Raman spectroscopic investigation of Gd-substituted lead magnesium niobate ceramics Pb1−xGdx(Mg1+x/3Nb2−x/3)O3 (0 ≤ x ≤ 0.1)

Vibrational properties of Gd substituted lead magnesium niobate (PGMN, Pb1−xGdx(Mg1+x/3Nb2−x/3)O3 with 0 ≤ x ≤ 0.1) ceramics have been investigated by using Raman spectroscopy, which were correlated to the enhancement in dielectric relaxor behaviour. Raman active stretching modes of NbOMg (A1g, ∼780 cm−1) and NbONb (∼500-600 cm−1) have been used as a probe to measure the effect of Gd on the size of chemical ordered regions (CORs) and polar nano-regions (PNRs) in the PGMN ceramics. The red shift in A1g mode is explained by considering the random site model used to elucidate B-site cation ordering in the chemical ordered regions (CORs). Similarly, the red shift in the NbONb stretching mode has been related to off-centre displacement of Nb-ion in the polar nano-region (PNRs), which is reduced with increasing content of Gd. Along with this, the decrease in lattice parameter, as well as the blue shift in stretching mode of PbO (∼150 cm−1) and bending mode of OBO (∼260 cm−1), confirms the Gd-substitution to be at Pb-site. An increase in the width of distribution of the relaxation time, characterized by the Cole-Cole fitting parameter α at Tm(100Hz)+15 °C, and reduction in interaction between the PNRs, characterized by the stretched exponent fitting parameter p, is consistent with the Raman studies. The presence of multiple heterogeneities, i.e., site disorder, presence of the CORs, off-center ionic displacements in the PNRs, are responsible for high dielectric relaxation, as well as the broadening and overlapping of the Raman active modes.