NaCl and filled Re₃B-type structures for two metastable chromium carbides

Abstract

New metastable chromium carbides are formed by crystallization of C-rich Cr_1−yCy amorphous alloys with 0.33≤ y ≤0.50; namely Cr₃C_2−x and CrC_1-z Cr₃C_2−x has a filled-up Re₃B-type structure, of space group Cmcm. Its crystals shows pseudo-tenfold twinning with various growth morphologies. CrC_1-z, with nanometre-sized domains, has a NaCl type structure (a =0.402 nm) with (100) planar defects at the domain boundaries. These chromium carbides offer either octahedral interstitial sites to C atoms (CrC_1-z) or both octahedral and prismatic interstitial sites for Cr₃C_2−x, contrary to all previously known chromium carbides. This suggests that more than one type of local order (octahedral and/or prismatic) may exist in amorphous Cr-C alloys according to the C content.