  Microstrctural, vibrational, ferroelectric and dielectric properties
investigation reveals non-stoichiometric chemical ordered regions (CORs) and
relaxor like dielectric characteristics in lead magnesium niobate (PMN)
ceramics uniaxially hot pressed (HP) at 800 oC to 1200 oC. Occurance of (1/2
1/2 1/2) superlattice reflections along <111> in <110> zone axis SAED pattern
as well as the presence of bright nano-meter regions in corresponding
dark-field images in 800 oC hot pressed PMN demonstrates formation of the CORs
as soon as perovskite phase is formed during calcination and remains unaffected
with HP sintering tempreature. Separation between two broad peaks of Nb-O-Nb
stretching mode and the red-shift of O-B-O bending modes suggest an increase in
the polar nano-size regions. Grain size dependence of the em and shifting of
the Tm from 278 K to 263 K is explained by the core-shell model. Polarization
switching and impedance spectroscopy is observed to be consistent with the
grain size analysis.
