Combustion synthesis, structural, magnetic and dielectric properties of Gd3+-doped lead molybdato-tungstates

Gd3+-doped lead molybdato-tungstates with the chemical formula of Pb1-3x & x25a1;Gd-x(2x)(MoO4)(1-3x)(WO4)(3x) (where x = 0.0455, 0.0839, 0.1430, corresponding to 9.53, 18.32, 33.37 mol% of Gd3+, respectively, as well as & x25a1; denotes cationic vacancies) were successfully synthesized via combustion route. The XRD and SEM results confirmed the formation of single-phase, tetragonal scheelite-type materials (space group I4(1)/a) with the uniform, spherical and oval grains ranging from 5 to 20 mu m. Individual grains are strongly agglomerated into big clusters with the size even above 50 mu m. The magnetic measurements as well as the Brillouin fitting procedure showed paramagnetic state with characteristic superparamagnetic-like behaviour and the short-range ferromagnetic interactions. The electrical and broadband dielectric spectroscopy studies revealed insulating properties with the residual electrical n-type conduction of 2x10(-9) S/m and low energy loss (tan delta approximate to 0.01) below 300 K. Dielectric analysis showed that no dipole relaxation processes in the Gd3+-doped materials were observed. A fit of dielectric loss spectra of Gd3+-doped samples by sum of the conductivity and the Havriliak-Negami, Cole-Cole, and Cole-Davidson functions confirmed this effect.