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

Amorphous TiNiSn films were prepared at T ≈ 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 T c increases with increasing annealing temperature, being the maximum T c 3 = 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 T c , indicating that amorphous TiNiSn is an intermediate electron–phonon coupled superconductor. After annealing up to T = 850 K, the amorphous sample crystallizes at T K ≈ 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 F 4 ¯ 3 m , with a TiNiSn atomic order in the unit cell instead of the common NiTiSn type.