Zero thermal expansion and ferromagnetism in cubic sc_{1- x}M_xF₃ (M = Ga, Fe) over a wide temperature range

The rare physical property of zero thermal expansion (ZTE) is intriguing because neither expansion nor contraction occurs with temperature fluctuations. Most ZTE, however, occurs below room temperature. It is a great challenge to achieve isotropic ZTE at high temperatures. Here we report the unconventional isotropic ZTE in the cubic (Sc_1-xM_x)F₃ (M = Ga, Fe) over a wide temperature range (linear coefficient of thermal expansion (CTE), _l = 2.34 × 10^-7 K^-1, 300-900 K). Such a broad temperature range with a considerably negligible CTE has rarely been documented. The present ZTE property has been designed using the introduction of local distortions in the macroscopic cubic lattice by heterogeneous cation substitution for the Sc site. Even though the macroscopic crystallographic structure of (Sc_0.85Ga_0.05Fe_0.1)F₃ adheres to the cubic system (Pm3m) according to the results of X-ray diffraction, the local structure exhibits a slight rhombohedral distortion. This is confirmed by pair distribution function analysis of synchrotron radiation X-ray total scattering. This local distortion may weaken the contribution from the transverse thermal vibration of fluorine atoms to negative thermal expansion, and thus may presumably be responsible for the ZTE. In addition, the present ZTE compounds of (Sc_1-xM_x)F₃ can be functionalized to exhibit high-T_c ferromagnetism and a narrow-gap semiconductor feature. The present study shows the possibility of obtaining ZTE materials with multifunctionality in future work.