p–n Transition in Thermoelectric Semiconductor Eskebornite

Eskebornite (CuFeSe₂) is a I-III-VI₂ semiconductor with a tetragonal crystal structure, known for its intriguing electrical and magnetic properties. However, experimental studies on this material remain scarce. In this study, Ni-doped eskebornite, Cu_1-xNi_xFeSe₂ (x = 0.02-0.06), was synthesized via solid-state methods by substituting Ni²⁺ for Cu⁺. Mechanical alloying was employed to prepare the compounds, followed by hot pressing. X-ray diffraction analysis revealed the eskebornite phase alongside a minor secondary phase, identified as penroseite (NiSe₂) with a cubic crystal structure. Thermoelectric properties were measured over the temperature range of 323-623 K. The Seebeck coefficient exhibited p-type behavior at low temperatures but transitioned to n-type at higher temperatures, indicating a temperature-dependent p-n transition due to changes in the dominant charge carriers. With increasing Ni doping, the Seebeck coefficient increased positively at low temperatures and negatively at high temperatures, with the p-n transition temperature shifting to lower values. Electrical conductivity decreased with higher Ni doping levels, while its positive temperature dependence became more pronounced, reflecting non-degenerate semiconductor behavior. Thermal conductivity showed a negative temperature dependence but increased with higher Ni content. The highest thermoelectric performance was observed for Cu_0.98Ni_0.02FeSe₂, achieving ZT_p = 0.30 × 10^-3 at 523 K, and for Cu_0.94Ni_0.06FeSe₂, achieving ZT_n = 0.55 × 10^-3 at 623 K, where ZT_p and ZT_n represent the dimensionless figure of merit for p-type and n-type thermoelectric materials, respectively.