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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on Nb6C5 by Materials Project

Nb6C5 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. there are two inequivalent Nb+3.33+ sites. In the first Nb+3.33+ site, Nb+3.33+ is bonded to five C4- atoms to form a mixture of edge and corner-sharing NbC5 square pyramids. There are a spread of Nb–C bond distances ranging from 2.12–2.25 Å. In the second Nb+3.33+ site, Nb+3.33+ is bonded to five C4- atoms to form a mixture of edge and corner-sharing NbC5 square pyramids. There are a spread of Nb–C bond distances ranging from 2.15–2.29 Å. There are three inequivalent C4- sites. In the first C4- site, C4- is bonded to six Nb+3.33+ atoms to form a mixture of edge and corner-sharing CNb6 octahedra. The corner-sharing octahedral tilt angles are 5°. In the second C4- site, C4- is bonded to six Nb+3.33+ atoms to form a mixture of edge and corner-sharing CNb6 octahedra. The corner-sharing octahedra tilt angles range from 5–7°. In the third C4- site, C4- is bonded to six Nb+3.33+ atoms to form a mixture of edge and corner-sharing CNb6 octahedra. The corner-sharing octahedra tilt angles range from 6–7°.

36 MATERIALS SCIENCE↗
NASA NTRS1972

Ordering Effects in NbC and TaC

By means of transmission electron microscopy and electron diffraction, evidence has been obtained for the existence of long range carbon atom ordering in single-crystal niobium carbide that has a carbon-to-metal ratio close to the integral composition Nb6C5. The ordering, which gives rise to superlattice and domain structures similar to those observed in V6C5, appears, however, only in samples that have been cooled slowly through the order-disorder temperature of 1025 C. In TaC of similar composition, the ordering, although present, remains very imperfect even after the crystals are subjected to the same thermal treatment. The results are interpreted in terms of the electronic structure of the transition metal carbides as it is currently understood, and their relevance to the mechanical properties of NbC and TaC are discussed.

Venables, J. D.↗