Sagrario M. Montemayor

Lithium insertion in two tetragonal tungsten bronze type phases, M8W9O47 (M=Nb and Ta)

A study of lithium insertion in two tetragonal tungsten bronze (TTB) type phases of general formula M 8 W 9 O 47 (M=Nb and Ta), is presented. The electrochemical insertion of up to 20 lithium atoms per formula unit in Nb 8 W 9 O 47 (Li/ΣM=1.2) proceeds through a reversible reaction with several single phase and one two-phase domains, while in Ta8W9O47 the reversibility of lithium insertion is limited to 15 atoms (Li/Sigma;M=0.9). Structural changes on Nb 8 W 9 O 47 as a function of the number of lithium atoms inserted have been studied by X-ray powder diffraction.

A Critical Review of Existing Criteria for the Prediction of Pyrochlore Formation and Stability

Depending on intrinsic (e.g., radius ratio rule rLn/ rZr) and extrinsic factors (e.g., processing conditions), pyrochlore-type Ln2Zr2O7 oxides achieve variable degrees of structural disorder. We report on a systematic study of the structural and microstructural characteristics of the Gd2- xLn xZr2O7 system, exploring the effect of replacing Gd with a wide range of homovalent lanthanide ions (Ln = Nd, Sm, Dy, Ho, Y, and Er; x = 0.20 and 0.80). All compositions were prepared via a mechanochemical reaction between the corresponding oxides and characterized by X-ray diffraction (standard and synchrotron sources) using the Rietveld method, as well as by Raman spectroscopy. Irrespective of chemical composition, this study reveals that all compositions exhibit a fluorite-like structure. Furthermore, by firing each sample at 800 and 1400 °C, we are able to analyze the transition to pyrochlore-like structures, featuring different degrees of disorder, in all but Gd1.20Y0.80Zr2O7, which retains the fluorite structure even after heating. The structural data are used to assess the existing criteria for predicting the formation and stability of the pyrochlore structure; according to this analysis, the simple radius ratio rule ( rLn/ rZr), provides a useful and sufficiently robust criterion. Because the pyrochlore structure has a strong tendency to disorder, it is not possible to define an empirical index similar to the Goldschmidt tolerance factor for perovskite.