂O₉, (La,Nd)₂Mo₂O₉ and La₂(Mo,W)₂O₉ phases have been studied by high resolution X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermal analysis to give a further insight of these structures.