We report the crystal growth, structure, physical properties, and first-principles calculations of a vanadium-based oxytelluride ${\mathrm{Cs}}_{1-\delta }{\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$. The material possesses two-dimensional ${\mathrm{V}}_2\mathrm{O}$ square nets sandwiched by tellurium layers, with local crystallographic symmetry satisfying the spin symmetry for a $d$-wave altermagnet. An antiferromagnetic transition at 293 K is unambiguously evidenced from the measurements of magnetic susceptibility and specific heat. In addition, a secondary transition at $\sim 70\mathrm{K}$ is also observed, possibly associated with a Lifshitz transition. The first-principles calculations indicate robust Néel-type collinear antiferromagnetism in the ${\mathrm{V}}_2\mathrm{O}$ plane. Consequently, spin splittings show up in momentum space, in relation to the real-space mirror/rotation symmetry. Interestingly, the V-$d_{yz}/d_{xz}$ electrons, which primarily contribute to the quasi-one-dimensional Fermi surface, turns out to be fully orbital and spin polarized, akin to the case of a half metal. Our work lays a solid foundation for the potential applications utilizing altermagnetic properties in vanadium-based oxychalcogenides.

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• Electronic structure
• Magnetic susceptibility
• Structural properties
• Altermagnets
• Single crystal materials
• Density functional calculations