Systematically theoretical research was performed on the monazite- and zircon-structure RXO(4) (R = Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; X = P, As) series by using the chemical bond theory of dielectric description. The chemical bond properties of R-O and X-O bonds were presented. In the zircon phase, the covalency fractions of X-O bonds increased in the order of V-O < As-O < P-O, which was in accordance with the ionic radii and electronegative trends, and the covalency fractions of R-O bonds varied slightly due to the lanthanide contraction. While in the monazite phase, both R-O and X-O bonds were divided into two groups by their covalency fractions. The contributions from the bond to the lattice energy, linear thermal expansion coefficient (LTEC), and bulk modulus were explored. The X-O bonds with short bond lengths and high chemical valence made greater contributions to the lattice energy and performed nearly rigidly during the deformation. A regular variation of lattice energy, LTEC, and bulk modulus with the ionic radii of the lanthanides was observed in both monazite and zircon phases.

Three series of R 2 MoO 6 :Eu (R ) La, Gd, and Y) phosphor powders were synthesized by using the sol-gel method, and then the powder was annealed at a gradually decreased temperature from 1200 to 700 °C. The structures and spectra of the as-prepared phosphors were characterized by powder X-ray diffraction and photoluminescence measurements, respectively. The profile of the Y 2 MoO 6 :Eu particles was observed by means of field emission scanning electron microscopy. The excitation of the Mo-O groups yields the emission of Eu 3+ ions, indicating that an energy transfer process takes place from the Mo-O groups to Eu 3+ ions. With a decrease in annealing temperature, the intensities of the excitation and emission peaks decrease; at the same time, the position of the Mo-O excitation band shifts to the short wavelength direction. The variation of the Mo-O excitation band is discussed from three aspects: crystalline size, coordination environment of Mo, and structural symmetry of the host crystal.

No abstract