The XRD pattern of the MgO sample calcined at 500 °C is presented in Fig. 3(1). All the peaks in Fig. 3(1) could be in- dexed to cubic MgO crystallites with the lattice parameter a =
0.421 nm using the standard values of JCPDS 45-946. No peak arising from an impurity was observed in Fig. 3(1).