The 12442-type Fe-based superconductor is the only system that possesses two FeAs layers between neighboring insulating layers, which is worth the in-depth investigations. In this work, millimeter-sized single crystals of KCa2Fe4As4F2 were grown using a self-flux method. The chemical compositions and crystal structure were characterized carefully. Superconductivity with the critical transition T c = 33.5 K was confirmed by both the resistivity and magnetic susceptibility measurements. Moreover, the upper critical field H c2 was studied by the resistivity measurements under different magnetic fields, where an anisotropy of 8 was revealed near the superconducting transition. Importantly, a rather steep increase for the in-plane H c2 ab with cooling, dμ0 H c2 ab /dT| T c = −50.9 T/K, was observed. This value is several times higher than that of other systems of Fe-based superconductor and indicates an extremely high upper critical field. Possible origins for this behavior were discussed. The finding in our work is a great promotion both for understanding the physical properties and for the high-field applications of 12442-type Fe-based superconductors.

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