Vibrational spectra and factor-group analysis of double arsenates of zirconium and alkali metal MZr2(AsO4)3 (MÂ =Â Li-Cs)

Complex phosphates and arsenates of alkali metal and zirconium with NASICON and Sc(WO4)3 structures are fast ion conductors. In this context, double arsenates MZr2(AsO4)3, where M = Li, Na, K, Rb, and Cs, have been synthesised as powders through a precipitation method and investigated by Raman and infrared spectroscopy in combination with factor-group analysis to assign the bands. The stretching and bending vibrations of the AsO43− units and external modes, as well as the Zr4+ and M+ (Li-Cs) translations, have been determined for these compounds, which crystallised in the R3¯c (D3d6) and P1121/n (C2h5) space groups (LiZr2(AsO4)3). The correlations between the spectra and the nature of the M+ cation are revealed. Some external modes were identified by studying mass effects (Zr-Hf, Na-Cs). The bands for the symmetrical bending vibrations of the AsO43− units and some of the external modes shift towards higher wavenumbers with the increasing size of the alkaline cations due to the predominant direction of these vibrations and translations along the a axis; the M+ cation size increases as much as parameter a decreases. The differences observed in the vibrational spectra of the arsenates containing alkaline elements and zirconium with different space groups are caused by the reduced symmetry.