Mössbauer-Effect Study of Calciomagnetites

Ca$\text{}_{x}$Fe$\text{}_{3-x}$O$\text{}_{4}$ samples, with x ≤ 0.28 , prepared from α-Fe$\text{}_{2}$O$\text{}_{3}$ and CaO by the Bridgman method were investigated by means of $\text{}^{57}$Fe Mössbauer spectroscopy. Room temperature spectra gave evidence that the samples were multiphase, the calciomagnetite being the dominant phase with a contribution of 73% to 99%, depending on the sample. Analysis of the subspectra associated with the calciomagnetite gave evidence that Ca-ions reside on the tetrahedral (A) sites and have not random distribution. Their presence on those sites causes a reduction of the hyperfine field at Fe nuclei occupying the octahedral positions (B) by 12.3(1.0) kOe per one Ca-ion and a decrease in the average hf field, 〈H$\text{}_{B}$〉/dx, at the rate of 29 kOe/mol% for x ≤ 0.07 and of 387 kOe/mol% for larger x. In addition, a linear correlation between 〈H$\text{}_{B}$〉 and the average isomer shift, 〈I$\text{}_{B}$〉, was revealed to hold. It indicates that addition of Ca-ions into the magnetite causes a decrease in the spin-density and an increase in the charge-density at Fe nuclei occupying the B-sites.