Formation mechanism of Cr⁴⁺ ions in PbWO₄ single crystals

The behaviors of Cr ions in heavily Cr-doped and Cr, La-codoped PbWO₄ crystals are studied by structural, optical and dielectric methods, which demonstrate the interaction between defects in PbWO₄ (PWO). For La-free sample, it shows a double-doping process that Cr enters the PWO predominantly as Cr³⁺ at the Pb site and a small part as Cr⁴⁺ at the W site. The (Cr_Pb³⁺)^̇ is achieved at expense of large lattice relaxation. Two dielectric relaxation peaks are observed and their activation energies E_α=0.61±0.02 and E_β=0.46±0.01, are determined, which are attributed to [2(Cr_Pb³⁺)̇−(V_Pb)^″] and [2(Cr_Pb³⁺)̇−(Cr_W⁴⁺)^″] associations, respectively. The remarkable increase of Cr⁴⁺ in PWO due to the co-addition of La³⁺ is reflected by the optical absorption spectra on the ground that La³⁺ is supposed to preferentially occupy the Pb site and Cr is forced into the W site as Cr⁴⁺. As a result, (V_Pb)^″ defects are dramatically suppressed and the observed dielectric relaxation is ascribed to [2(La_Pb³⁺)^̇−(Cr_W⁴⁺)^″] clusters with an activation energy of 0.35±0.02 eV.