Thermal conductivity of entropy stabilized oxides: approaching the minimum limit in single crystal solid solutions through charge induced atomic-scale disorder
Gheorghe Stan, Jeffrey Braun, Christina Rost, Mina Lim, Ashutosh Giri, David Olson, George Kotsonis, Donald Brenner, Jon-Paul Maria, Patrick Hopkins
Manipulating a crystalline materials configurational entropy though the introduction of unique atomic species can produce novel materials with desirable mechanical and electrical properties. From a thermal transport perspective, large differences between elemental properties such as mass and interatomic force can lead to strong phonon scattering that can drive down the lattice thermal conductivity beyond what is predicted from phonon scattering theory. To show this experimentally, we study the thermal properties of a new class of mixed oxide solid solutions, entropy stabilized oxides (ESOs), characterized by their high configurational entropy that leads to the structural and chemical stabilization through a local minimization of Gibbs free energy. These entropy stabilized oxide thin films are composed of oxide anions that create a fixed cubic sublattice as part of a larger rocksalt lattice. Contrary to analytical theory, the thermal conductivity of these ESOs drops by nearly a factor of two when adding an additional cation species to a 5-cation crystal, regardless of the mass added. Using extended X-ray absorption fine structure, we isolate the mechanism of this reduction to atomic level disorder that results from charge differences among cation/anion pairs, creating disorder within interatomic forces that bind the atoms. This local atomic disorder manifests itself in an observable distortion of the oxygen sublattice while preserving long range crystallographic order measured with X-ray diffraction. This finding is further corroborated by molecular dynamics simulations that account for differences among interatomic forces though heterogeneous integration of atomic charges based on Bader charges taken from density functional theory calculations. The thermal conductivities of these single crystalline entropy stabilized oxides demonstrate similar values and temperature trends to their amorphous counterparts.