Crystal Structures of Cubic and Trigonal Yttrium Hypocarbides; A Dimorphically Interphased Single-Crystal Study
X-ray structure determinations on the cubic YCx (x ≈ 0.3 - 0.7, stable above about 900°C) and the trigonal Y2C (stable below 900°C) have been carried out using a quenched "single crystal" in which a transient state in the midst of the cubic-to-trigonal transition has been arrested. Our "single crystal" is composed of one cubic and four trigonal single crystals, the trigonal c axis being parallel to one of four cubic body-diagonal axes. The cubic structure is of NaCl type with random holes in the carbon-atom sites and with a = 5.115 ± (2) Å for YC0.44. The ordering of the carbon atoms creates the anti-CdCl2-type trigonal Y2C with the yttrium positional parameter z = 0.2585 ± (3) in the hexagonal-based unit cell, a = 3.617 ± (2) and c = 17.96 ± (1) Å, containing three Y2C formula units. Mo Kα x-ray was employed, and consequently the dispersion corrections on the atomic scattering factor of Y amount to 10%-20% of the nondispersion value and are considerably larger than the carbon-scattering factor. The structure analyses with and without the dispersion corrections demonstrate that even these unusually large corrections do not modulate the Y and C positional parameters significantly. The x-ray single-crystal results were further substantiated by the neutron-powder analysis. The present work confirmed unambiguously not only the reported x-ray powder results as well as the neutron-powder data of the isostructural Ho2C, but also yielded the structural parameters with a much higher accuracy, hence providing detailed insights into the chemical-bonding correlation in the order-disorder transition.