Giant Negative Thermal Expansion in Ultralight NaB(CN)₄

Negative thermal expansion (NTE) is crucial for controlling the thermomechanical properties of functional materials, albeit being relatively rare. This study reports a giant NTE (α V ∼−9.2 ⋅ 10 −5 K −1 , 100–200 K; α V ∼−3.7 ⋅ 10 −5 K −1 , 200–650 K) observed in NaB(CN) 4 , showcasing interesting ultralight properties. A comprehensive investigation involving synchrotron X‐ray diffraction, Raman spectroscopy, and first‐principles calculations has been conducted to explore the thermal expansion mechanism. The findings indicate that the low‐frequency phonon modes play a primary role in NTE, and non‐rigid vibration modes with most negative Grüneisen parameters are the key contributing factor to the giant NTE observed in NaB(CN) 4 . This work presents a new material with giant NTE and ultralight mass density, providing insights for the understanding and design of novel NTE materials.