Antimony desinsertion reaction from Sb x CoSb 3− x

The compound Sb x CoSb 3 - x was produced at 7.7 GPa and 550 ∘ C in a self-insertion reaction from the binary skutterudite CoSb 3 . This self-insertion reaction is characterized by the collapse of some framework Sb atoms into the cages formed by the Co and Sb atoms in the skutterudite structure, as was further confirmed by Bader's analysis of maximum-entropy charge density maps obtained from synchrotron radiation x-ray powder diffraction data. The opposite reaction (i.e., Sb desinsertion, Sb x CoSb 3 - x → CoSb 3 ) occurs when Sb x CoSb 3 - x is heated above 180 ∘ C at ambient pressure. This desinsertion reaction was followed by means of differential scanning calorimetry, x-ray diffraction, and electrical resistivity measurements. Differential scanning calorimetry measurements revealed the presence of two thermal events in samples rich in the Sb x CoSb 3 - x phase. An endothermic peak around 150 ∘ C was assigned to a small change in the position of the guest Sb atoms inside the cages of Sb x CoSb 3 - x . This assignment was based on the analysis of charge density maps obtained from synchrotron x-ray diffraction measurements carried out both at room temperature and at 155 ∘ C . Accordingly, the guest Sb atoms in Sb x CoSb 3 - x shift from the 12d (x,0,0) site of Im 3 ¯ space group (in a position distant about 0.35 Å from the center of the cages), at room temperature, to the 2a (0,0,0) site (i.e., to the center of the cages) above 150 ∘ C . An exothermic event starting at 180 ∘ C is the thermal signature of the desinsertion of guest Sb atoms from the Sb x CoSb 3 - x skutterudite cages, as confirmed by x-ray diffraction analysis and further verified by electrical measurements. After heating to 350 ∘ C , Sb x CoSb 3 - x samples fully convert back to CoSb 3 . The Sb desinsertion reaction from Sb x CoSb 3 - x follows a first order kinetics, with a transition enthalpy of approximately 21 kJ/mol and an activation energy of 83 kJ/mol.