Mihailova

1. Berichtskolloquium des SPP 1236, 01.10.07 – 03.10.07, Hünfeld
High pressure-induced pressure induced renormalization phenomena
in lead scandium niobates and tantalates
with relaxor behaviour
Principal investigator: Boriana Mihailova
Co-investigator: Ulrich Bismayer
(project MI 1127/2-1)
Universität Hamburg
Mineralogisch-Petrographisches Institut

Co-workers Co workers and collaborators
University of Hamburg
Prof Dr Boriana Mihailova (JP), Prof Dr Ulrich Bismayer
PhD student Anna-Maria Welsch
Dr Carsten Paulmann, Dr Thomas Malcherek
Virginia Tech
Prof Dr Ross Angel
Dr Jing Zhao, Dr Jens Engel
Bulgarian Academy of Sciences
Prof Dr Marin Gospodinov
PTB Braunschweig
Dr Bernd Güttler, Dr Rainer Stosch

Outline
1. Relaxor ferroelectrics
structural peculiarities and open questions
2. Pressure-induced structural transformations
PbSc 0.5Ta 0.5O 3 and Pb 0.78 Ba 0.22 Sc 0.5 Ta 0.5 O 3
(AP - 9 GPa, RT)
3. Our research plan for next 12 months

Relaxor ferroelectrics vs proper ferroelectrics
proper FE PbTiO3 10 3 ε
8
6
4
2
673
FE
773
Temperature / K
dielectric permittivity ε
PE
873
relaxor FE
10 3 ε
Temperature / K
PbMg 1/3Nb 2/3O 3
high ε in relatively wide T range near RT
→ multifunctionality!

Relaxor ferroelectrics – complex nanoscale structure
Proper ferroelectrics
T c ~ T m
paraelectric
ferroelectric
long-range ferroelectric ordering
(length of coherent atomic shifts >> a 0)
T c ≤ T m
Relaxor ferroelectrics
T B
T m
Δα r ~ 0.1°
pseudo-PE
uncoupled atomic shifts
Burns temperature (non-linearity in n(T))
randomly oriented
polar nanoclusters
dielectric permittivity maximum
coupled polar
nanoclusters
canonical
relaxors

Relaxor ferroelectrics – open questions
Atomistic nature of relaxor behaviour
Can we tune relaxor properties
by tailoring polar nanoclusters?
nanoclusters?
• Which is the driving force for nucleation of polar nanoclusters at HT?
• What is the reason for suppression of ferroelectric long-range ordering at LT?
local electric fields due to charge imbalance
local elastic fields due to local structural distortions
The majority of relaxors :
Pb-based perovskite-type oxides
(Pb,A”)(B’,B”)O 3
performance
structure
properties
processing

Our general goals
Raman scattering and X-ray ray diffraction on single crystals
PbSc0.5Ta0.5O3 (PST)
and
PbSc0.5Nb0.5O3 (PSN)
A-site-doped PST and PSN,
substitution of Ba 2+ , La 3+ , Bi 3+ for Pb 2+
Nb-doped PST, substitution of Nb 5+ for Ta 5+
High-pressure
High pressure studies on relaxors of type PbB’ 0.5B” 0.5O3 :
- atomic clustering under pressure
- pressure-induced phase transitions
AB’ 1/2 B” 1/2 O 3
AB’ 1/3 B” 2/3 O 3
- changes in the compression behaviour upon doping

Relaxors – phonon anomalies at ambient p and T
prototype structure Fm3m
F1u F1u Pb-localised
BO3 translation vs Pb
B-cation localised
Pb-O bond stretching
BO6 bending
p-pol c-pol
F 1u
F 1u
E g
BO 6 stretching
A 1g
F 2g
F 2u
Pb-localised
BO3 translation vs Pb
B-localised
Pb-O bond stretching
BO6 bending
BO6 str.
bold: E i || [100]
thin: E i || [110]
F 2g
PST
PSN
300-350 cm -1 (Pb-O stretching):
correlated ferroic Pb-O species
Raman-activity near 250 cm -1 :
off-centre shifts of B-cations

PbSc 0.5Ta 0.5O 3 (PST) and Pb 0.78 Ba 0.22 Sc 0.5 Ta 0.5 O 3 (PBST) at AP
Why those two compounds?
PST
PBST
relaxor with long-range FE ordering at LT canonical relaxor

PST and PBST at HP
Raman scattering, X-ray X ray diffraction and diffuse scattering
AP - 9 GPa : • enhancement of the anisotropy in the Pb-O system
• suppression of B-cation off-centre shifts
PbSc 0.5Ta 0.5Ta0.5
0.5 O3 new! new!
Pb 0.78Ba 0.78Ba0.22Sc
0.22Sc0.5Ta
0.5Ta0.5
0.5 O3 a sequence of local structural changes between 1.2 - 3.5 GPa
reversible phase transition near 2.0 GPa
(submitted for publication)
new canonical relaxor! relaxor
two types of Pb-O bonds revealed by pressure
stiff octahedral distortions near the incorporated Ba cations
responsible for the suppression of long-range FE ordering
(to be submitted for publication)

Our research plan (next 12 months)
HP Raman spectroscopy on PST and Ba-PST up to 15 GPa
(an attempt to collect polarised spectra < 4 GPa)
Analysis of the XRD data on
PST and Ba-PST collected at DESY
Pressure-induced structural transformations in PSN and Nb-doped PST
(Raman scattering and XRD analysis)
Ba-PSN and Ba-PST-PSN
(Raman scattering)
La-PST and La-PSN
(Raman scattering)

Acknowledgements:
Financial support by the DFG (MI 1127/2-1) 1127/2 1)
Ross Angel, Jing Zhao, Jens Engel, Virginia Tech
Alexandra Friedrich, Uni Frankfurt
Clivia Hejny, Hejny,
Uni Innsbruck
Hans Ahsbahs, Ahsbahs,
Uni Marburg
Karsten Knorr, Knorr,
Uni Kiel
Leonid Dubrovinski,
Dubrovinski,
Uni Bayreuth
Tiziana Boffa Ballaran, Ballaran,
Uni Bayreuth
Organisers and Lecturers of the Workshop in Bayreuth, 24.-26.04.2007
24. 26.04.2007