Antimony desinsertion reaction from SbxCoSb3-x

The compound SbxCoSb3-x was produced at 7.7 GPa and 550 ºC in a self-insertion reaction from the binary skutterudite CoSb3. 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, Sbx CoSb3-x→CoSb3) occurs when SbxCoSb3-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 SbxCoSb3-x phase. An endothermic peak around 150 ºC was assigned to a small change in the position of the guest Sb atoms inside the cages of SbxCoSb3-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 ºC. Accordingly, the guest Sb atoms in SbxCoSb3-x shift from the 12d (x,0,0) site of Im3 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 ºC. An exothermic event starting at 180 ºC is the thermal signature of the desinsertion of guest Sb atoms from the Sbx CoSb3-x skutterudite cages, as confirmed by x-ray diffraction analysis and further verified by electrical measurements. After heating to 350 ºC, SbxCoSb3-x samples fully convert back to CoSb3. The Sb desinsertion reaction from Sbx CoSb3-x follows a first order kinetics, with a transition enthalpy of approximately 21 kJ/mol and an activation energy of 83 kJ/mol