To further understand the dielectric behavior of the PZT–PCN
system, the permittivity of a first-order normal ferroelectric can
be described by the Curie–Weiss law and a second-order relaxor
ferroelectric can be described by a simple quadratic law[21]. This
arises from the fact that the total number of relaxor contributing to
the permittivity response in the vicinity of the permittivity peak is
temperature dependent, and the temperature distribution of this
number is given by a Gaussian function about a mean valueT0with
a standard deviationd. The relative permittivity can be derived via
using the following equation:
em
eðf;TÞ
¼1þ
ðTTmðfÞÞ
c
2d
2
ð1Þ
whereemis the maximum value of the permittivity atT=Tm(f). The
value ofcis the expression of the degree of dielectric relaxation,
whiledis used to measure the degree of diffuseness of the phase
transition. In a material with a ‘‘pure” diffuse phase transition, c
is expected to be 2 [10]. As listed in Table 2, the values of cvary
with Zr/Ti ratio between 1.52 and 1.89, which confirms that diffuse
phase transition occurs in the PZT–PCN system. It is important to
note that in perovskite ferroelectrics it has been established thatc
anddcan be affected by Zr/Ti ratio and structure of the materials.
The values ofcandddecrease with increasing Zr from 0.46 to
0.50 mol%, and then increase when Zr content increases further,
as tabulated inTable 2. It is interesting to observe that the phase
transition is least diffused at MPB, possibly due to a sharp increase
of dielectric permittivity at MPB. A similar behavior has also been
observed in PZT–PZN systems[16]