Alloying effects on structural, magnetic, and electrical/thermal transport properties in MAX-phase Cr2−MGeC (M= Ti, V, Mn, Fe, and Mo)

Herein we systematically investigated the alloying effects on structural, magnetic, and electrical/thermal transport properties in MAX-phase Cr2−xMxGeC (M = Ti, V, Mn, Fe, and Mo). The alloying of M with the larger covalent radius than that of Cr increases lattice constants (a and c) as well as unit cell volume (V) of Cr2−xMxGeC, and vice versa. However, the c/a ratio monotonously decreases with increasing alloying level x, which is due to a larger change of a than that of c. The Pauli paramagnetic ground state of Cr2GeC is confirmed by magnetic measurements and low-temperature specific heat analysis. Interestingly, ferromagnetism can be introduced in Cr2−xMxGeC by doping magnetic elements (Mn and Fe) and non-magnetic elements (Ti and Mo), which may be due to a reconstruction of the Fermi surface caused by chemical doping. All our samples show a metal-like electrical transport behavior, and the residual resistivity ratio decreases with increasing alloying concentration, which are mainly attributed to the disorders induced by alloying. The change of electron specific heat coefficient is consistent with the change of density state of Fermi surface in Cr2−xMxGeC. In addition, solid-solution scattering is the dominant factor for the behavior of thermal conductivity k(T) in Cr2−xVxGeC, while enhanced phonon scattering induced by alloying is the decisive factor for the change of k(T) in Cr2−xMoxGeC. The positive Seebeck coefficient of Cr2−xVxGeC and Cr2−xMoxGeC may be close related to the decrease of structural anisotropy.