MÖSSBAUER STUDIES OF NATURAL AND SYNTHETIC FERROUS PHOSPHATES OF THE HOMOLOGOUS SERIES Fe2+3(PO4)2(H2O )n

Abstract

The present work reports successful results on the synthesis of the hydrated ferrous phosphates of the homologous series Fe2+3(PO4)2(H2O)n. Three compounds occur as mineral : vivianite n = 8, ludlamite n = 4 and phosphoferrite n = 3. By hydrothermal synthesis with vivianite as starting material we have synthetised the other minerals as well as three new species : (A) Fe3(PO4)2(H2O)4, (A) Fe3(PO4)2(H2O)3, dimorphous to the minerals ludlamite and phosphoferrite respectively, and the monohydrate Fe3(PO4)2(H2O). Mossbauer spectra shows that in the three mineral species and in (A) Fe3(PO4)2(H2O)3 the Fe2+ ion occupies two crystallographic non-equivalent sites with relative population 2 : 1. In the synthetized compounds (A) Fe3(PO4)2(H2O)4, Fe3(PO4)2(H2O) the three sites of the Fe2+ ion are none quivalent. Of the new synthetized species only for the monohydrate the three dimensional structure has been determined (P.B. Moore, personal communication). The three sites for Fe2+ are non-equivalent, Fe(1) and Fe(3) are distorted octahedra but Fe(2) is only five coordinated, a distorted square pyramid. Octahedra edge sharing is one of the main features of this unusual structure. The correlation of the Mossbauer parameters (Q.S.I.S.) with the hydration number n indicates that with decreasing number of coordinated H2O molecules the parameters also decrease, as is observed in other series of hydrated iron compounds. With Fe3(PO4)2 . 3 H2O on starting material we have prepared by oxydation a polycrystalline sample of synthetic Krizhanovskite Fe2+Fe3+2(PO4)2(OH)-2(H2O). In the temperature range between 80 K and 300 K the Mossbauer spectra show a perfect cationic ordering, with the Fe2+ and Fe3+ ions occupying different crystallographic sites. At temperatures below TN = 162.5 K, the spectra show the presence of a long range magnetic ordering together with a local quadrupolar interaction at the Fe2+ sites of the same order of magnitude. The spectra have been interpreted following the analytical method proposed by Williams and Bancroft (Chem. Phys. Lett. 2 (1969) 110) and recalculated by us for the case I = 3/2 leading to slightly different final equations. The temperature dependence of the hyperfine magnetic fields H for the Fe3+ and Fe2+ follows the familiar law H ∝ (1 - T/TN)β. The experimental data can be fitted with TN = 162.5 ± 2.5 K and β = 0.25 ± 0.01. It is important to note that the values of these parameters are the same, for the two sets of Fe ions, within the accuracy of our experimental data.