Raman study of LiTaO3-related non-stoichiometric solid solutions isolated inside the ternary systems Li2O–Ta2O5–(M′O)2 with M′=Mn, Co

We have characterized by Raman spectroscopy the disorder and the local modifications of cation environment in the lithium tantalate structure resulting from the incorporation of bivalent cations Co2+ and Mn2+ as potential substitutes for Li and/or Ta ions. Frequency and damping of the E(TO1), E(TO6) and E(TO8) phonon modes of ceramic powders are studied along seven lines in the ternary phase diagrams Li2O–Ta2O5–(M′O)2 with M′=Mn and Co, and compared to those of the pure stoichiometric LiTaO3. Raman spectroscopy is found to be very sensitive to the substitution ions and defects generated in the lattice vibration. Dopants occupy primarily the Li site in the region of lithium oxide excess. The site of Ta becomes progressively implicated in the substitution process when the concentration of dopant increases. In the Li-poor region of the ternary-phase diagram, corresponding to under-stoichiometric compositions, we retain the charge compensation mechanism involving both Li and Ta site according to: 3Li++Ta5+→4M′2+ with M′2+=Mn2+ or Co2+.