The equilibrium phase of a material is only one of many structures that can potentially form. As structure plays the critical role in determining many material properties, accessing these non-equilibrium metastable phases can dramatically expand the design space of materials. While metastable materials are in higher free energy states than the stable phase, the differences are often small compared to the free energy change during crystallization from either a liquid or amorphous precursor. Consequently, during thermal annealing metastable phases can readily nucleate and grow. At slow thermal quench rates, subsequent transformations often relax the materials to the equilibrium structure. However, if thermal quench rates are sufficiently high, these interesting metastable phases can be retained to low temperatures. In this work, we report the development of a high throughput process to characterize metastable phase formation utilizing millisecond time scale thermal anneals with spatial temperature gradients. This technique, lateral gradient laser spike annealing (lgLSA), provides a robust and rapid platform to develop time and temperature maps of phase formation in a broad range of material systems ranging from metals to complex oxides. The formation and trapping to room temperature of numerous metastable metal oxide phases are reported. As a potentially important ion conductor, we investigated metastable phase formation in the Bi2O3 system. In equilibrium, Bi2O3 transforms to a high oxygen ion conductive phase (δ-phase) at temperatures above 730°C. This phase cannot be quenched to room temperature with standard processing, transforming at 640°C during furnace quenches. Using the lgLSA technique, annealing times and temperatures which result in the formation of α, β, and δ phase Bi2O3 were identified. The stable and metastable α, β, and δ phases were observed to form by solid-solid transformations from the initially amorphous precursor. For thermal anneals above the melting temperature, large grain δ-phase was observed to nucleate and was retained to room temperature at quench rates on the order of 105 K/sec. A second system, MnTiO3, was also extensively investigated due to predictions by the “materials by design” community of enhanced piezoelectric and multiferroic properties. During this investigation, a previously unreported phase of MnTiO3 was discovered. Formation of this new phase, and other known phases, were characterized as a function of annealing time and temperature. This new phase was not observed for anneals longer than ~1 ms, and at longer anneals only previously reported phases are observed. It was only under the high quench rates possible during lgLSA that this new phase could be kinetically trapped and stabilized to room temperature.
This work demonstrates the ability of lgLSA to serve as a foundation for exploring and understanding metastable phase formation. Coupled with high throughput characterization techniques, the complex time and temperature phase space for a wide range of material systems can be rapidly assessed.