Negative Thermal Expansion in a Versatile Family of A2M2O7: Materials, Mechanism, and Application

Abstract

Negative thermal expansion (NTE) materials are attracting increasing attention for thermal-expansion control in aerospace, electronic packaging, and precision engineering. Among the known NTE families, A2M2O7 (A = Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn; M = P, V, As) compounds are particularly attractive because they combine large phase-transition-driven NTE, compositional flexibility, facile synthesis, and excellent thermal-compensation efficiency. Owing to these advantages, they have become one of the most intensively investigated phase-transition-type NTE materials in recent years. However, despite substantial experimental and theoretical advances, a unified understanding of the NTE behavior across the A2M2O7 family has remained elusive. This has hindered the transition from empirical optimization to mechanism-guided composition design. Recent studies increasingly suggest that the diverse phase-transition and thermal-expansion behaviors of A2M2O7 compounds can be understood within a common structural-electronic framework. This framework involves cooperative AO6 distortion networks associated with Jahn–Teller (JT) or pseudo-Jahn–Teller (PJT) effects and their thermally driven evolution across the phase transition. This mechanistic framework establishes connections among crystal structure, electronic instability, phase-transition behavior, and macroscopic thermal expansion, providing a conceptual basis for composition design through both structural-distortion engineering and electronic-structure regulation. Future progress will require a quantitative understanding of distortion energetics and electron–lattice coupling across different compositions. Such a quantitative understanding may enable predictive design of high-performance NTE materials and thermal-expansion compensators for advanced engineering applications.