Studies on structural and thermo-chemical behavior of MFe 12O 19(s) (M = Sr, Ba and Pb) prepared by citrate–nitrate gel combustion method

Abstract

The citrate–nitrate gel combustion route was used to prepare SrFe 12O 19(s), BaFe 12O 19(s) and PbFe 12O 19(s). The compounds were characterized by Rietveld profile analysis of the X-ray powder diffraction data. The thermal expansion coefficients of MFe 12O 19 (M = Sr, Ba and Pb) were determined using high temperature powder X-ray diffraction data. The lattice parameters of these compounds were fitted into polynomial expressions as a function of temperature. Melting points of MFe 12O 19 (M = Sr, Ba and Pb) were determined using high temperature calorimeter and the values are; 1632 K, 1611 K and 1538 K, respectively. Standard molar heat capacities of BaFe 12O 19(s) and PbFe 12O 19(s) were determined from 130 K to 850 K using a heat flux type differential scanning calorimeter (DSC) and heat capacity anomalies were observed at 725 K and 718 K, respectively. This heat capacity anomaly is due to the magnetic order–disorder transition from ferrimagnetic state to paramagnetic state.