Tuning properties through disorder: structural, optical, and mechanical insights into novel medium-entropy rare-earth hafnate pyrochlores

Abstract Entropy-stabilized oxides find application in several areas due to their interesting properties which comes from the four core attributes—entropy-stabilization effect, severe lattice distortion effect, sluggish diffusion effect and cocktail effect. Five novel Medium-Entropy Pyrochlore Oxides (MEPO) are developed, with four rare-earth (RE) elements and a transition metal (TM), forming a general structure of the form 4RE₂TM₂O₇. The design, synthesis and characterizations were performed to understand their structural, optical and mechanical properties. A four-element combination from La, Ce, Pr, Nd and Gd are selected as REs and Hf is taken as the TM. The five novel phase-pure pyrochlore oxides were synthesized using solution combustion method with urea as fuel. The oxides display excellent phase stability up to calcination temperatures of 1600 °C. The developed oxides display a narrower band gap when compared to the individual rare-earth hafnates, due to severe lattice distortion effect. The oxides also exhibited good IR absorption from 800–2000 nm. MEPO-2 variant ((La_0.25Ce_0.25Pr_0.25Gd_0.25)₂Hf₂O₇) having the highest size disorder of 3.66% displays the highest hardness of 11.59 GPa and fracture toughness of 3.68 MPa.m^1/2, resulting in 18.15% and 62.11% enhancement respectively when compared to La₂Hf₂O₇. Thus, in this work, the effect of size and mass disorder on the optical and mechanical properties of medium-entropy rare-earth pyrochlore systems are investigated.