Dielectric permittivity of some novel copper/cobalt and rare-earth metal tungstates

Polycrystalline samples of new copper and samarium tungstates (Cu3Sm2W4O18 and CuSm2W2O10) and cobalt and europium tungstate (CoEu4W3O16) have been successfully synthesized by the solid state reaction of d-electron metal tungstates MWO4 (M = Cu, Co) with corresponding rare-earth metal tungstate RE2WO6 (RE = Sm and Eu). Broadband dielectric spectroscopy provides experimental evidence that in tungstates under study both relative dielectric permittivity (ɛr) and loss tangent (tan δ) strongly depend on the temperature and frequencies. Maximal relative permittivity value ɛr = 42 for CuSm2W2O10, ɛr = 30 for Cu3Sm2W4O18, and ɛr = 63,000 for CoEu4W3O16 at low frequency (ν = 0.1 Hz) and at 373 K indicates that only these ions which have the large number of unpaired and unscreened electrons on the unfilled shells are responsible for the colossal dielectric effect.