Difference in physical properties of MAX-phase compounds Cr2GaC and Cr2GaN induced by an anomalous structure change in Cr2GaN

Abstract MAX-phase compounds with layered structure have been investigated extensively due to their excellent metallic and ceramic properties. Among them, Cr 2 GaC and Cr 2 GaN possess the same crystal structure and similar electronic structure, whereas Cr 2 GaN undergoes a spin-density-wave transition around 170 K but Cr 2 GaC is Pauli paramagnetism without any magnetic transition. To understand what make these happen, we studied the crystal structure between 300 and 35 K for both Cr 2 GaC and Cr 2 GaN. As the temperature decreases, the lattice constants (both a and c ) decline monotonically in Cr 2 GaC, while an abnormal negative thermal expansion of c axis was observed below 170 K in Cr 2 GaN. The increase of c/a induced by abnormal increase of c leads to the enhancement of localization of Cr 2 N plane and causes more Cr-3 d electrons confinement in Cr 2 N plane of Cr 2 GaN. This further facilitates the reconstruction of density of states near the Fermi surface and even forms the Fermi surface nesting, resulting in the spin-density-wave transition as well as the abrupt changes of resistivity, specific heat, and density of carriers around 170 K in Cr 2 GaN.