Intellectual Merit:

The researchers further developed and used a high-throughput epitaxial film growth methodology to establish general descriptors of crystal growth, map out phase stability across growth conditions, and generate new complex oxides of interest as electronic materials. 

The method is called combinatorial substrate epitaxy (CSE), and investigators used this to study local epitaxial growth on hundreds to thousands of different kinds of surfaces. Using CSE, the researchers demonstrated that several important generalizable observations concerning crystal and film growth, regardless of crystal systems. 

Grain-over-grain film growth occurs on well-polished polycrystalline substrates. This opens wide the door for exploring epitaxy in complex systems and developing new materials.

Similar growth and property results are found for films deposited on commercial single crystalline or in-house prepared polycrystalline substrates. This further demonstrate new materials can be developed and optimized on polycrystalline substrates, as opposed to the few commercially available substrates.

Local epitaxy dominates crystal growth across all substrate orientation space. This indicates that interface driven crystallization occurs on all orientations, regardless of the local surface orientation and vastly increases our understanding of crystal growth using CSE.

Epitaxy is largely governed by a single preferred orientation relationship between crystals, regardless of the substrate / interface plane and governed by the crystal systems. The most general orientation relationship (OR) is the alignment of close-packed planes and directions, we call the eutactic OR. This observation may be generalized to orientation relationships across most interfaces in materials. 

In complex systems, such as layered oxides in the Ruddlesden-Popper family of materials, thermodynamic (special low energy surfaces) and kinetic (temperatures unable to support out-of-plane diffusion) effects lead to only a small number of secondary orientations.

Epitaxial phase selection can be mapped out across thermodynamic growth parameters, such as temperature, pressure, and film composition, as well as substrate parameters, such as composition, misfit strain, and local surface orientation. All of these parameters influence polymorph selection and CSE allows one to quantify them efficiently. 

Based on these results, the researchers developed a computational first-principles approach to rationalize and predict synthesis conditions, especially substrate orientation, that lead to preferred polymorph selection. Computationally Guided Epitaxial Synthesis (CGES) uses density functional theory to quantify thermodynamic values in standard continuum level models of crystallization, for the preferred ORs observed in CSE. The preferred substrate orientation was consistent when describing anatase versus rutile selection of TiO₂ on (Ba,Sr)TiO₃ and cubic versus hexagonal (4H) SrMnO₃ on SrTiO₃. These results promise that versions of predictive synthesis via epitaxy are readily achievable.

The investigators targeted the discovery of several specific breakthrough materials expected to be exciting catalysts, electrodes, ferroelectrics, and multiferroics. Investigations were carried out in (Ti,Sn,Ru)O₂ in the anatase/rutile/columbite structures, Ln₂BO₄ (B=Ni, Ru, Cu) in the T- and T'-polymorphs related to catalysis and superconductivity; Ln₂(Ru,V,Ti)₂O₇ in the (110)-layered perovskite and pyrochlore structures, and order or disordered double perovskites in the Sr₂FeWO₆ system.

Broader Impacts:

This program supported the education and training of 1 Ph.D. (a US female), four research based MS (one a US female, one a foreign national female, and two foreign national males), and three undergraduate / MS (2 female and one male) coursework students.

Epitaxial film growth and crystal nucleation / growth are important technological processes and the continued development of CSE will impact many industries. CSE and CGES together contribute to the materials genome initiative goals to accelerate the discovery, design, development, and deployment of new, advanced materials. It is readily implementable in laboratories across the globe.

Researchers maintained close ties with Kennametal Inc., transferring knowledge and quantifying the use of CSE methods on some of their materials. Researchers collaborated with experts in EBSD to improve the automated indexation of EBSD patterns in complex crystal systems and thin films. They implemented a computational approach called dictionary indexing (or DI) and assessed DI for its utility in transmission Kikuchi diffraction using commercial nanoscale films. Initial work through this program led to follow on funding to interact with Kennametal and to improve EBSD and TKD phase assignments in commercial films.

The researchers collaborated with a CMU expert in DFT to help develop CGES, as well as with international researchers in France, Poland, and the United Kingdom to expand the use of, and materials targeted for synthesis by, CSE.

The investigators developed traditional and technology-enhanced learning tools for delivery of primary content and practice of essential skills. A collaboration with learning software developers exposed the real lack of appropriate content to populate learning apps. Thus, a draft of a textbook on point defects was developed. A unified chemical thermodynamic description of point defects was incorporated for improved pedagogy, along with an extensive number of examples and problems. The book and problems were used in a CMU MSE course and distributed to over 100 undergraduate students.

Last Modified: 07/27/2021
Modified by: Paul A Salvador