IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
VI.6. Atomic Scale Study of Negative Thermal
Expansion in Zirconium Tungstate
Vast majority of materials
have a positive coefficient of
expansion and their volume
increases upon heating.
Zirconium tungstate (ZrW 2 O 8)
exhibits negative thermal
expansion (NTE) isotropically
over the temperature range 4 to
1000 K, whose mechanism is
yet to be completely
understood. It was earlier
understood that a large
transverse vibration of the
oxygen atom in the middle of
the W-O-Zr linkage, which
requires corresponding
rotations of the polyhedra, is
the primary origin of NTE in this
material due to unconstrained
feature of the structure. It
becomes essential to study the
local structure at Zr and /or W
sites for obtaining the
structural variations of
ZrO 6 /WO 4 polyhedra towards
a better understanding of NTE
in this system
ZrW 2 O 8 is a cubic compound
consisting of a three
dimensional network of corner
linked ZrO 6 octahedra and
WO 4 tetrahedra, with the latter
having one non-bridging W-O
bond. We probe at Zr sites (as
WO 4 are more rigid than ZrO 6 )
by looking at the variation of
quadrupole parameters of
probe atoms occupying Zr sites
using Perturbed Angular
Correlation (PAC) which is a
hyperfine interaction technique.
We have carried out 181 Hf/ Ta
based PAC spectroscopy on this
system to probe local structure
at ZrO 6 . Using this technique
Electric Field Gradient (EFG)
tensors at the site(s) of
occupancy of probe atoms are
deduced. Principal component
of EFG tensor is proportional to
the quadrupole frequency (νQ)
and is inversely proportional to
Zr-O bond length. Asymmetry
parameter (ratio of relative
0.1
0.0
-0.1
-0.2
0.1
0.0
-0.1
-0.2
T m
=250 K
T m
=560 K
R(t)
0 20 40
time (ns)
change of other two
components of EFG and the
principal component) is
sensitive to orientation
distortion if any associated with
ZrO 6 .
A few representative time
dependent anisotropy spectra
R(t) and their Fourier transform
P(ω) are shown in Fig.1.
Analysis of the spectra shows
that there are four distinct Zr
sites experiencing distinct
quadrupole parameters such as
quadrupole frequency,
asymmetry parameters. The
fraction which is predominant
0.05
0.00
0.03
0.02
0.01
0.00
P(ω)
0 2
ω (GHz)
Fig.1 Representative TDPAC spectra and their fourier transforms for
measurements carried out at 250 K and 560 K.
BASIC RESEARCH 155