Broad Negative Thermal Expansion Operation-Temperature Window Achieved by Adjusting Fe–Fe Magnetic Exchange Coupling in La(Fe,Si)₁₃ Compounds

Cubic La­(Fe,Si)₁₃-based compounds have been recently developed as promising negative thermal expansion­(NTE) materials, but the narrow NTE operation-temperature window­(∼110 K) restricts their actual applications. In this work, we demonstrate that the NTE operation-temperature window of LaFe_13–xSi_x can be significantly broadened by adjusting Fe–Fe magnetic exchange coupling as x ranges from 2.8 to 3.1. In particular, the NTE operation-temperature window of LaFe_10.1Si_2.9 is extended to 220 K. More attractively, the coefficients of thermal expansion of LaFe_10.0Si_3.0 and LaFe_9.9Si_3.1 are homogeneous in the NTE operation-temperature range of about 200 K, which is much valuable for the stability of fabricating devices. The further experimental characterizations combined with first-principles studies reveal that the tetragonal phase is gradually introduced into the cubic phase as the Si content increases, hence modifies the Fe–Fe interatomic distance. The reduction of the overall Fe–Fe magnetic exchange interactions contributes to the broadness of NTE operation-temperature window for LaFe_13–xSi_x.