Effects of gas pressure and temperature on the synthesis and formation mechanism of hollow carbon spheres (HCSs) prepared in argon atmosphere using ferrocene and ammonium chloride as reactants are investigated. The reactions occurred in the process as well as the formation mechanism of the HCSs is also studied. Samples are characterized by X-ray diffraction, field-emission scanning electron microscopy, differential thermal analysis, differential scanning calorimetry analysis and thennogravimetric analysis. It is found that the quasi-static atmosphere is crucial for the formation of the HCSs. The yield of HCSs can be enhanced by increasing the initial argon pressure and reaches a maximum value at 2 MPa. When the initial argon pressure is 2 MPa, HCSs can be fabricated at 450 degrees C and yield of the HCSs is increased with temperature up to 600 degrees C. However, the diameters of the HCSs are several micrometers and independent on the argon pressure and temperature in the given ranges. Several iron compounds, including Fe(NH3)(2)Cl-2, Fe(NH3)(6)Cl-2, (NH4)FeCl5, NH4FeCl3 and FeCl2, are formed in different temperature ranges. The spherical Fe(NH3)(2)Cl-2 droplets formed in the process is thought to serve as the core templates for the formation of the HCSs. (c) 2008 Elsevier B.V. All rights reserved.