Abstract
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 $[\ensuremath{\varepsilon}(0)\ensuremath{\sim}100]$ 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.
Highlights
The presence of orthogonal threefold and twofold rotation axes in the P321 space group sets the stage for a number of intriguing functional properties expressed across a range of real material compounds
We found that the lattice dynamics in this frequency range do not alter much at low temperatures and that this procedure does not significantly affect our observations in the far-infrared range below 650 cm−1
Pure and rare-earth substituted crystals of the langasite (La3Ga5SiO14) family were investigated by Fourier transform infrared (FTIR) reflection and THz magneto-optical spectroscopy, and compared to capacitive results approaching the static response
Summary
The presence of orthogonal threefold and twofold rotation axes in the P321 space group sets the stage for a number of intriguing functional properties expressed across a range of real material compounds. A prime example is the diversity of phenomena exhibited by the langasite family, with parent chemical formula La3Ga5SiO14 (LGS) and P321 space-group symmetry [1]. Langasites are constructed by two alternating layers, each with threefold symmetry, stacked along the c direction. The first layer consists of La3+ decahedra (3e sites with C2 symmetry) and Ga3+ octahedra (1a sites with D3 symmetry). The second layer consists of Ga3+ (3 f sites with C2 symmetry) and Si4+/Ga3+ (2d sites with C3 symmetry) tetrahedra. The oxygen ions are coordinated at the vertices of each polyhedra
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