Zero thermal expansion in high-entropy molybdate

Abstract

Zero thermal expansion is of great significance for materials science and industrial technology. However, achieving this property remains highly challenging. Here, we introduced the concept of high-entropy to design a novel oxide, KMnSc_0.25In_0.25Lu_0.25Yb_0.25(MoO₄)₃ (KMHO), which displays zero thermal expansion (α_V = 1.53 × 10⁻⁶ K⁻¹) over 300 to 800 K. The underlying mechanism was elucidated through synchrotron X-ray diffraction and high-pressure Raman spectroscopy. KMHO crystallizes in the NZP-type structure, where the positive thermal expansion along the c-axis is associated with the weak bonding at the K⁺ site, while the negative thermal expansion within the ab-plane originates from polyhedral rotations. High-pressure Raman spectroscopy further confirmed the contribution of polyhedral rotations to the negative thermal expansion. Moreover, it also exhibits typical semiconducting behavior, with an ionic conductivity reaching a maximum value of 1.62 × 10⁻³ S cm⁻¹ at 1023 K. This work not only reports a novel high-entropy oxide with zero thermal expansion but also expands the potential applications of high-entropy materials in semiconducting and ionic conducting systems.