Jon-Paul Maria

Epitaxial growth of (001)-oriented and (110)-oriented SrBi2Ta2O9 thin films

Epitaxial SrBi2Ta2O9 thin films have been grown with (001) and (110) orientations by pulsed laser deposition on (001) LaAlO3–Sr2AlTaO6 and (100) LaSrAlO4 substrates, respectively. Four-circle x-ray diffraction and transmission electron microscopy reveal nearly phase pure epitaxial films. Minimization of surface mesh mismatch between the film and substrate (i.e., choice of appropriate substrate material and orientation) was used to stabilize the desired orientations and achieve epitaxial growth.

Origin of preferential orthorhombic twinning in SrRuO3 epitaxial thin films

In order to elucidate the driving forces which promote oriented in-plane crystallographic texture in SrRuO3 thin films deposited on stepped SrTiO3 substrates, a high-temperature x-ray analysis of both SrRuO3 thin films and powders was conducted. Structural phase transitions were found at temperatures near 350u200a°C and slightly above 600u200a°C. The transitions are tentatively indexed as orthorhombic to tetragonal and tetragonal to cubic, respectively. These results suggest that SrRuO3 thin films grow with cubic symmetry. As such, film–substrate interfacial characteristics, rather than a preferred growth direction, are believed to determine the orientation of orthorhombic twins.

Charge-Induced Disorder Controls the Thermal Conductivity of Entropy-Stabilized Oxides

Manipulating a crystalline materials configurational entropy through 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 reduce the rate at which phonons carry heat and thus reduce the thermal conductivity. Recent advances in materials synthesis are enabling the fabrication of entropy-stabilized ceramics, opening the door for understanding the implications of extreme disorder on thermal transport. Measuring the structural, mechanical, and thermal properties of single-crystal entropy-stabilized oxides, it is shown that local ionic charge disorder can effectively reduce thermal conductivity without compromising mechanical stiffness. These materials demonstrate similar thermal conductivities to their amorphous counterparts, in agreement with the theoretical minimum limit, resulting in this class of material possessing the highest ratio of elastic modulus to thermal conductivity of any isotropic crystal.