Lattice Contributions To The Anisotropic Dielectric Response Of Rare-Earth Langasites

We investigate the phonon spectrum of pure and rare-earth substituted langasites. For electric fields oriented along the c axis, a strong low-frequency phonon is observed, softening at lower temperature in all samples studied. The dielectric contribution and temperature dependence of this mode is the primary source of the large [epsilon(0) similar to 100] static dielectric permittivity observed in these systems. Softening of this mode with increasing mass of the rare-earth substitutes further suggests that langasites are close to a structural instability at low temperatures. The application of a magnetic field to a diluted Ho-langasite sample reveals a shift in the strong absorption band at THz frequencies associated with this soft lattice mode, suggesting spin-lattice effects may also be at play in these systems.