Circumventing Diffusion in Kinetically Controlled Solid-State Metathesis Reactions

Solid-state diffusion is often the primary limitation in the synthesis of crystalline inorganic materials and prevents the potential discovery and isolation of new materials that may not be the most stable with respect to the reaction conditions. Synthetic approaches that circumvent diffusion in solid-state reactions are rare and often allow the formation of metastable products. To this end, we present an in situ study of the solid-state metathesis reactions MCl2 + Na2S2 → MS2 + 2 NaCl (M = Fe, Co, Ni) using synchrotron powder X-ray diffraction and differential scanning calorimetry. Depending on the preparation method of the reaction, either combining the reactants in an air-free environment or grinding homogeneously in air before annealing, the barrier to product formation, and therefore reaction pathway, can be altered. In the air-free reactions, the product formation appears to be diffusion limited, with a number of intermediate phases observed before formation of the MS2 product. However, grinding the reactants in air allows NaCl to form directly without annealing and displaces the corresponding metal and sulfide ions into an amorphous matrix, as confirmed by pair distribution function analysis. Heating this mixture yields direct nucleation of the MS2 phase and avoids all crystalline binary intermediates. Grinding in air also dissipates a large amount of lattice energy via the formation of NaCl, and the crystallization of the metal sulfide is a much less exothermic process. This approach has the potential to allow formation of a range of binary, ternary, or higher-ordered compounds to be synthesized in the bulk, while avoiding the formation of many binary intermediates that may otherwise form in a diffusion-limited reaction.

Pyrite formation via kinetic intermediates through low-temperature solid-state metathesis

The preparation of materials with limited phase stabilities yet high kinetic activation barriers is challenging. Knowledge of their possible formation pathways aids in addressing these challenges. Metathesis reactions present an approach to circumvent these barriers; however, solid-state metathesis reactions are often too rapid from extensive self-heating to understand the reaction. The stoichiometric reaction of MCl2 salts (M = Mn, Fe, Co, Ni, Cu, Zn) with Na2S2 enables the formation of pyrite (FeS2), CoS2, and NiS2 at low temperatures (250-350 °C). Na2S2 has the same polyanionic dimer as found in the pyrite structure, which would suggest the possibility of a facile ion-exchange reaction. However, from high-resolution synchrotron X-ray diffraction and differential scanning calorimetry, the energetic driving force does not appear to result solely from NaCl formation but also from formation of intermediate and pyrite phases. It is apparent that the reaction proceeds through polyanionic disproportionation and formation of a low-density alkali-rich intermediate, followed by anionic comproportionation and atomic rearrangement into the pyrite phase. These results have profound implications for the use of low-temperature metathesis in achieving materials by design.