Abstract:
Rare-earth tri-iodides (𝑅I3) 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 𝑅I3 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 𝑅I3 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 𝑅I3 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.