Structural stability and thermal oxidation of CrSBr crystals in air studied at microscopic scales

2D materials with intrinsic magnetic ordering opened new grounds in spintronics and quantum information processing. Out of these CrSBr has been suggested to be an air stable 2D magnet. Herein, we ponder into the mechanisms behind structural stability and thermal oxidation of bulk CrSBr crystals in air. Our XAS and XPS studies show that long-time storage of this material in air produces Br vacancies as well as near-surface defective S and Cr atoms. Experimental AFM, XAS and XPS studies supported by DFT calculations show that heating CrSBr crystals produces vacancies first and then some surface-bound oxides/hydroxides, which increase the surface micro-roughness. In particular, fifteen minutes heating at 270 °C leads to Br and S vacancies as well as presence of sulfates at the surface but without any Cr oxides. However, 15 min heating at 330 °C induces oxidation of Cr atoms into Cr2O3 and/or Cr(OH)3. Furthermore, defective CrSBr undergoes a unique thermal oxidative etching with formation of sub-surface square etch pits. This leads to occasional bursts of the top-most layers once a maximum strain within the CrSBr membrane has been met. The DFT calculations support favorable oxygen incorporation underneath the top-most layers through the Br vacancies and passivation of the S vacancies in the top-most layer.