Controllable thermal expansion and crystal structure of (Fe_1−xNi_x)ZrF₆ solid solutions

Most materials expand on heating and contract on cooling. In the recent years, however, some compounds have been found to exhibit abnormal negative thermal expansion (NTE) behavior; this presents an opportunity to adjust the coefficient of thermal expansion (CTE) of such materials. It is especially important to obtain controllable thermal expansion in isotropic compounds. Herein, we report the preparation, crystal structure, and controllable thermal expansion in double ReO₃-type (Fe_1−xNi_x)ZrF₆ solid solutions. (Fe_1−xNi_x)ZrF₆ exhibits full range solubility. A controllable thermal expansion of (Fe_1−xNi_x)ZrF₆ could be achieved by the chemical substitution of Ni²⁺ for Fe²⁺ over a wide range of CTE from −3.24 × 10⁻⁶ to +18.23 × 10⁻⁶ K⁻¹ (300–675 K). In particular, zero thermal expansion was obtained for the composition (Fe_0.5Ni_0.5)ZrF₆. As a kind of typical framework structure, the transverse thermal vibrations of fluorine atoms are expected to play a critical role in the thermal expansion behavior of double-ReO₃ compounds. This study presents a potential method to tune the thermal expansion of NTE (negative thermal expansion) families which have an open framework structure.