The mechanism of formation of a chemical compound is a key component to the understanding of controlling the reaction path in chemical syntheses. While many mechanisms of molecular reactions have been discovered, the mechanistic investigation of solid state reactions is still in its infancy due to the thermodynamic control of most systems, the complex heterogeneous nature of reacting systems, the reactions occurring at interfaces, and the small ratio of interfaces to the bulk, which makes suitable experimental probes difficult to find. Ferecrystals are layered compounds with the general formula [(MSe)1+δ]m[TSe2]n (M = RE,Bi,Pb,Sn; T = transition metal) where m MSe and n TSe2 units are stacked along the c axis. They are synthesized by physical vapor deposition via a multi-layer precursor consisting of layers of the individual elements with only a few Ångströms in thickness. Short diffusion lengths and designable layering sequences in the precursor lead to a large variety of kinetic compounds with more than 20,000 potential compounds for m,n ≤ 10 for each combination of M and T. The precursors for ferecrystals have a large interface to bulk ratio and the interfaces occur periodically, which makes them a good candidate for mechanistic studies since processes at the interface can be probed much better than in traditional solids. Analogous to molecular chemistry, knowing the mechanism of formation of these compounds could lead to the targeted synthesis of new materials with desired properties. The compound [(SnSe)1.15][VSe2] was chosen as a model system for our mechanistic investigations. Specular XRD shows the formation of superlattice peaks forming between 100 °C and 200 °C. DSC shows an exothermal event at around 130 °C, indicating a single nucleation event. To investigate the formation more in-depth, the precursor was annealed at different temperatures (100 °C, 200 °C, 300 °C, and 400 °C) and analyzed using various diffraction, microscopy and spectroscopy techniques. In-plane x-ray diffraction data which shows the simultaneous growth of SnSe and VSe2 hk0 reflections that grow in intensity and get sharper until 400 °C. Additionally, reflections of SnSe2 can be observed which vanish at 400 °C, coinciding with the loss of Se according to EPMA. EXAFS and TEM will be used to gain information about local structures to get detailed insight about the structure of the intermediates.

Mechanistic studies in kinetically controlled solid state synthesis: The case of [(SnSe)1.15][VSe2]

249th National Meeting (2015)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.