Superconductivity in the amorphous phase of the half-Heusler TiNiSn alloy

Amorphous TiNiSn films were prepared at T approximate to 4.2 K and resistance R(T) and thermopower S(T) have been investigated. R(T) shows that the sample is superconducting and the transition temperature Tc increases with increasing annealing temperature, being the maximum Tc3 = 4.19 K. The electron-phonon coupling estimated from S(T) results is very close to the value calculated using McMillan equation for the superconducting transition Tc, indicating that amorphous TiNiSn is an intermediate electron-phonon coupled superconductor. After annealing up to T = 850 K, the amorphous sample crystallizes at TK approximate to 785 K. The atomic structure and high stability of the amorphous phase can be explained in the framework of electronically stabilized Hume- Rothery phases, where hybridization from Ti -d and Ni -d electrons plays an important role. The crystallized sample shows a well-ordered half-Heusler crystalline phase, space group F43m, with a TiNiSn atomic order in the unit cell instead of the common NiTiSn type.