Uniaxial thermal expansion behaviors and ionic conduction in a layered (NH4)2V3O8

The zero/negative thermal expansion (ZTE/NTE), an intriguing physical property of solids, has been observed in a few families of materials. The ZTE materials possess practical applications in the special circumstances, e.g., space apparatus, engineering structure and precision mechanical equipment. The NTE materials are generally used as additives to form a composite of ZTE material with the positive thermal expansion matter. It is still a tremendous challenge to design new families of ZTE/NTE materials. Herein, we present temperature dependent single crystal structure analysis in 110−300 K for a layered (NH4)2V3O8, which crystallizes in tetragonal space group P4bm, consists of mixed-valence [V3O82−]∞ monolayers and NH4+ residual in the interlayer spaces. Along c-axis, (NH4)2V3O8 shows uniaxial expansion behaviors, i.e., ZTE with αc = −1.10×10-6 K−1 in 110−170 K and NTE with αc = −16.25×10-6 K−1 in 170−220 K. Along a-axis, (NH4)2V3O8 exhibits ZTE with αa = +2.06×10-6 K−1 in 240−300 K. The mechanisms of ZTE and NTE are explored by structural analysis. The conduction of NH4+ ions in the interlayer space were investigated, indicating that the conductivity rises rapidly with the expansion of interlayer space at the temperature above 293 K. This study discloses that layered vanadates are promising ZTE/NTE materials.