Crystallographic investigation of the iron phosphate tungsten bronze (Fe-PWB)

Abstract

In this paper, 12-tungstenphosphoric acid (PWA) was synthesized in combination with FeCl3 at room temperature (25 °C). At such manner, Fe3+ ion exchange gave new 12-tungstenphosphoric salt of the transition metal iron (FePW12O40×nH2O; Fe-PWA). Thermal analysis determined the temperature of about 596 °C of the phase transition, i.e., the temperature at which the structure of the Kegin anion is disturbed. Therefore, it was chosen temperature above the breakdown of the Kegin anion of 650 °C, and which is required to obtain phosphate tungsten bronzes (PWB) doped with iron (Fe-PWB). The sample was kept in the oven for 10 min. Such obtained new Fe-PWB doped bronze was further investigated by the X-ray powder diffraction (XRPD) and Rietveld methods. The XRPD patterns of Fe-PWA and Fe-PWB were taken in the 3–90° 2θ angle range, and clearly reveal crystallographic and structural differences between these two phases. Determined unit-cell parameters of Fe-PWB obtained by the Rietveld method in the monoclinic crystallographic system are as following: a0 = 7.53(2) Å; b0 = 7.51(1) Å; c0 = 7.64(1) Å; β0 = 89.7(2)° and V0 = 431(2) Å3. These unit-cell parameters were compared with PWB, as well as other previously characterized doped bronzes (LiPWB and Ca-PWB). It can be concluded that inserting of the Fe3+ ion into the PWB’s structure was undoubtedly proven, and have the most influence to the axis a0 (i.e. it significantly increased), angle β0 (i.e., it significantly decreased), and volume V0 (i.e., it significantly increased). On the other hand, influence to the axis c0 is quite smaller (i.e. it slightly decreased), whereas influence to the axis b0 is minor. Such behavior is also very different in comparison to the Li-PWB and Ca-PWB.