Realization of Z2 Topological Metal in Single‐Crystalline Nickel Deficient NiV2Se4

Temperature‐dependent electronic and magnetic properties are reported for nickel‐deficient NiV2Se4. Single‐crystal X‐ray diffraction shows it to crystallize in the monoclinic Cr3S4 structure type with space group I2/m$I2/m$ and vacancies on the Ni site, resulting in the composition Ni0.85V2Se4 in agreement with our electron‐probe microanalysis. Structural distortions are not observed down to 1.5 K. Nevertheless, the electrical resistivity shows metallic behavior with a broad anomaly around 150–200 K that is also observed in the heat capacity data. This anomaly indicates a change of state of the material below 150 K. It is believed that this anomaly could be due to spin fluctuations or charge‐density‐wave fluctuations, where the lack of long‐range order is caused by vacancies at the Ni site of Ni0.85V2Se4. The non‐linear temperature dependence of the resistivity as well as an enhanced value of the Sommerfeld coefficient γ=104.0(1)$\gamma = 104.0\,(1)$ mJ mol−1 K−2 suggest strong electron–electron correlations in this material. First‐principles calculations performed for NiV2Se4, which are also applicable to Ni0.85V2Se4, classify this material as a topological metal with Z2=(1;110)$Z_2 = (1;110)$ and coexisting electron and hole pockets at the Fermi level. The phonon spectrum lacks any soft phonon mode, consistent with the absence of periodic lattice distortion in the present experiments. Growth of single crystals of nominal NiV2Se4 has produced nickel‐deficient Ni0.85V2Se4. Vacancies at the nickel site explain the absence of a charge‐density‐wave (CDW) transition, while the temperature dependencies of physical properties exhibit broad anomalies, indicative of spin or CDW fluctuations. Density‐functional‐theory calculations reveal that Ni0.85V2Se4 is a Z2 topological metal.