Magnetic, Mössbauer and optical spectroscopic properties of the AFe3O(PO4)3 (A = Ca, Sr, Pb) series of powder compounds

AFe₃O(PO₄)₃ (A = Ca, Sr and Pb) powder compounds were studied by means of X-ray diffraction (XRD), electron-probe microanalysis (EPMA) coupled with wavelength dispersion spectroscopy (WDS), Raman and diffuse reflectance spectroscopies, specific heat and magnetic properties measurements. Magnetization, magnetic susceptibility and specific heat measurements carried out on AFe₃O(PO₄)₃ (A = Sr, Ca and Pb) powders firmly establish a series of three ferromagnetic (FM)-like second order phase transitions spanned over the 32-8 K temperature range. Room temperature Mössbauer spectroscopy and associated DFT calculations confirm the existence of three crystallographically non equivalent Fe³⁺ sites in the three compounds. Mössbauer spectra recorded as a function of temperature in the PbFe₃O(PO₄)₃ compound also establishes the occurrence of two purely magnetic and reversible phase transitions at 32 and 10 K. Diffuse reflectance measurements reveal two broad absorption bands at 1047 and 837 nm, in both PbFe₃O(PO₄)₃ and SrFe₃O(PO₄)₃ powders, with peak cross sections ∼10^-20 cm² typical of spin-forbidden and forced electric dipole intraconfigurational transitions.