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.