Determination of Stoichiometry Variations in LiNbO 3 and LiTaO 3 by Raman Powder Spectroscopy

Raman spectroscopy of powders is shown to be a sensitive technique for detecting stoichiometry variations. The method makes use of the broadening of the Raman‐active modes which occurs when the translational symmetry of the lattice is reduced on deviation from simple, fixed cation ratios in the structure. Application of the technique to the LiNbO 3 ( ss ) system shows that the room‐temperature solid‐solution range does not exceed 50±0.5 mol% Nb 2 O 5 and that congruent LiNbO 3 ( ss ) at 51.4 mol% Nb 2 O 5 is thermodynamically unstable below ≅800°C. Similarly, application of the Raman method to the LiTaO 3 ( ss ) system indicates that congruent LiTaO 3 ( ss ) at 51.0 mol% Ta 2 O 5 is unstable at temperatures <1000°C.