Nd3+, Eu3+ and Yb3+ Ions as Structural Probes in the Scheelite-Type Cadmium Molybdates with Vacancies

Abstract

The main aim of this research is to demonstrate how important are the laser techniques like site selective spectroscopy and time-resolved spectroscopy at low temperature in determining the structure of compounds in complementarity of the usual XRD diffraction technique. Basing on the analyse of specific lines like the 5D0 ↔ 7F0 of Eu3+ ion, the 4I9/2 → 2P1/2, the 4I9/2 → 4F3/2 absorption transitions of Nd3+, ion as well as the zero-phonon line of the 2F5/2 ↔ 2F7/2 of Yb3+ ion we get precise information about the active metal site in the CdMoO4 host lattice. As a good example we have chosen the series of rare earth-doped vacancied scheelite-type cadmium molybdate micro-crystals as Cd1-3xRE2x□xMoO4 (where RE = Nd3+, Eu3+ and Yb3+) solid solutions prepared by a high-temperature solid state method. The monophasic samples crystallize in the tetragonal scheelite-type structure (the space group I4 1 /a, with point symmetry S4) when the concentration of optical activator is between 0 and 66.67 mol% of Nd3+ or Eu3+ ions and 33.36 mol% of Yb3+ ions. The substitution of divalent Cd2+ by trivalent RE3+ cations leads to the formation of cationic vacancies in the framework (which are denoted in the chemical formula as □), due to the charge compensation: 3Cd2+ → 2YRE3+ + □ vacancy. We check the relationship between structural and spectroscopic properties of Nd3+, Eu3+ and Yb3+ rare earth ions as structural probes in the scheelite-type CdMoO4 with vacancies mainly by confirming the presence of multi-sites with each dopant. We also analyze how the vacancy concentration dependence may enhanced the optical parameters and brings some originality to this research program in optical materials. We show that RE3+ -doped CdMoO4 investigated materials are interesting for potential applications like promising phosphors activated by Eu3+ ions under excitation by UV light with the very strong red luminescence, potential laser materials with Nd3+ and Yb3+ ions by the values of the absorption cross-section, the strong emission intensity and laser parameters.