Low-temperature Mossbauer study of heterosite, (Fe, Mn)PO4.

Abstract

The heterosite phase occurring in a pegmatitic rock sample was characterized by X-ray diffraction, by energy-dispersive X-ray spectroscopy and by Mossbauer spectroscopy. The orthorhombic unit-cell parameters, expressed in A˚ , were found as a = 9.733 (1), b = 5.837 (1) and c = 4.776 (1). The composition was determined to be (Fe0.54Mn0.43Mg0.04)PO4. Mossbauer spectra recorded at temperatures T of 65K and higher consist of two broadened quadrupole doublets. Their isomer shifts ı are both diagnostic for the ferric state. The dominant doublet (∼60% of total area) exhibits an average quadrupole splitting _EQ,av of 1.62mm/s at room temperature, while the weaker broader doublet has _EQ,av = 0.68 mm/s. For temperatures T≤60K the spectra are composed of a broad sextet and a central quadrupole doublet. The doublet persists down to the lowest applied temperature of 17 K. It is concluded that this doublet is due to an Fe-bearing phase other than heterosite and which gives rise to the inner doublet appearing in the spectra recorded at T≥65 K. The broad sextets, attributable to the heterosite phase, were fitted with model-independent hyperfine-field distributions. However, it was consistently experienced that using the common Lorentzian-shaped elementary sextets composing the distribution, could not adequately reproduce the observed line shapes. Instead, the calculations had to be based on the diagonalization of the complete hyperfine-interaction Hamiltonian. This is due to the unusually strong quadrupole interaction. The as-such calculated hyperfine parameters of the heterosite phase at 17K may be summarized as follows: maximum-probability hyperfine field Bhf,m = 473 kOe, isomer shift ıFe = 0.54 mm/s, average quadrupole coupling constant ½e2qQ = 1.50 mm/s, asymmetry parameter of the EFG _ = 0.80, and polar angles of the hyperfine field with respect to the EFGs principal axes frame˝=40◦ and _ =90◦. The temperature variation of the hyperfine field was interpreted in terms of the Bean–Rodbell (BR) model. The BR parameter, _BR, was found to be 0.90, indicating a first-order magnetic transition at TN = 59.7 K. The temperature variation of the isomer shift is explained by the second-order Doppler shift ıSOD. Using the Debye model for the lattice vibrational spectrum for calculating ıSOD, the characteristic Mossbauer temperature _M was found to be 400 K, which is unusually low for a ferric compound.