Crystal growth and magnetic behavior of quasi-two-dimensional van der Waals rare-earth tri-iodides

Abstract

Rare-earth tri-iodides (𝑅⁢I3) are a series of two-dimensional van der Waals (vdW) magnetic materials comprising a layered honeycomb structure. Both the growing interest in vdW magnetic systems and the honeycomb layered structure of 𝑅⁢I3 compounds, which is essential for realizing Kitaev physics, motivated us to grow high-quality single crystals of this series of materials and investigate their ground-state magnetic properties. Here, we report the crystal growth, magnetic, and thermodynamic properties of large (centimeter-sized), high-quality crystals of 𝑅⁢I3 compounds. The crystal growth uses the physical vapor transport method via self-transport reaction using iodine as a transporting agent. The growth parameters are meticulously reported. The crystallographic parameters are obtained using single-crystal x-ray diffraction. While the 𝑅⁢I3 compounds for the lighter rare earths (𝑅 = La and Ce) crystallize with an orthorhombic (Cmcm) structure, the structure for the heavier rare earths is trigonal ( 𝑅⁢‾‾3 ), analogous to the extensively investigated Kitaev material 𝛼−RuCl3. The density functional theory–based calculations are performed to obtain the eigenfrequencies/eigenvectors of the Raman-active phonon modes for both structure types and compared with the experimental Raman spectra of CeI3 (Cmcm) and HoI3 ( 𝑅⁢‾‾3 ). The magnetic behavior examined for the heavier rare-earth-based compounds indicates no signs of long-range magnetic ordering down to 2 K. However, signatures of short-range correlations are seen below 5 K, both in the magnetic susceptibility and specific heat of all the compounds.