The presence of Laue oscillations in a film grown on a solid surface is broadly taken as indicating a high quality, crystallographically aligned film of the targeted compound. In this paper we briefly review the origins of both Laue oscillations and Kiessig fringes and show how they can be used together to determine if extra thickness exists above or below the coherently diffracting domains. The differences between experimental and “ideal” films are discussed and the effect of structural features (roughness, different thickness coherently diffracting domains and thickness in addition to the coherently diffracting domains) are illustrated with experimental and simulated data for metal and mixed-metal chalcogenide films of titanium, bismuth, vanadium/iron, and bismuth/molybdenum. Examples are given showing how quantitative information can be extracted from experimental diffraction patterns.

Three metastable compounds predicted to be kinetically stable using an “island” approach were successfully synthesized from designed modulated elemental reactants. Fe0.8V0.2Se2 was synthesized by depositing ultrathin elemental layers in a V|Fe|Se sequence to control the local composition. An alloyed rock salt structured Pb3Mn2Se5 constituent layer, which does not exist as a bulk compound, was synthesized in the heterostructure (Pb3Mn2Se5)0.6VSe2 by depositing a precursor with a V|Se|Pb|Se|Mn|Se|Pb|Se|Mn|Se|Pb|Se sequence of elemental layers that mimicked the compositional profile of the targeted heterostructure. The heterostructure (PbSe)1+δ(FeSe2)2 was prepared by depositing a precursor with a repeating layering sequence of Fe|Pb|Fe|Se, where each sequence contains the number of atoms required to form a single unit cell. In all three systems, the local compositions in the layer sequence kinetically favored the nucleation and growth of the targeted products during the deposition. The diffusion lengths to form the targeted compounds were short, and the diffusion was limited by postdeposition low temperature annealing to favor the growth of the targeted compounds and avoid the decomposition into a mixture of thermodynamically stable compounds.

Predicting new, stable materials and their properties is an important approach to accelerate the innovation cycle. A major remaining challenge is the lack of experimental approaches to synthesize them. While attempting the synthesis of the predicted compounds PbFe 2 Se 3 , Pb 2 FeSe 3 , and Pb 2 FeSe 4 using the modulated elemental reactant approach, we observed the formation of a new Pb 1−x Fe x Se phase that crystallizes in a primitive tetragonal unit cell. Pb 1−x Fe x Se decomposes when annealed at 200 °C into a mix of PbSe and known binary iron selenides. We suspect that this new phase forms because the low diffusion rates at low temperatures prevent disproportionation. Since it has a much simpler unit cell than the predicted ternary phases, it is easier to nucleate under the experimental conditions. These results support the idea that the ability to synthesize a given compound depends on its free energy relative to other potential compounds, the depth of the local free-energy minima, and the volume of configuration space occupied by the local free-energy minima.