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

Abstract

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.

The role of average atomic volume in predicting negative thermal expansion: The case of REFe(CN)6

“…Due to their flexible networking architecture, many cyanides/Prussian blue analogues (PBAs), such as Zn(CN) 2 [29], ZnPt(CN) 6 [30], LaCo(CN) 6 [31] and ScCo(CN) 6 [32] , display a fascinating NTE behavior. Gao et al [33] reported the large NTE behavior in YFe(CN) 6 , whose AAV is 22.58 Å 3 , well above the critical point (assessed to ~16 Å 3 by Gao et al [28]). The framework structure of isostructural materials with formula REFe(CN) 6 consists of REN 6 trigonal prisms alternating with FeC 6 octahedra, as shown in Fig.…”

“…Recently, Gao et al [28] proposed a new concept of average atomic volume (AAV) by summarizing the structural characteristics of isotropic framework NTE materials and successfully discovered the NTE material AgB(CN) 4 . The concept of AAV is a simple parameter to indicate the average volume of atoms in a unit cell.…”

“…The formula is AAV = V/(Z × N), where V is the unit cell volume, Z is the cell formula units, and N is the number of atoms in the chemical formula. Here, with the help of AAV, one could evaluate the thermal expansion behavior [28]. It should be noted that the number of anisotropic materials is much larger than that of isotropic ones.…”

Theoretical Study of Abnormal Thermal Expansion of CuSCN and Effect on Electronic Structure

“…Recently, Gao et al suggested a simple rule based on the concept of average atomic volume (AAV) according to the statistical data of NTE materials, where they proposed a critical AAV of 16 Å 3 for the framework compounds with NTE property, and they believed that the flexibility of the structure is the key factor for the NTE characteristics (Gao et al, 2020c). It provides a simple way to search for and discover new NTE materials.…”

Controlling thermal expansion within mixed cocrystals by tuning molecular motion capability

“… 1 Positive TE (PTE) is the most common behavior in materials 2 , 3 and involves an increase in size upon heating. Alternatively, negative TE (NTE) is rarer 4 – 6 and corresponds to a decrease in size upon heating. Lastly, near-zero or zero TE materials are quite rare 7 – 9 and are characterized by minimal to no change in response to temperature.…”