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 (alpha(V)similar to-9.2 & sdot; 10(-5) K-1, 100-200 K; alpha(V)similar to-3.7 & sdot; 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 Gr & uuml;neisen 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.