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

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 circle 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 circle 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. (c) 2022 Elsevier B.V. All rights reserved.