Synthesis and structural study of a new NASICON-type solid solution:: Li1-xLax/3Zr2(PO4)3

A new complete solid solution of NASICON-type compounds between LiZr2(PO4)(3) and La1/3Zr2(PO4)(3) was evidenced with the general formula Li1-xLax/3Zr2(PO4)(3) (0 <= x <= 1). These phases were synthesized by a complex polymerizable method and structurally characterized from Rietveld treatment of their X-ray and neutron powder diffraction data. This solid solution results from the substitution mechanism Li+ -> 1/3La(3+) +2/3 leading to an increase of the vacancies number correlated to an increase of the La content. According to this substitution mechanism, the general formula can then be written Li1-xLax/3 Zr-2x/3(2)(PO4)(3) (0 <= x <= 1) in order to underline the correlation between the La content and the vacancies rate. For all the compounds, the structure is clearly related to that of the NASICON family with three crystallographic domains evidenced. For 0 <= x <= 0.5, all the members adopt at high temperature the typical NASICON-type structure (s.g. R3c), while at lower temperature, their structure distorts to a triclinic form (s.g. C 1), as observed for LiZr2(PO4)3 prepared above 1100 degrees C. Moreover, in this domain, the reversible transition is clearly soft and the transition temperature strongly depends of the x value. For 0.6 <= x <= 0.9, the compounds crystallize in a rhombohedral cell (s.g. R3), while for x = 1, the phase La1/3Zr2(PO4)(3) is obtained (s.g. P3, Z = 6, a = 8.7378(2) angstrom, c = 23.2156(7) angstrom). This paper is devoted to the structure analysis of the series Li1-xLax/3Zr2(PO4)(3) (0 <= x <= 1), from X-ray and neutron powder thermo diffraction and transmission electron microscopy (TEM) studies. (c) 2007 Elsevier Inc. All rights reserved.