The research activity in this field focuses mainly on the study of the structure--properties correlation in oxygen and proton conducting oxides. Materials are prepared by means of solid-state synthesis and sol-gel synthesis and are characterized by means of in-house techniques and advanced scattering methods. In particular, we carry out extensive in-situ and time-resolved studies by means of Neutron and Synchrotron Diffraction under "operating-conditions", i.e., at working temperatures and under various and pertinent gas fluxes. An important part of our research in this filed concerns the investigation of this class of materials by means of x-ray and neutron Pair Distribution Function (PDF) analysis in order to study the role of the local structure on the diffusion mechanisms and conductivity. The PDF investigation is often completed by DFT and MD modelling.
Highlights and Recent Results
Local Structure of Proton-Conducting Lanthanum Tungstate La28−xW4+xO54+δ
Lanthanum tungstate (La28−xW4+xO54+δ) is a good proton conductors and exhibits a complex fluorite-type structure. To gain further understanding of the short-range order in the structure we correlate the optimized configurations obtained by density functional theory (DFT) with the experimental atomic pair distribution function analysis (PDF) of time of-flight neutron and synchrotron X-ray data, collected at room temperature. The local atomic arrangements cannot be described by means of any average symmetric structure. Tungsten forms WO6 octahedra in alternating directions, La1 is mainly 8-fold coordinated in relatively symmetric cubes, and La2 is coordinated with 6 or 7 oxygens in heavily distorted cubes. Both DFT and PDF confirm that the excess tungsten (x) is incorporated in La2 (1/4, 1/4, 1/4) sites in the La27W5O55.5 composition. This additional tungsten can be considered as a donor self dopant in the material and has implications to the conducting properties and the defect structure.
FIGURE 1 FIGURE 2
Figure 1. Calculated relaxed unit cell of La54W10O111 bettere describing the experimental data. Figure 2. PDF refinement by means of DFT calculated local structure of Figure 1. Blue circles: experimental data; red line: calculated PDF; and horizontal green line: residual
Liv-Elisif Kalland, Anna Magrasó, Alessandro Mancini, Cristina Tealdi, and Lorenzo Malavasi, Chem. Mater. (2013) 25 2378. [LINK]
Pair Distribution Function Investigation of LAMOX
We have provided one the first application of the atomic-pair distribution function analysis to the study of an oxygen fast-oxide ion conductor of actual interest in the SOFC community. Our results have shown that a clear and reliable description of the local atom arrangement in LAMOX structure can be only achieved through the application of a local probe such as the PDF. This allowed us to directly determine that the transition from the monoclinic to the cubic phase of LAMOX is a transition from a static to a dynamic distribution of the oxygen defects while preserving the monoclinic local structure. The application of the PDF analysis to the solid state ionics materials can allow a more detailed description of their local structure, particularly in the highly conductive phases, and leads to a better comprehension of the structure-property correlation, which is the starting point for the design of new and optimized functional materials.
Figure: Experimental and calculated PDFs for the alfa and beta phases of LAMOX. Panel A: comparison between the experimental PDFs of LAMOX at 500°C (blue line) and 600°C (red line) and their difference (black line). Panel B: comparison between the calculated PDFs of LAMOX from the monoclinic (blue line) and cubic (red line) models.
L. Malavasi, H. Kim, Simon J. L. Billinge, Th. Proffen, C. Tealdi, G. Flor, J. Am. Chem. Soc. (2007) 129 6903-6907. [ LINK ]