Then, the precursor powders were subjected to heat treatment at 350 C for 4 h in the air at a heatingrate of 1 C min1 and further calcination at 500, 700, 800, 850,900 C and 950 C for 12 h at a heating rate of 5 C min1 (CPsample). For comparative studies, a precursor without theapplication of PVP during the synthesis (CA sample) and aprecursor without citric acid and PVP during the synthesis (NCsample) were also prepared in a similar manner, and theproportion and pre-calcination conditions were the same asthose mentioned above for the CP sample.