Lower‐Temperature Modifications of Nb ₂ C and V ₂ C

Abstract

The orthorhombic, low‐temperature α‐modification of Nb 2 C has a structure similar to that of ζ‐Fe 2 N (a = 12.36 A, b = 10.89 5 A, c = 4.96 8 A) and transforms at ∼1200°C into the hexagonal ε‐Fe 2 N type. A second transition at approximately 2500°C is associated with the destruction of long range order in the carbon sublattice. Alpha‐divanadium carbide (orthorhombic, a = 11.49 A, b = 10.06 A, c = 4.55 A) is isostructural with α‐Nb 2 C and transforms at ∼800°C into the hexagonal high‐temperature modification. The structures of α‐V 2 C and α‐Nb 2 C are distorted modifications related to the ε‐Fe 2 N type.