Pressure as a Probe of the Physics of Relaxor Ferroelectrics Page: 4 of 6
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* S
25
2015 -
101-
5 s-
0
1. 20 kbar P 6f6/35 .
3. 1 kHZ
,1.a - - - 10 kHz
--+--100 kHz
2.0 -----t MH z
280 320 360 400 '
T (K)
0 kbar
Ferroelectrkic
10 kbar
- /
20 kbar
Relaxor
- e - a -300 350 400 450 500 550
T (K)
Fig. 1. Pressure-induced FE-R crossover in PLZT 6/65/35.
below Tm indicating that the sample is on the verge of entering
the R phase. On further cooling, FE behavior with
rhombohedral symmetry is observed. This response can be
understood as follows. On cooling in the PE phase, the
disorder-induced polar nanodomains grow and become large
and nearly static (hence the weak frequency dispersion at T <
Tm,) but not sufficiently large to condense a full FE state at Tm .
Continued correlation among the dynamically slowed-down
domains due to the high polarizability of the host lattice results
in their further growth and condensation into macro-domains
and an FE state below Tm.
Pressure causes large decreases in the amplitude for the
'(T) anomaly at Tm and in the transition temperatures and
induces full relaxor character for the PE-R transition (Fig. 1).
The inset in Fig. 1 provides an expanded view of the 20 kbar
response and shows the classic dipolar, glass-like relaxor
response.
In KTN with small Nb content, the Nb+ ion occupies an
off-center position leading to a large dipole moment.4 Above -2
at % Nb these materials exhibit normal FE transitions, but below
2 at % relaxor behavior sets in. Figure 2 shows the E'(T)
response of our sample. At 1 bar the response is the classic FE
signature with s' essentially independent of frequency. Up to
-6.5 kbar the main influence of pressure is a decrease in T as is
commonly observed for many soft mode FEs. At - 7 kbar there
is a qualitative change in the '(T) response signifying an FE-to-
R crossover as seen in the 7.7- and 8.5-kbar isobars.
As already noted, we have now observed this pressure-
induced FE-to-R crossover in many compositionally-disordered
ABO3 oxides and believe it to be a general phenomenon in soft
phonon mode systems. The crossover can be explained in terms
of a large decrease in the correlation length, r , among polar14
12
100
r8
64
21
0
0
20
40
T (K)60
Fig. 2. Pressure-induced FE-R crossover in KTN.
nanodomains with pressure - a unique property of soft FE
mode systems where the soft mode frequency, cos, controls
the polarizability of the host lattice and thereby re.3
Specifically, in 'the high temperature PE phase ws(r )
decreases (increases) with decreasing temperature, but
increases (decreases) with increasing pressure. The effects
are quite large; e.g., for our KTN crystal we estimate that at
44 K rc decreases by a factor of ~8 between 0 and 8.5 kbar,
and the corresponding correlation volume decreases by a
factor of over 500.a
Physically, we envision each disorder-related dipole
inducing polarization (or dipoles) in adjoining unit cells of
the highly polarizable host and forming a dynamic
"polarization cloud" whose extent is determined by r . At
high temperatures r is small, and the polarization clouds are
effectively polar nanodomains. With decreasing T at low
pressures, the rapidly increasing r couples these
nanodomains into rapidly growing polar clusters and
increases their Coulombic interactions. Ultimately, these
clusters percolate (or permeate) the whole sample and
precipitate a static, cooperative long-range ordered FE state
at T < T . At sufficiently high pressure, on the other hand,
the clusters increase in size on decreasing T in the PE phase,
but do not become large enough to permeate the whole
sample (or grains) to precipitate a FE transition. Rather, the
clusters exhibit dynamic "slowing down" of their fluctuations
at T < Tm leading to the observed relaxor behavior. Because
re decreases continuously with increasing pressure, the polar
clusters become smaller with increasing pressure - a fact that
accounts for the observed increase in frequency dispersion
and the suppression of the dielectric anomaly (Figs. 1 and 2).'p
x
W- . - . lbar-
7.5 kbar 4 kbar
.'
.1 . -
- ./I
- 8.3 kbar-
;;
- iKTN:Ca
2.3 at.% Nb).
-"- 102 Hz
3
-10
"--- 10s
10680
s - : - a - . - . - .
e
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Samara, George A. Pressure as a Probe of the Physics of Relaxor Ferroelectrics, article, August 9, 1999; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc794033/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.