In-Depth Analysis of the Species and Transformations during Sol Gel-Assisted V₂PC Synthesis

The sol-gel reaction mechanism of 211 MAX phases has proven to be very complex when identifying the intermediate species, chemical processes, and conversions that occur from a mixture of metal salts and gelling agent into a crystalline ternary carbide. With mostly qualitative results in the literature (Cr₂GaC, Cr₂GeC, and V₂GeC), additional analytical techniques, including thermal analysis, powder diffraction, total scattering, and various spectroscopic methods, are necessary to unravel the identity of the chemical compounds and transformations during the reaction. Here, we demonstrate the combination of these techniques to understand the details of the sol-gel synthesis of MAX phase V₂PC. The metal phosphate complexes, as well as amorphous/nanocrystalline vanadium phosphate species (V in different oxidation states), are identified at all stages of the reaction and a full schematic of the reaction process is suggested. The early amorphous vanadium species undergo multiple changes of oxidation states while organic species decompose releasing a variety of small molecule gases. Amorphous oxides, analogous to [NH₄][VO₂][HPO₄], V₂PO₄O, and VO₂P₂O₇ are identified in the dried gel obtained during the early stages of the heating process (300 and 600 °C), respectively. They are carbothermally reduced starting at 900 °C and subsequently react to crystalline V₂PC with the excess carbon in the reaction mixture. Through CHN analysis, we obtain an estimate of left-over amorphous carbon in the product which will guide future efforts of minimizing the amount of carbon in sol gel-produced MAX phases which is important for subsequent property studies.