Abstract
Mechanochemistry, as a synthesis tool for inorganic materials, became an ever‐growing field in material chemistry. The direct energy transfer by collision of the educts with the milling media gives the possibility to design environmental‐friendly reactions. Nevertheless, the underlying process of energy transfer and hence the kinetics of mechanosynthesis remain unclear. Herein, we present in situ synchrotron X‐ray diffraction studies coupled with pressure measurements performed during the formation of ZnS and the subsequent phase transition (PT) from the hexagonal to the cubic modification. Milling Zn and S8 results in the sublimation of S8, observed by a sudden pressure increase. Simultaneously, the hexagonal metastable ZnS‐modification (wurtzite) forms. Via detection of the pressure maximum, the exact start of the wurtzite formation can be determined. Immediately after the formation of wurtzite, the structural PT to the thermodynamic stable cubic modification sphalerite takes place. This PT can be described by the Prout‐Tompkins equation for autocatalytic reactions, similar to thermally induced PT in sulfur vapor at high temperatures (T>1133 K). The increase in the reactivity of the wurtzite formation is explained by the reaction in sulfur vapor and the induction of defect structures by the collisions with the milling media.
Highlights
Mechanochemistry, which is already known since the stone age,[1] is defined as the induction of a reaction via the direct absorption of the mechanical energy transferred from the milling media to the reactants.[1,2]
An extensive decomposition reaction would be expected.[1b,2b] local heating, possible eutectic melting, the generation of new surfaces as well as of defects, improved contacts between solids and diffusing atoms can be considered effective for all mechanically synthesized solids.[1b,3a,7] This results in different reaction pathways to those observed in classical reactions.[3d]. The tetragonal α-PbO transforms upon thermal annealing to the orthorhombic β-PbO around 763–813 K,[8] while by heating β-PbO to 673 K no evidence for a phase transition to α-PbO is observed even after 24 h.[9]
The metastable wurtzite structure can be synthesized via mechanochemistry in a shaker mill in an enormously fast reaction
Summary
Problematic and in addition, recovery rates usually do not exceed 50–80 %.[2b]. Because of these advantages, mechanochemical reactions are nowadays applied in various fields in chemistry to synthesize materials for applications, such as solidstate hydrogen storage, catalysis, electrodes/ electrolyte for solid batteries, fuel cells, or functional ceramics.[1b]. Temperatures above 104 °C can be generated, associated with transient plasmas and the ejection of energetic species including free electrons.[2b,6] These models are not suitable to describe mechanisms involved in the synthesis of molecular organic compounds or metalorganic framework materials For such reactions, an extensive decomposition reaction would be expected.[1b,2b] local heating, possible eutectic melting, the generation of new surfaces as well as of defects, improved contacts between solids and diffusing atoms can be considered effective for all mechanically synthesized solids.[1b,3a,7] This results in different reaction pathways to those observed in classical reactions (temperature- or pressure-induced).[3d] The tetragonal α-PbO transforms upon thermal annealing to the orthorhombic β-PbO around 763–813 K,[8] while by heating β-PbO to 673 K no evidence for a phase transition to α-PbO is observed even after 24 h.[9] During ball milling, no phase transformation of α- to βPbO is observed, whereas the phase transformation from β- to α-PbO can be detected already at room temperature in both planetary and shaker mills.[4b,10] The phase transition of β- to αPbO is even observed upon grinding in a mortar.[9] CaCO3 crystallizes in three polymorphic structures, the metastable hexagonal vaterite, the low temperature-stable orthorhombic aragonite, and the hexagonal calcite.[11] Thermally, both vaterite (T = 676–769 K) and aragonite (T = 730 K) transform to calcite as the most stable phase. In situ XRPD analyses enable real-time measurements[20b] and the study of the underlying reaction kinetics of the transformation of wurtzite to sphalerite
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