  Phase, microstructural, dielectric and ferroelectric properties investigation
clearly reveal that the charge imbalance created due to alio-valent Gd3+-ion
substitution at Pb2+ site in lead magnesium niobate should be compensated by
changing B-site cation ratio instead of creating A-site or B-site vacancies.
Microstructure analysis along with elemental mapping exhibit segregation of
magnesium oxide (MgO) and gadolinium niobate (GdNbO4) phase, which is observed
to remain invariant to the charge compensation mechanism. Fitting of frequency
dependent temperature of dielectric constant maximum Tm (temperature of emax)
and the Mydosh parameter K calculated from the emax (T) clearly reveals
critical slowing down of polar nano-regions (PNRs) dynamics leading to
super-dipolar glass state in Gd3+-ion substituted PMN, when the charge
imbalance is compensated by creating B-site vacancy or changing B-site cations
ratio. Precipitation of secondary (pyrochlore) phase is resulted in Gd3+-ion
substituted PMN when the charge imbalance is compensated by creating A-site
vacancies, which causes reduction in interaction among the PNRs and
circumventing the super-dipolar glass state.
