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_𝑐3 = 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 𝐹43𝑚, with a TiNiSn atomic order in the unit cell instead of the common NiTiSn type.