Between carbide and nitride MAX phases: sol–gel assisted synthesis and characterization of the carbonitride phase Cr2GaC1−xNx

Abstract

MAX phases are almost exclusively known as carbides, while nitrides and carbonitrides form a significantly underrepresented subgroup even though they have been shown to possess enhanced properties in comparison to their carbide counterparts. One example is the nitride phase Cr₂GaN which exhibits a spin density wave magnetic state below T = 170 K, while the metallic carbide phase Cr₂GaC follows the MAX phase-typical Pauli-paramagnetic behavior. To investigate the influence on the materials/functional properties of mixing carbon and nitrogen on the X-site, this study aims to synthesize and comprehensively characterize the hitherto unknown carbonitride phase Cr₂GaC_1−xN_x and compare it to the parent phases. Due to the challenging synthesis of (carbo)nitrides in general, a sol–gel-assisted approach is applied which was recently developed by our group. This process was further improved by using time-efficient microwave heating, leading to a highly phase pure product. STEM-EDX analyses reveal a C/N ratio of roughly 2 : 1. Temperature-dependent XRD measurements confirm the literature-known magnetic phase transition of the parent nitride phase Cr₂GaN, while the incorporation of carbon suppresses the latter. Nonetheless, magnetic characterization of the phases reveals that the magnetic behavior can be specifically influenced by changing the composition of the X-site, resulting in an increase of the susceptibility by increasing the nitrogen amount. Overall, these findings further substantiate the big potential in nitrogen-containing MAX phases, which will also serve as starting materials for future doping studies, i.e. on the M- and A-site, and as precursors for novel 2D MXenes.