Controlling structural instability in the Cu$_{12-x}$Ni$_x$Sb$_4$S$_{13}$ ($x$\,=\,0, 0.05, 0.2, 0.5) tetrahedrites

Abstract

Cu12−xNixSb4S13 (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 1st order at Tk=75−85,K, which is accompanied by strong changes in electronic density of states (EDOS) and entropy for x=0,0.05,0.2 whereas only weak changes were observed at x=0.5. However, further temperature dependent synchrotron high-resolution powder X-ray diffraction (HR PXRD) studies indicated compounds with Ni-content x=0 and 0.5 to remain body-centered cubic [space group (SG) I4¯3m, a≈10.4(1),\AA] down to 10,K, whereas crystal structures of x=0,0.05 and 0.2 became tetragonal, (SG P4¯c2, atetr≈acub2, ctetr≈acub) below Tk. Comparing changes of EDOS at the Fermi level [ΔN(EF)] deduced from magnetic and thermodynamic data with those calculated from density functional theory (DFT) assuming the validity of the rigid-band approximation, we found ΔNtheor(EF)≈0 when compared with cubic and tetragonal arragements for Cu11.5xNi0.5Sb4S13, which indicate the latter to be unstable. This study shows that SPT in tetrahedites is facilitated by the presence of minor amount of a dopant.