Ni/SnSe2 interfacial reactions and Ni–Se–Sn phase equilibria

SnSe2 is a promising thermoelectric compound. Ni is a commonly used barrier layer material. To provide fundamental information, Ni–Se–Sn phase equilibria isothermal sections and Ni/SnSe2 interfacial reactions at 500 °C and 300 °C are determined. Significant and peculiar interfacial reaction results are observed. The reaction zone has several phase regions, and within each phase region there are various phases, and yet these regions have clear boundaries with different average compositions. In the Ni/SnSe2 couples reacted at 500 °C, there are three phase regions with average compositions equal to Ni5SnSe2, Ni3SnSe2 and NiSnSe2. In those reacted at 300 °C, there are four phase regions with average compositions of Ni5SnSe2, Ni3SnSe2, Ni2SnSe2 and NiSnSe2. The significant interfacial reactions indicate Ni is not a suitable barrier layer for SnSe2. The reaction phase regions all have planar interfaces and fine microstructures, but they are not single-phase. The experimental results show the microstructures are unaffected by the quenching rates and become coarser with longer annealing time. These results indicate the different phases within each phase region are not formed during quenching. The mechanisms of the peculiar phenomena need to be further explored. Yet, preliminary speculation is metastable ternary phases are formed with the rapid diffusion of Ni atoms into the SnSe2 substrate. The reaction phase regions grow with the formation of the metastable ternary phases, and the thin metastable ternary phases decompose into stable phases with fine microstructures.