Tuning structural instability in Cu12−xNixSb4S13 (x = 0, 0.05, 0.2, 0.5) tetrahedrites

Abstract

Cu_12−xNi_xSb₄S₁₃ (x = 0, 0.05, 0.2, 0.5) tetrahedrites were synthesized by the polyol method. Measurements of magnetic susceptibility, specific heat capacity as well as electrical and thermal transport properties indicated a structural phase transition (SPT) of 1^st order at T_k = 75–85 K, which is accompanied by strong changes in electronic density of states (EDOS) and entropy for x = 0, 0.05, and 0.2, whereas only weak changes were observed at x = 0.5. However, further temperature-dependent synchrotron high-resolution powder X-ray diffraction (HR PXRD) studies revealed compounds with Ni-content x = 0 and 0.5 to remain body-centered cubic [space group (SG) I3m, a ≈ 10.4(1) Å] down to 10 K, whereas crystal structures of x = 0.05 and 0.2 became tetragonal (SG Pc2, , c_tetr ≈ a_cub) below T_k. Comparing changes of EDOS at the Fermi level [ΔN^(E_F)] deduced from magnetic and thermodynamic data with those calculated from density functional theory (DFT) in a rigid-band approximation, we found that decreases with Ni content and vanishes for x = 0.5, thus explaining the absence of the tetragonal instability at higher Ni dopings. This study shows that the SPT in tetrahedrites is facilitated by the presence of a minor amount of dopant.