Thermochimica Acta 581 (2014) 1–13
Contents lists available at ScienceDirect
Thermochimica Acta
j ourna l h om epage: www.elsevier.com/locate/tca
Thermodynamic studies of thorium phosphate diphosphate and
phase investigations of Th-P-O and Th-P-H
2
O systems
Deepak Rawat, Smruti Dash
∗
, A.R. Joshi
Product Development Division, Bhabha Atomic Research Centre, Mumbai 400085, India
a r t i c l e i n f o
Article history:
Received 25 July 2013
Received in revised form 5 February 2014
Accepted 8 February 2014
Available online 18 February 2014
Keywords:
Solution calorimeter
Enthalpy of formation
Heat capacity
Phase diagram
Phosphate
Actinide storage material
a b s t r a c t
The standard molar enthalpy of formation of thorium phosphate diphosphate, -Th
4
(PO
4
)
4
P
2
O
7
(cr) has
been determined using an isoperibol solution calorimeter, a differential scanning calorimeter and phase
transition data of high temperature calorimeter. The enthalpy of precipitation of thorium phosphate-
hydrogenphosphate hydrate, Th
2
(PO
4
)
2
(HPO
4
)·H
2
O(cr), was measured at 298.15 K using an isoperibol
solution calorimeter. Heat capacities of -Th
4
(PO
4
)
4
P
2
O
7
(cr) and -Th
4
(PO
4
)
4
P
2
O
7
(cr) were measured
using a Differential Scanning Calorimeter. Combining these experimental data, and other auxiliary data
from the literature, thermo-chemical reaction scheme was devised to calculate the standard molar
enthalpy of formation of -Th
4
(PO
4
)
4
P
2
O
7
(cr) to be {-10565.5 ± 13.6} kJ mol
-1
. The thermodynamic
functions for -Th
4
(PO
4
)
4
P
2
O
7
(cr) have been computed using
f
H
o
m
(298.15 K), C
o
p,m
(T ) and literature
data. The Gibbs phase diagram and predominant area diagrams for Th-P-O system have been computed
using the thermodynamic information of the various phase present in Th-P, P-O and ThO
2
-P
2
O
5
systems.
E
H
–pH diagram of Th-P-H
2
O system has been computed to determine stability of Th
4
P
6
O
23
(cr) in ground
water.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
The disposal of radioactive wastes is a serious environmental
problem for which complete satisfactory solution is not available.
Intensive research is being carried out for the storage and disposal
of the minor actinides and fission products in stable and suitable
matrices. A vast variety of studies have been reported for search
of alternate suitable host matrices to borosilicate glass. Several
phosphate ceramics [1–15] have been investigated for long-term
storage of nuclear waste. Zirconium phosphate, thorium phosphate
diphosphate (TPD) [1], apatites [M
10
(PO
4
)
6
X
2
, where M = Ca, Sr, Ba;
X = OH
-
, Cl
-
, F
-
, CO
3
2-
] [2], monazites [3,4], cheralite [5–7], zir-
conium phosphate [Zr
2
O(PO
4
)
2
] [8], sodium zirconium phosphate
[9], lead phosphate [10] and iron phosphate [11] matrices as well as
some other materials such as synroc, zircon and pyrochlore are also
proposed as ceramic matrix for the storage of radioactive waste.
Thorium phosphate diphosphate (-Th
4
(PO
4
)
4
P
2
O
7
(cr), -TPD,
-Th
4
P
6
O
23
(cr)), is a potential actinide-bearing phase, because
of its high-capability to immobilize tetravalent actinides [12,13],
high-resistance to aqueous corrosion and high-thermal stabil-
ity [14]. However, the self-irradiation of -Th
4
(PO
4
)
4
P
2
O
7
(cr) by
∗
Corresponding author. Tel.: +91 22 2559 3709.
E-mail address: smruti@barc.gov.in (S. Dash).
-decays due to actinides loading could modify these initial per-
formances, especially its chemical reactivity by modifying the
thermodynamic stability of the material [15]. Hence, thorough
study of the thermodynamic properties of -Th
4
(PO
4
)
4
P
2
O
7
(cr)
is of great importance to evaluate the long-term durability and
reactivity. The high temperature thermodynamic property of -
Th
4
(PO
4
)
4
P
2
O
7
(cr) has not been reported in the literature. The
present study is focused on the measurements of standard molar
enthalpy of formation and heat capacity of -Th
4
(PO
4
)
4
P
2
O
7
(cr).
The heat capacity gives essential information about the phys-
ical properties of materials. Its knowledge is indispensable for
determination of other thermodynamic quantities at high tem-
perature. In addition, heat capacity is also related to the thermal
conductivity of the material. The thermodynamic functions for -
Th
4
(PO
4
)
4
P
2
O
7
(cr), Gibbs phase diagram and predominant area
diagrams for Th-P-O system and E
H
–pH diagram for Th-P-H
2
O sys-
tem have been computed using FactSage programme.
2. Experimental
2.1. Preparation
-Th
4
(PO
4
)
4
P
2
O
7
(cr) has been prepared from thorium phos-
phate-hydrogen phosphate hydrate (Th
2
(PO
4
)
2
(HPO
4
)·H
2
O(cr)).
Low-temperature phase, thorium phosphate-hydrogen phosphate
http://dx.doi.org/10.1016/j.tca.2014.02.005
0040-6031/© 2014 Elsevier B.V. All rights reserved.