Discovering Large Isotropic Negative Thermal Expansion in Framework Compound AgB(CN)4 via the Concept of Average Atomic Volume

• Published in: Journal of the American Chemical Society 2020-04, Vol.142 (15), p.6935-6939
• Main Authors: Gao, Qilong, Wang, Jiaqi, Sanson, Andrea, Sun, Qiang, Liang, Erjun, Xing, Xianran, Chen, Jun
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.0c02188

Exploring isotropic negative thermal expansion (NTE) compounds is interesting, but remains challenging. Here, a new concept of “average atomic volume” is proposed to find new NTE open-framework materials. According to this guidance, two NTE compounds, AgB­(CN)4 and CuB­(CN)4, have been discovered, of which AgB­(CN)4 exhibits a large NTE over a wide temperature range (α v = −40 × 10–6 K–1, 100–600 K). The analysis by extended X-ray absorption fine structure spectroscopy and first-principles calculation indicate that (i) the NTE driving force comes from the transverse vibrations of bridge chain atoms of C and N, corresponding to the low-frequency phonon modes; and (ii) the same transverse vibration direction of C and N atoms is a key factor for the occurrence of strong NTE in AgB­(CN)4. The present concept of “average atomic volume” can be a simple parameter to explore new NTE compounds especially in those open-framework materials.