Transition-metal carbides exhibit high melting points, high hardness, and metallic conductivity that make these materials attractive candidates for different technological applications and for material science studies. Carbon has the capability of forming various bonding states affecting the structures and properties of transition metal carbides. High pressure can be used to alter bonding patterns, leading to new compounds with unusual bonding states and properties.
The yttrium carbide family shows a large variety of possible phases at ambient pressure with different stoichiometry (e.g., Y2C, Y4C5, Y3C4, Y2C3, Y4C7, YC2). Here, we report results on the synthesis and characterization of a previously unknown high-pressure yttrium carbide, HP-Y4C5, which displays very unusual crystal chemistry. The HP-Y4C5 phase was synthesized by the chemical reaction of pure metallic yttrium and paraffin oil at P ~ 44 GPa and T ~ 1500 K in a laser-heated diamond anvil cell, and was characterized by synchrotron single-crystal X-ray diffraction and Raman spectroscopy. On decompression, diffraction lines of HP-Y4C5 were traced down to at least 16(2) GPa, and the characteristic Raman modes of the phase were observed to ~ 2 GPa.
The HP-Y4C5 phase crystallizes in the orthorhombic system (space group Cmce, Z = 16) with the unit cell parameters a = 12.1834(8) Å, b = 7.6592(4) Å and c = 8.8584(11) Å at 44 GPa (Fig. 1). Notable structural elements of HP-Y4C5 are [C2] dumbbells and [C3] trimers. While the first coordination sphere Y-C distances (2.24-2.69 Å) are typical for yttrium carbides, the C-C distances in the [C2] dimers (1.40(2) Å) and the [C3] trimers (1.43(1) Å) are unusual: they are significantly larger than expected for double-bonded carbon atoms (~1.33 Å) and much shorter than for single-bonded (~1.54 Å).
Phonon dispersion calculations show that HP-Y4C5 is dynamically stable at 44 GPa. The electron localization function (ELF) demonstrates strong covalent bonding between carbon atoms within the carbon dumbbells and trimers, and covalent bonds between Y and C. According to our analysis of the charge distribution in the ionic approximation, the HP-Y4C5 phase contains the anions [C2]5- and [C3]6-, with a bond order of 1.5 in both cases. The calculated electronic density of states shows that yttrium carbide HP-Y4C5 is metallic and that the main contribution at the Fermi level comes from Y 4d and C 2p states.
Fig. 1: Crystal structure of Y4C5 at 44 GPa. The grey, blue and red spheres represent yttrium and carbon atoms in dimers and trimers, respectively. The [C2]5- and [C3]6- species with their bond lengths and angles are indicated on the right side.