IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
f 1
(dl/l) PAC
f 2
f 3
f 4
1.0
0.8
0.6
0.4
0.5
0.4
0.3
0.2
0.1
0.2
0.04
0.02
0.00
-0.02
-0.04
0.4
0.3
0.2
0.1
240 280 320 360 400 440
Temperature (K)
at 250 K is interpreted to be
due to probe atoms occupying
regular ZrO 6 . This is supported
by a point charge model based
computation of EFG carried out
by us. Based on the values of
quadrupole parameters it is
interpreted that the other three
sites are associated with probe
atoms occupying contracted
and distorted ZrO 6 . An
increase in the fractions
experiencing
higher
quadrupole frequncies at
higher temperature is seen in
Fig 2.
280 350 420 490
Temperature (K)
Fig.2 Variation of fractions of probe atoms associated with distinct ZrO 6
octahedra. Notice the increase in the fraction corresponding to distorted
octahedra, f 2 with increase of temperature. Shown in the
inset is the fractional change in the average Zr-O distance,
dl/l with temperature.
Shown in Fig.2 is the variation
of the fractions
with the measurement
temperature. f 1 representing the
regular ZrO 6 octahedra
decreases (Cf. Fig.2) while the
other fractions increase with
temperature. PAC spectra and
the results in terms of variation
of quadrupole parameters with
temperature are repeatable at
room temperature after in-situ
measurements till 600 K. This
means that the observed
quadrupole parameters are
due to different equilibrium
configurations that ZrO 6 take
as a consequence of inward
rotations of polyhedra. Plotted
at the top inset in Fig. 2 is the
variation of dl/l with
temperature. The mean value
of Zr-O bondlength is obtained
as the average of the negative
cubic root of quadrupole
frequencies weighted with
respect to corresponding values
of the fractions. Slope of the
plot represents the coefficient
of thermal expansion which is
found to be negative
having a value of
approximately -6x10 -5 K -1 .
This implies that the existence
of higher frequency
components and importantly an
increase in the magnitudes of
these fractions is essential to
obtain a negative slope in the
variation of mean Zr-O
bondlength with temperature.
This means that the existence of
fractions other than f1 which
represent contracted and
distorted ZrO 6 octahedra
caused by the inward rotations
of polyhedra is essential for
understanding negative thermal
expansion in zirconium
tungstate. Thus this atomic
scale study provides a proof
that an effective contraction of
distinct ZrO 6 octahedra due to
inward rotations of polyhedra
accompanied by distortions is a
very important factor
contributing for the observed
NTE in these systems.
156 BASIC RESEARCH