Giant Negative Thermal Expansion in Ultralight NaB(CN)₄

Abstract

Negative thermal expansion (NTE) is crucial for controlling the thermomechanical properties of functional materials, albeit being relatively rare. This study reports a giant NTE (aV~‐9.2·10‐5 K‐1, 100‐200 K; aV~‐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.