Thermochimica Acta 478 (2008) 13–16
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Thermochimica Acta
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Heat capacity of Sr
10
(PO
4
)
6
Cl
2
and Ca
10
(PO
4
)
6
Cl
2
by DSC
R. Venkata Krishnan
a
, Hrudananda Jena
b
, K.V. Govindan Kutty
b
, K. Nagarajan
a,∗
a
Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
b
Liquid Metals and Structural Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
article info
Article history:
Received 21 May 2008
Received in revised form 12 August 2008
Accepted 20 August 2008
Available online 28 August 2008
Keywords:
Heat capacity
Chloroapatites
Radioactive waste immobilization
Apatites
DSC
abstract
Strontium and calcium chloroapatites were synthesized by wet chemical method, characterized by X-ray
diffraction and are found to be phase pure materials. The measured room temperature lattice parameter
of Ca
10
(PO
4
)
6
Cl
2
is a = 9.523 Å, c = 6.855 Å and for Sr
10
(PO
4
)
6
Cl
2
is a = 9.876 Å, c = 7.188 Å. Heat capacity
measurements were carried out on Ca
10
(PO
4
)
6
Cl
2
and Sr
10
(PO
4
)
6
Cl
2
by DSC in the temperature range
298–800 K. The heat capacity values of Sr
10
(PO
4
)
6
Cl
2
is higher at all temperatures than Ca
10
(PO
4
)
6
Cl
2
.
Enthalpy and entropy increments were computed. Heat capacity values of Ca
10
(PO
4
)
6
Cl
2
and Sr
10
(PO
4
)
6
Cl
2
at 298 K are 758 and 868 J K
-1
mol
-1
, respectively.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Several studies are conducted worldwide to find new host forms
for immobilizing radioactive wastes [1–9]. The candidature of a
material for radioactive waste immobilization is decided based
on its properties such as resistance to thermal degration, abil-
ity to accommodate significant quantities of radioactive nuclides
in their structure, cost effectiveness, etc. [3]. Apatites are consid-
ered to be among suitable solid host matrices for immobilizing
chloride wastes generated by pyrochemical reprocessing of spent
fuel. Apatites are naturally occurring minerals having hexagonal
crystal structure (P6
3
/m space group) with a unit cell formula of
M
5
(PO
4
)
3
(X), M = Ca, Sr, Ba, and (X = OH, Cl or F) [10,11]. These
apatites can immobilize highly active and heat generating radioac-
tive elements such as
90
Sr,
137
Cs,
99
Tc, etc. and halide ions [12] by
accommodating them in their crystal lattice by forming substitu-
tional solid solution [13]. The attractive feature of the apatites for
nuclear waste immobilization is their ease of preparation at low
temperature. Thermodynamic data such as heat capacity, thermal
conductivity, etc. of the chloroapatites are essential to understand
their phase behavior after immobilizing the chloride, alkali and
alkaline earth fission products into the apatite structure. There are
no measured heat capacity data for these compounds. Therefore,
∗
Corresponding author at: Fuel Chemistry Division, Indira Gandhi Centre for
Atomic Research, Chemistry Group, Kalpakkam 603102, India.
Tel.: +91 44 27480500x24289; fax: +91 44 27480065.
E-mail address: knag@igcar.gov.in (K. Nagarajan).
in the present study, heat capacity measurements were carried out
on the phase pure Ca
10
(PO
4
)
6
Cl
2
and Sr
10
(PO
4
)
6
Cl
2
by DSC and the
experimental results are discussed in this paper.
2. Experimental
2.1. Synthesis and characterization
SrCl
2
, CaCl
2
and NH
4
H
2
PO
4
of 99.99% purity supplied by M/s.
SD-Fine Chemicals, India were used for the preparation of the sam-
ples. Stiochiometric quantities of SrCl
2
, CaCl
2
and NH
4
H
2
PO
4
were
dissolved in deionized water, and mixed and stirred. The pH of
the mixture was maintained in the range of 9–10, by addition
of aqueous ammonia. The white precipitate formed was filtered,
washed with deionized water and oven dried. The dried powders of
Ca
10
(PO
4
)
6
Cl
2
and Sr
10
(PO
4
)
6
Cl
2
formed were characterized by X-
ray powder diffraction, employing a Siemens D-500 powder X-ray
diffractometer in the step scan mode using Cu K radiation. Purity
of the samples were estimated by using ICP-MS. The total impuri-
ties in both Ca
10
(PO
4
)
6
Cl
2
and Sr
10
(PO
4
)
6
Cl
2
are less than 0.1 mol.%.
Ca
10
(PO
4
)
6
Cl
2
and Sr
10
(PO
4
)
6
Cl
2
powders were then compacted
into pellets of 5 mm in diameter and sintered by heating at 1273 K
for 10h in air. The sintered pellets were used for heat capacity
measurements.
2.2. Heat capacity measurements
A heat flux type differential scanning calorimeter (model num-
ber DSC821e/700 of M/s. Mettler Toledo GmbH, Switzerland) was
0040-6031/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2008.08.009