Multiscale study on the formation and evolution of the crystal and local structures in lanthanide tungstates Ln2(WO4)3

Abstract

The influence exerted by the specific type of the lanthanide cation and calcination temperature on the crystal and local structures of Ln2(WO4)3 tungstates (Ln = La–Dy) prepared by a coprecipitation is studied using synchrotron X-ray diffraction, X-ray absorption fine structure (XAFS) spectroscopy, Fourier transform infrared (FT-IR) and Raman spectroscopies, photoluminescence, simultaneous thermal analysis, and inductively coupled plasma atomic emission spectroscopy. The combination of these experimental techniques enabled a structural insight into Ln tungstates at multiple characteristic scales, i.e. short-range of metal atom coordination (XAFS), medium-range of the network of chemical bonds (FT-IR and Raman spectroscopies), and long-range or 3D periodicity within crystallites (XRD). It is found that the onset of amorphous precursor crystallization is observed at 575–600 ∘C/3 h and leads to the formation of Ln2(WO4)3 nanocrystalline powders with a monoclinic (sp. gr. C12/c1 (15)) structure. An increase in the calcination temperature leads to the growth of crystallite size and a decrease in microstrains. In the case of Dy2(WO4)3 an additional orthorhombic phase emerges (sp. gr. Pbcn(60)) at 1000 ∘C. It is shown that the local structure of all well-crystallized compounds being studied contains lanthanide ions in the form of Ln3+ and tungsten ions in the form of WO42− tetrahedra. The local structure in the monoclinic phase can be represented as a superposition of two non-equivalent tungstate tetrahedra: W(1)O4 (C2 site symmetry) and W(2)O4 (C1 site symmetry). The LnO8 polyhedra are strongly irregular, and the Ln3+ cations occupy low-symmetry sites.