Skutterudites: Their structural response to filling

The skutterudite structure, M₄Pn₁₂ (cubic Im-3̄), where M = Group VIII transition metal and Pn = pnicogen, can be filled to varying degree by up to one atom per formula unit of a lanthanide, actinide, alkaline earth, alkali, or thallium atom, A_1-xM ₄Pn₁₂. Skutterudite phases are of interest as promising thermoelectric materials. The available database of all reported crystal structure refinements of filled and unfilled skutterudites are analyzed to identify common structural responses to filling. As the filling fraction increases, the cell volume increases by about 3.6%, both the y and z positional parameters of the 24g Wyckoff sites occupied by the pnicogen increase, the four-membered pnicogen ring becomes more square, and the mean-square displacement of the filling atom decreases. Details of the structural response to filling can be correlated with physical properties and used as input to build more accurate theoretical models. Crystal structure refinements of neutron powder diffraction data also are reported for three Tl-filled skutterudites, Tl_0.25Co₄Sb₁₂, Tl_0.51Co ₄Sb_11.5Sn_0.5, and Tl_0.75Co ₃FeSb₁₂.