Improving thermoelectric performance of CuCrSe₂ by manipulating the precipitated ferromagnetic spinel phase

As a superionic conductor, CuCrSe2 has recently attracted renewed attention in thermoelectric community for its phonon-liquid electron-crystal transport behaviours. Here, we demonstrate that the usually undesired second phase of ferromagnetic spinel CuCr2Se4 can play a significantly positive role in synergistically optimizing the thermoelectric properties of the CuCrSe2 compound. After systematically investigating the substitution behaviour of Ni for Cr, it is found that the contents of the ferromagnetic spinel phase can be controlled by virtue of the unique occupation preference of Ni in the tetrahedral and octahedral sites of CuCrSe2. A small amount of ferromagnetic CuCr2Se4 spinel phase essentially does not affect the weighted mobility but can act as carrier reservoirs in its ferromagnetic state, releasing additional holes into the matrix in the paramagnetic state. Furthermore, the presence of CuCr2Se4 precipitations in the CuCrSe2 matrix can drastically reduce the lattice thermal conductivity by generating extra phonon scattering mechanisms. Consequently, a zT value around 0.95, which is 33.8% higher than the absolutely pure sample, is achieved in the CuCr0 center dot 99Ni0 center dot 01Se2 sample with 3.64 wt % ferromagnetic spinel impurity.