Structural and spectroscopic characterization of double perovskites La$_{2}$Co$_{4/3}$M$_{2/3}$O$_{6}$ (M= V, Nb or Ta) and La$_{4/3}$Bi$_{2/3}$Co$_{4/3}$Ta$_{2/3}$O$_{6 }$

We report the synthesis of the title compounds and the refinement of their structures using Rietveld analysis of powder XRD data. All belong to the monoclinic space group P2$_{1}$/n. While the crystallographic formula can be written as La$_{2}$(Co)$_{2a}$(Co$_{1/3}$M$_{2/3})_{2b}$O$_{6}$ for M= Nb or Ta, indicating the highest possible order for the cations, we find La$_{2}$(Co$_{2/3}$V$_{1/3})_{2a}$(Co$_{2/3}$V$_{1/3})_{2b}$O$_{6 }$for V replacement indicating the highest disorder configuration. The cell volume for La$_{2}$Co$_{4/3}$M$_{2/3}$O$_{6}$ decreases as the ionic radius of the M$^{5+}$ cation decreases. These compounds were previously shown to be ferrimagnetic with Curie temperatures in the range 60-70 K. Since the incorporation of cations with the sp hybridized lone pair can potentially induce ferroelectric properties, we also synthesized La$_{4/3}$Bi$_{2/3}$Co$_{4/3}$Ta$_{2/3}$O$_{6}$ where we found a partial cations disorder. Crystal and magnetic structures refined with powder neutrons diffraction measurements as well as the infrared spectroscopic characterization, now in progress, will be presented.