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.

Automated summary

The study revealed a low-frequency phonon in langasites that softens at lower temperatures, with its dielectric contribution and temperature dependence being the main factors contributing to the high static dielectric permittivity observed in these systems. Increasing mass of rare-earth substitutes causes further softening of this phonon mode, suggesting langasites may be close to a structural instability at low temperatures. Additionally, applying a magnetic field to a diluted Ho-langasite sample resulted in a shift in the strong absorption band at THz frequencies associated with this soft lattice mode, indicating possible spin-lattice effects in these systems.