Design of Giant Negative Thermal Expansion Based on a Fluorite Structure

Negative thermal expansion (NTE) is a unique phenomenon in which materials contract upon heating, and thus, there is a strong demand for rational synthetic strategies and the design of giant NTE materials. In tetracyanoborate salts, however, the formation of interpenetrated structures suppresses structural flexibility and limits NTE behavior. Herein, we demonstrated that constructing a fluorite structure is an effective strategy for enhancing NTE through the synthesis of Ba[B(CN) 4 ] 2 with toluene as a template molecule. Density functional theory calculations predicted that Ba[B(CN) 4 ] 2 exhibited pronounced volume contraction at elevated temperature among M[B(CN) 4 ] 2 (M = Ca 2+ , Sr 2+ , and Ba 2+ ). In situ X‐ray diffraction measurements showed that Ba[B(CN) 4 ] 2 exhibited giant NTE with a volumetric thermal expansion coefficient of α v = –122.0 × 10 −6 K −1 over 620–657 K after the removal of the template molecules. Phonon calculations revealed that giant NTE in Ba[B(CN) 4 ] 2 originates from a non‐rigid unit mode with strongly negative Grüneisen parameters. These findings provide a new design principle for synthesizing open‐framework materials with precisely controlled thermal expansion properties.