Mechanistic Insights into the Nonconventional Sol-Gel Synthesis of MAX Phase M₂GeC (M = V, Cr)

Despite their intriguing properties, MAX phases to date remain a class of materials overwhelmingly synthesized and studied with conventional approaches that date back to their discovery. With an ever-increasing demand for new and better materials and areas of application, developing new synthesis techniques must be at the forefront of our scientific efforts and cannot be overlooked. Sol-gel chemistry, while being a very traditional approach (especially for oxides), has so far hardly been leveraged within the MAX phase community. As a newly emerging technique to access nonoxide compounds, such as MAX phases, it offers a variety of advantages over classical solid-state chemistry, namely, milder reaction conditions and greater processibility (as previously shown for Cr2GaC). Here, the sol-gel synthesis of the two MAX phase members V2GeC and Cr2GeC, in combination with both conventional and nonconventional (microwave) heating techniques, is presented. In all instances, high yields were achieved, with only minor impurities remaining in the product. This expansion of the method to other members (apart from Cr2GaC) is a critical milestone in proving the technique's viability. Additionally, using simultaneous calorimetry and mass spectrometry, first insights into the underlying carbothermal reduction reaction are presented. Understanding the chemistry and formation mechanism will help broaden the sol-gel-based synthesis technique and increase its applicability.