The role of lattice defects for structural, mechanical, and physical properties of HPT processed p-type skutterudite DD0.7Fe3CoSb12

HPT (high pressure torsion)-processed samples of p-type skutterudite DD_0.7Fe₃CoSb₁₂ were systematically investigated for clarifying the mechanisms behind the enhancements of mechanical properties and particularly of the thermoelectric figure of merit ZT. For the first time we combined experiments (differential scanning calorimetry, X-ray diffractometry, energy dispersive spectroscopy, and scanning electron microscopy) with calculations by density functional theory (DFT). The results demonstrated that the individual thermal stabilities of lattice defects from HPT-processing with special respect to their densities and arrays are responsible for the properties observed. The mechanical properties are mainly governed by dislocations and grain boundaries, while the thermoelectric figure of merit ZT is affected by the generation and annihilation of vacancy type defects: These allow for the formation of low-angle grain boundaries out of the HPT induced dislocations with increasing misorientation between the grains as a function of deformation and annealing temperature. In contrast to simply entangled dislocation cell walls, these low/high angle grain boundaries account for a minimum of the product of resistivity and thermal conductivity, and thus of a maximum of ZT. DFT calculations not only provided formation energies of vacancies for the three atom sites in NdFe₄Sb₁₂ and NdCo₄Sb₁₂, but also insight on the stabilities of these compounds. The Nd vacancy formation in NdCo₄Sb₁₂ is the least energy-demanding, which makes the structure more stable than e.g. the Nd vacancy formation in NdFe₄Sb₁₂, where decreasing electron deficit competes with increasing structure distortion.