Lattice dynamics and structure of the new langasites Ln3CrGe3Be2O14 (Ln = La, Pr, Nd): Vibrational spectra and ab initio calculations

We present experimental (by means of the infrared absorption and Raman scattering measurements at room temperature) and first-principles studies of the lattice dynamics of the recently discovered new compounds La3CrGe3Be2O14, Pr3CrGe3Be2O14, and Nd3CrGe3Be2O14 belonging to the langasite series and constituting a new class of low-dimensional antiferromagnets. The observed vibrational modes of all the three compounds are consistent with the results of the group-theoretical analysis for the langasite structure (sp. gr. P321) established earlier for La3CrGe3Be2O14 by the Rietveld refinement. Vibrational frequencies are uniformly distributed over the spectral range ~80–840 cm−1. Frequencies and intensities of the modes were satisfactory modeled in the framework of density functional theory and MO LKAO approach using the B3LYP hybrid functional which takes into account the non-local exchange in the Hartree-Fock formalism. Analyzing the displacement vectors obtained in the ab initio calculations, we estimate the degree of participation of each ion in each vibrational mode. Information on the lattice dynamics of the title compounds is essential for a further research on the properties and functionalities of these new low-dimensional magnets with frustrated interactions.