Dielectric, optical and EPR studies of PbMoO4 single crystals, pure and doped with cobalt ions

Lead molybdate single crystals, pure and doped with Co, grown by the Czochralski method were investigated by dielectric, optical. and electron paramagnetic resonance spectroscopy methods. Detailed studies of dielectric permittivity epsilon ', conductivity and loss tangent (tan delta) of the compound, both as a function of temperature (290-400 K) at 10 kHz and frequency (1Hz-100 kHz) showed clear influence of Co substitution on the dielectric properties of PbMoO4 crystals. Conductivity activation energy of pure and PbMoO4 crystals doped with Co was established as being equal to 0.74 and 0.66 eV, respectively. Optical studies revealed absorption in a VIS range at an approximate wavelength of 530 nm, corresponding to (4)A(2)-T-4(1) (P) of Co2+ in the garnet tetrahedral site. Moreover, the 860 nm absorption band was observed, possibly due to F-type (oxygen) and/or Pb vacancy centers. We also observed a triplet absorption band in the IR centered at 4039, 4233 and 4460 nm. The absorption coefficients of the band maxima had the following values: 0.91 cm(-1), 2.27 cm(-1), and 0.88 cm(-1) respectively. It indicated that a MoO4- type center appeared in lead molybdate crystals. EPR measurements confirmed the presence of antiferromagnetically coupled Co2+ ions in the analyzed crystals. The EPR spectra disappeared at a temperature of about 35 K as a result of the onset of fast spin-lattice relaxation. (C) 2008 Elsevier B.V. All rights reserved.