Effect of Ce
4þ
-substitution at A and B sites of Nd
2
Zr
2
O
7
: A study for
plutonium incorporation in pyrochlores
Chiranjit Nandi
a, *
, A.K. Poswal
b
, Mohsin Jafar
c, e
, Swayam Kesari
d, e
, V. Grover
c, e, **
,
Rekha Rao
d, e
, Amrit Prakash
a
, P.G. Behere
a
a
Radiometallurgy Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India
b
Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India
c
Chemistry Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India
d
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India
e
Homi Bhabha National Institute, Mumbai, 400 094, India
article info
Article history:
Received 1 April 2020
Received in revised form
5 June 2020
Accepted 19 June 2020
Available online 25 June 2020
Keywords:
Ce-substituted Nd
2
Zr
2
O
7
Pyrochlores
Inert matrix fuel
Nuclear waste immobilization
XRD
Raman spectroscopy
XANES
abstract
Zirconate pyrochlores have been envisaged as promising candidates for inert matrix fuel applications and
immobilization of nuclear waste. Therefore, Ce
4þ
-substituted Nd
2
Zr
2
O
7
samples with general formula
[Nd
1-x
Ce
x
]
2
Zr
2
O
7þd
(0.0 x 0.5) and Nd
2
[Zr
1-x
Ce
x
]
2
O
7
(0.0 x 1.0) were synthesized by solid state
route and thoroughly characterized by X-ray diffraction (XRD) in order to mimic plutonium incorporation
at A- and B-sites of Nd
2
Zr
2
O
7
pyrochlore which are crystallographically distinct. The local structure has
also been probed by Raman spectroscopy and X-ray absorption near edge structure (XANES) spectros-
copy. Interestingly, the substitution of Ce
4þ
at different sites yields distinct phase relations. Ce
4þ
-sub-
stitution at A- (Nd
3þ
) site retains pyrochlore-type phase up to nominal composition with x ¼ 0.4 and F-
type phase was observed for composition with x ¼ 0.5. However, Ce
4þ
-substitution at B- (Zr
4þ
) site
demonstrated transformation of pyrochlore-type phase to F-type phase through a biphasic phase field
consisting of these two phases. Raman spectroscopic studies on Nd
2
[Zr
1-x
Ce
x
]
2
O
7
(0.0 x 1.0) system
clearly indicated existence of C-type ordering in largely F-type phases for compositions with x ¼ 0.6 and
beyond. XANES studies exhibited a change in Zr local coordination upon successive Ce
4þ
-substitution at
‘Nd’ site and pointed towards increase in local disorder across the pyrochlore phase regime. Ce L
III
edge
XANES also revealed significant changes in ‘Ce’ local environment in Ce-substituted samples.
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
Zirconate pyrochlores with general formula Ln
2
Zr
2
O
7
(Ln:
trivalent rare-earth cation) are widely known multifunctional
materials because of their diverse applications in scientific and
industrial fields. They have been extensively used in catalysis [1],
magnetism [2], photoluminescence [3], thermal barrier coatings [4]
and solid oxide fuel cells [5] etc. In addition, pyrochlore materials
are of prime interest in nuclear industry because of their applica-
tions as nuclear waste immobilization matrices [6e9] and inert
matrices (IMF) for plutonium (Pu) and minor actinides (MAs)
transmutation [8, 10e13]. Zirconate pyrochlores exhibit excellent
chemical durability and radiation stability [14]. Also zirconia based
materials have low solubility in water [15]. All these properties
make pyrochlores very attractive host matrix for immobilization of
nuclear waste in deep geological repository. Low neutron absorp-
tion cross section, high melting point and good compatibility of
zirconia-based materials with reactor components are conducive to
their application as inert matrices for Pu and MA transmutation
[15]. In particular, Nd
2
Zr
2
O
7
pyrochlore has been investigated as
host for immobilization of Pu and MAs [16] while Nd
2
Zr
2
O
7
eMgO
composites have been investigated as promising IMF candidates by
many researchers [10e12]. For both these applications, it is
essential to understand the structural changes brought about by
incorporation of Pu in proposed Nd
2
Zr
2
O
7
host material.
This work aims to investigate solubility of PuO
2
employing its
non radioactive surrogate CeO
2
in Nd
2
Zr
2
O
7
for both transmutation
* Corresponding author.
** Corresponding author. Chemistry Division, Bhabha Atomic Research Centre,
Mumbai, 400 085, India.
E-mail addresses: cnandi@barc.gov.in (C. Nandi), vinita@barc.gov.in (V. Grover).
Contents lists available at ScienceDirect
Journal of Nuclear Materials
journal homepage: www.elsevier.com/locate/jnucmat
https://doi.org/10.1016/j.jnucmat.2020.152342
0022-3115/© 2020 Elsevier B.V. All rights reserved.
Journal of Nuclear Materials 539 (2020) 152342