IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
particles of σ = 2.7 µC/cm 2 as
compared to that for particles
with σ = 0.3 C/cm 2 .
Measured experimental U(r)
data was found to fit to the
following empirical expression
U(r)=A(exp(-κr)/r) - B exp(-κr),
where the constants A, B
represent the strength of
repulsive and attractive
components of the interaction,
respectively and κ -1 represents
the range of interaction.
Present measurements thus
confirm the existence of a
long-range attractive term in
the effective pair-potential
between like-charged colloids.
The monovalent counterion (H +
ions) concentration is increased
in the system by increasing the
charge density on the particles.
The attraction is believed to
arise from the counterions
mediation between the
macroions.
VI.5. Soft Modes and NTE in Zn(CN) 2 from Raman
Spectroscopy and First Principles Calculations
Vibrational spectroscopy is an
excellent tool to study the
mechanism of thermal
expansion, especially in
materials that exhibit the
anomalous behaviour of
negative thermal expansion
(NTE). Zn(CN) 2 is reported to
exhibit one of the largest NTE
coefficient over a wide
temperature range. As some
vibrational modes in a material
can be optically inactive, or
have low Raman scattering
cross section, leading to low
intensity Raman lines, it is not
always possible to study the
complete set of vibrational
modes of a material
experimentally. Hence, we need
to simulate the phonon
dispersions at different
pressures using ab-initio
methods and calculate the
mode Gruneisen parameters of
all the phonons as required for
calculation of thermal
expansion coefficient.
First-principles, ab-initio
density functional calculations
are performed at different
pressures and phonon
dispersions calculated using
frozen phonon approximation
with SIESTA code. Norm
conserving pseudopotentials in
the generalized gradient
approximation are used in
calculations. A X 3 X 3 X 3
supercell of Zn(CN) 2 unit cell is
used for determining the
Fig.1 Behaviour of observed Raman modes at different pressures.
Inset shows pressurevariation for the calculated mode frequencies.
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