Effect of stoichiometry on thermodynamic and thermal transport properties of entropy-stabilized oxide MgCoNiCuZnO5

Lattice distortion in a crystal have strong effect on its thermal transport properties. Using a neuroevolution machine learning potential and molecular dynamics simulations, we demonstrate that lattice thermal conductivity of MgCoNiCuZnO5 (J14) drops from κ ≃ 2.60 {plus minus} 0.20 Wm-1K-1 at 300 K to κ ≃ 2.07 {plus minus}0.05 Wm-1K-1 at 900 K. The reduction in lattice thermal conductivity as a function of temperature in J14 is due to a decreased mean free path of propagons. We investigated the bounds on κ when reducing or increasing distortions. This is done by varying stoichiometry to make atomic size distribution of transition metal elements more or less uniform: the composition of zinc and nickel in J14 was varied by 30% to make atomic sizes more and less uniform respectively. While adding more Zn increases distortions, adding Ni reduces it. As a result, we find that related changes in the thermal conductivity with 30% more zinc (Mg0.2Co0.2Ni0.14Cu0.2Zn0.26O) and 30% more nickel (Mg0.2Co0.2Ni0.26Cu0.2 Zn0.14O) are respectively 2.54 {plus minus} 0.20 and 2.77 {plus minus} 0.20 Wm-1K-1 at room temperature, which is correlated with lattice distoriton. Larger mean free paths (MFPs) is the less distorted (30% Ni) compound, while MFPs in the more distorted compound are shorter. The MFPs are associated with phonon-like vibrational modes (also called propagons). These results imply that propagons are mostly responsible for the changes in thermal conductivity. This is supported by slight decrease in the thermal conductivity with temperature, implying important contribution of propagons in thermal transport in these distorted high-entropy materials.