High Entropy Rare-Earth Molybdate Ceramics with Broad Operating-Temperature Window and Anti-Hygroscopicity Ability: A Promising Strategy for Negative Thermal Expansion Materials

Negative thermal expansion (NTE) has been an attractive topic in engineering application and functional materials. However, the narrow operating-temperature window and the hygroscopicity have become bottlenecks restricting the NTE behavior. In this work, a design strategy for effectively ameliorating the NTE property, rare-earth based high-entropy (Y 0.2 Yb 0.2 Er 0.2 Tm 0.2 A 0.2 ) 2 Mo 3 O 12 (A = Sc, Gd, Lu) (HE-RE 2 Mo 3 O 12 ) ceramics, is demonstrated for the first time. The as-synthesized HE-RE 2 Mo 3 O 12 ceramics exhibit an orthorhombic crystal structure with a uniform distribution of rare-earth elements. Furthermore, the coefficients of thermal expansion (CTEs) of (Y 0.2 Yb 0.2 Er 0.2 Tm 0.2 Sc 0.2 ) 2 Mo 3 O 12 and (Y 0.2 Yb 0.2 Er 0.2 Tm 0.2 Lu 0.2 ) 2 Mo 3 O 12 ceramics are −4.96 × 10 –6 K –1 and −4.58 × 10 –6 K –1 with high service temperatures (100–800 °C). Fascinatingly, the large ionic radius Gd(III) is incorporated into the orthorhombic (Y 0.2 Yb 0.2 Er 0.2 Tm 0.2 Gd 0.2 ) 2 Mo 3 O 12 ceramic, and a lower negative thermal expansion property (−0.76 × 10 –6 K –1 ) with broad operating-temperature window (100–800 °C) is achieved. Remarkably, the antihygroscopicity ability of the high-entropy specimens is improved, and the water absorption of such ceramics is reduced by 61.4%. Moreover, the improvements of the tunable thermal expansion and antihygroscopicity property of HE-RE 2 Mo 3 O 12 ceramics are ascribed to the lattice distortion effect. The present study provides a promising strategy for the development of NTE functional materials.