The effect of the synthesis conditions on the structure and phase transitions in Ln2(MoO4)3

The effect of the lanthanide cation type and calcination temperature on the structure and phase transitions in Ln(2)(MoO4)(3) molybdates was studied using synchrotron X-ray diffraction, infrared and Raman spectroscopy, photoluminescence, simultaneous thermal analysis and inductively coupled plasma atomic emission spectroscopy. It was found that the crystallization of the precursors at 500 degrees C/3 h results in the formation of nanocrystalline powders with the monoclinic structure (sp. gr. I112/b (15)) for Ln(2)(MoO4)(3) (Ln = La-Ho), and a mixture of orthorhombic (sp. gr. Pnca(60)) and tetragonal (sp. gr. I4(1)/a(88)) phases for Yb-2(MoO4)(3). A further increase in temperature to 1000 degrees C leads to a significant rearrangement of the crystal structure, which is largely determined by the lanthanide type. The diagram of phase relations in the Ln(2)(MoO4)(3) molybdates as a function of the Ln(3+) cation radius and the calcination temperature has been refined. According to the results of Raman and IR spectroscopy, MoO4 tetrahedra are the main structural units in Ln molybdates. The increased hygroscopicity of heavy lanthanide molybdates (Ln = Ho, Yb) is associated with the presence of empty cavities in the orthorhombic structure. The photoluminescence spectra have shown that EuOn polyhedra in Eu-2(MoO4)(3) are distorted, and Eu3+ ions occupy several nonequivalent crystallographically different sites with a low symmetry point group.