Synthesis of rare-earth metal compounds through enhanced reactivity of alkali halides at high pressures

Abstract

Chemical stability of the alkali halides NaCl and KCl has allowed for their use as inert media in high-pressure high-temperature experiments. Here we demonstrate the unexpected reactivity of the halides with metals (Y, Dy, and Re) and iron oxide (FeO) in a laser-heated diamond anvil cell, thus providing a synthetic route for halogen-containing binary and ternary compounds. So far unknown chlorides, Y₂Cl and DyCl, and chloride carbides, Y₂ClC and Dy₂ClC, were synthesized at ~40 GPa and 2000 K and their structures were solved and refined using in situ single-crystal synchrotron X-ray diffraction. Also, FeCl₂ with the HP-PdF₂-type structure, previously reported at 108 GPa, was synthesized at ~160 GPa and 2100 K. The results of our ab initio calculations fully support experimental findings and reveal the electronic structure and chemical bonding in these compounds.

Fig. 1. Chemical reactions between yttrium and NaCl detected from X-ray diffraction.

Powder X-ray diffraction patterns of the Y-NaCl sample before (black) and after (red) laser-heating at ~2000 K and 41 GPa. Insert shows 2D XRD patterns of the same sample before and after laser heating. The previously known (fcc-YC) and unknown (Y₂Cl, Y₂ClC, and Y₃C₄) compounds were identified using single-crystal XRD data; the ticks here are added according to their calculated powder XRD patterns.

Fig. 2. Crystal structures of novel chlorides of Y, Dy, and Fe.

a Stick-and-ball model of Y₂Cl structure at 41(1) GPa; b view of the Y metal framework in Y₂Cl along the c-direction; c square antiprisms geometry around the Cl center in Y₂Cl. d Polyhedral model of the crystal structure of DyCl at 40(1) GPa built of YCl₆ octahedra; e view of the structure along the c-direction: Cl atoms form hexagonal close packing (hcp) and Y atoms occupy the space in between. f Polyhedral model of FeCl₂ at 160(1) GPa. Y, Dy, Fe, and Cl atoms are shown in blue, purple, tan, and red colors, respectively.

Fig. 3. Crystal structure of Y₂ClC and Dy₂ClC synthesized at ~40 GPa.

a Ball model in a hexagonal setting. b Polyhedral model built of (YC₃Cl₃) octahedra; A, B, C letters highlight the ccp formed by Y/Dy atoms. c View of the structure along the c-direction; C and Cl atoms together forming the ccp. d Crystal structure in the rhombohedral setting. e Interatomic distances (in Å) in Y₂ClC (black numbers) and Dy₂ClC (red numbers). Y/Dy, Cl, and C atoms are shown in blue, red, and gray colors.

Fig. 4. Results of theoretical calculations.

a The total and projected densities of states (TDOS and PDOS) curves of Y₂ClC at 40 GPa; the Fermi energy level was set to 0 eV. b The 2D electron localization function (ELF) map of Y₂ClC in the (1 0 0) plane; c, d ELF with the isosurfaces value set as c 0.7 and d 0.25. Y, Cl, and C atoms are shown in blue, red, and gray colors. One Y₄ tetrahedron is highlighted in 4b, and the weak ELF value in its center is highlighted with the red dashed circle in d. These weak ELF values in the centers of the Y₄ tetrahedra form bridges connecting the C atoms (highlighted with the red dashed square in b).