<p>Ternary compounds of the Cu–X–Q system (where X = Fe, Sb, Sn and Q = S, Se), such as Cu₅FeS₄, Cu₃SbS₄, and Cu₂SnSe₃, have garnered considerable attention for their potential applications in electronics, optics, and energy technologies. These compounds are noted for their low thermal conductivity and narrow band gaps, making them promising candidates for thermoelectric materials. However, detailed experimental investigations into the phase transitions and thermoelectric properties of synthetic hakite, particularly with Ni substitution, have been limited. This study focused on synthesizing Ni-substituted hakite (Ni_xCu_12−xSb₄Se₁₃; x = 0.5–2) through mechanical alloying and hot pressing techniques, while also exploring the phase transitions and thermoelectric characteristics as a function of Ni content. Despite the charge compensation effect of Ni, a pure hakite phase could not be achieved. Instead, the resultant phases comprised mixtures of secondary phases including bytizite, pribramite, and permingeatite, or their composites. This indicates that the introduction of Ni into the system did not promote the formation of a single-phase hakite but rather stabilized a multi-phase system. The introduction of Ni resulted in a decrease in electrical conductivity across all specimens. Notably, the materials exhibited non-degenerate semiconductor behavior. The measured Seebeck coefficients were significantly high and positive, confirming p-type behavior. However, these coefficients decreased with increasing temperature. The thermal conductivity of the materials displayed minimal temperature dependence, consistently remaining below 0.65 Wm⁻¹&#xa0;K⁻¹. This low thermal conductivity is advantageous for thermoelectric efficiency, as it minimizes heat loss while maintaining charge transport. For the composition Ni_0.5Cu_11.5Sb₄Se₁₃, we achieved a maximum power factor of 0.09 mWm⁻¹&#xa0;K⁻² and a peak dimensionless figure of merit (ZT) of 0.18 at 623&#xa0;K.</p> Graphical Abstract <p></p>