Tunable thermal expansion from positive, zero to negative in RbMgInMo3O12 with NASICON structure

Precise control of thermal expansion is important for open-framework structure materials and full of challenges. The present work designs a feasible strategy to achieve the thermal expansion from positive to zero and negative in NASICON structure molybdates by part removing Rb ions in the RbMgInMo3O12 framework. The removal of alkali metal ions Rb+ reduces the number of Rb–O bonds that contribute to positive thermal expansion along the c-axis direction and expands the average atomic volume of the framework structure. This structural modification facilitates coupling oscillations between polyhedra, significantly reducing the coefficient of thermal expansion along the c-axis. Meanwhile, the coefficient of thermal expansion of the ab-plane remains essentially unchanged. Consequently, the thermal expansion of RbxMgxIn2-xMo3O12 is effectively modulated from positive (αv = 3.3 × 10−6 K−1) to zero (αv = 1.0 × 10−6 K−1) and negative thermal expansion (αv = −2.7 × 10−6 K−1). This work not only reports the interesting zero and negative thermal expansion functional materials but also gives one way to design more negative thermal expansion materials with the open-framework structure.