nor its MXene derivative have been reported; density functional theory (DFT) calculations suggest that this 312 MAX phase is dynamically unstable. The Nb MAX and MXene phases have various interesting properties and potential applications: Nb2CTx 24,27 and Nb4C3Tx have attracted recent interest as lithium-ion battery anodes, Nb4C3Tx is one of the most conductive and strongest MXene phases, which makes it promising for field-effect transistors and mechanical applications, and Nb2SnC 31 and Nb2AlC 32 are superconducting up to 7.8 K and 0.440 K, respectively. Superconductivity was recently observed in Nb2CTx at 4.5 to 7.1 K, the temperature depending on the surface termination. The structures of the Nb MAX and MXene phases are shown in Figure , with the firstshell coordination numbers and geometry given in Table . Nb4AlC3 and Nb4C3Tx contain two distinct crystallographic Nb sites and two C sites. Nb2 and C2 are the sites nearer to the Al/etched layer and will thus be referred to as 'external' while Nb1 and C1 are denoted 'internal' following the assignments in ICSD crystal structure files 160755 and 606236 .