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

Abstract

The zero/negative thermal expansion (ZTE/NTE), which is an intriguing physical property of solids, has been observed in a few families of materials. ZTE materials possess practical applications in specific circumstances such as space-related applications, engineering structures and precision instrument. Generally, NTE materials are used as additives to form a composite of the ZTE material with positive thermal expansion material. It is still a tremendous challenge to design new families of ZTE/NTE materials. Herein, we presented a temperature-dependent single crystal structure analysis in 110–300 K for a layered (NH₄)₂V₃O₈, which crystallizes in a tetragonal space group P4bm and comprises mixed valence [V₃O₈²⁻]_∞ monolayers and NH₄⁺ residual in the interlayer spaces. Along the c-axis, (NH₄)₂V₃O₈ demonstrated uniaxial expansion behaviors, i.e., ZTE with α_c = −1.10 × 10⁻⁶ K⁻¹ in 110–170 K and NTE with α_c = −16.25 × 10⁻⁶ K⁻¹ in 170–220 K. Along the a-axis, (NH₄)₂V₃O₈ exhibited ZTE with α_a = + 2.06 × 10⁻⁶ K⁻¹ in 240–300 K. The mechanisms of ZTE and NTE were explored using structural analysis. The conduction of NH₄⁺ ions in the interlayer space was studied, indicating that the conductivity rapidly rises with the expansion of interlayer space at temperatures of >293 K. This study discloses that layered vanadates are promising ZTE/NTE materials.