We report the realization of a metallic flat band in the marcasite compound ${\mathrm{MnSb}}_2$. The material was synthesized under high pressure, and its crystal structure was established by x-ray diffraction. Electrical and thermoelectric transport measurements reveal signatures of a flat band near the Fermi level that persist across three structural, magnetic, and electronic transitions. These observations are supported by extensive density-functional-theory calculations. Related marcasite diantimonides, including ${\mathrm{FeSb}}_2$ and ${\mathrm{CrSb}}_2$, also host flat bands, but these lie far from the Fermi level and therefore do not produce a metallic flat-band state. ${\mathrm{MnSb}}_2$ therefore emerges as the only known metallic marcasite antimonide in which a flat band lies in the vicinity of the Fermi level.

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• Electrical properties
• Flat bands
• Metals
• Crystallography
• Density functional theory
• Liquid helium cooling
• Magnetotransport
• Specific heat measurements
• Susceptibility measurements
• X-ray diffraction