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 of AgB(CN)4 and CuB(CN)4 have been discovered, of which AgB(CN)4 exhibits a large NTE over a wide temperature range (αv = -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.

中文翻译：

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通过平均原子体积的概念发现 AgB(CN)4 骨架化合物中的大的各向同性负热膨胀

探索各向同性负热膨胀 (NTE) 化合物很有趣，但仍然具有挑战性。在这里，提出了“平均原子体积”的新概念，以寻找新的 NTE 开放框架材料。根据该指南，已发现 AgB(CN)4 和 CuB(CN)4 两种 NTE 化合物，其中 AgB(CN)4 在较宽的温度范围内表现出较大的 NTE（αv = -40 * 10-6 K -1, 100-600 K)。扩展X射线吸收精细结构光谱分析和第一性原理计算表明：（i）NTE驱动力来自C和N桥链原子的横向振动，对应于低频声子模式；(ii) C和N原子的相同横向振动方向是AgB(CN)4中发生强NTE的关键因素。“平均原子体积”的现有概念