ИСТИНА

This talk focuses on the recently synthesized compounds Ln3CrGe3Be2O14 (Ln = La–Sm) as a new class of low-dimensional magnetics. They belong to a vast and interesting family of the so called langasites. The parent compound of the family, the langasite La3Ga5SiO14 with the Ca3Ga2Ge4O14 structure (sp. gr. P321, Z = 1), was discovered in 1980s in the Moscow State University. At present, more than 200 compounds and solid-solution phases of such and derivative structures are known. Langasites with 3d ions were shown to demonstrate interesting magnetic and magnetoelectric properties. The family of langasites is of great interest not only for fundamental physics (frustrated and low-dimensional magnetic interactions, chirality, phase transitions) but also for applications, due to such functional properties as piezoelectricity and multiferroicity. The recently discovered Ln3CrGe3Be2O14 (Ln = La–Sm) compounds have the ordered crystal structure with Cr3+ ions residing in oxygen octahedra (point symmetry 32) and Ln3+ ions occupying distorted Thomson cubes with symmetry 2. They represent the first example of two-sublattice magnets in the langasite family. We have performed the first studies of the magnetic properties of Ln3CrGe3Be2O14 (Ln = La–Sm) by means of the magnetic susceptibility, specific heat, and high-resolution optical spectroscopy measurements on polycrystalline samples in a wide temperature range. In La3CrGe3Be2O14, which contains only one (chromium) magnetic system, the shortest exchange paths are within isolated chromium chains along the c axis. The compound demonstrates one-dimensional magnetic behavior, i.e., an absence of a long-range order, at least down to 0.4 K, and short-range magnetic correlations with the antiferromagnetic exchange integral along the chains estimated as JCr = 4.0 ± 0.5 K. In Nd3CrGe3Be2O14 containing two magnetic subsystems, a splitting of the neodymium Kramers doublets at low temperatures indicates magnetic activity due to both short-range correlations and possible magnetic ordering at 2 K. Pr3CrGe3Be2O14 does not show any sign of a long-range order down to 0.4 K. Temperature behavior of the magnetic susceptibility and specific heat demonstrates broad maxima at 6.5 K and 12 K, respectively. On the basis of spectroscopic data on the energy levels of the Pr3+ ions, they were interpreted as Shottky-type anomalies connected with depopulation of the crystal-field energy levels of praseodymium.