Second-order Raman spectra of a LiNbO₃:Tb crystal

Additional bands have been discovered for the first time in the Raman spectra of terbium doped lithium niobate crystals. The bands have been determined in the high-frequency region and have been attributed to the overtone states of the second order of polar fundamental modes of the crystal lattice. It has been established that Raman spectrum of terbium doped lithium niobate contains bands with frequencies exceeding the overtones of the highest-frequency fundamental polar modes of A(1)(Z) and E(X, Y) symmetry types. The observed features of the second-order Raman spectra have been interpreted as a manifestation of bound states of phonons - biphonons. (C) 2021 Published by Elsevier B.V.