High-pressure structural changes in the Gd
2
Zr
2
O
7
pyrochlore
F. X. Zhang,
1
J. Lian,
1
U. Becker,
1
R. C. Ewing,
1,
* Jingzhu Hu,
2
and S. K. Saxena
3
1
Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
2
X17C, National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973, USA
3
The Center for Study of Matter at Extreme Conditions, Florida International University, Miami, Florida 33199, USA
Received 17 April 2007; revised manuscript received 28 September 2007; published 10 December 2007
Pressure-induced structural changes in Gd
2
Zr
2
O
7
pyrochlore have been investigated at pressures up to
43 GPa by synchrotron x-ray diffraction and Raman scattering measurements. With increasing pressure, the
ordered pyrochlore begins to transform to a disordered defect-fluorite-type cubic structure up to 15 GPa.
Above 15 GPa, a high-pressure HP phase forms that has a distorted defect-fluorite-structure of lower sym-
metry. Upon release of pressure, the HP phase is not stable and gradually transforms back to the cubic
defect-fluorite structure.
DOI: 10.1103/PhysRevB.76.214104 PACS numbers: 61.50.Ks, 61.10.Nz, 72.80.Ga
I. INTRODUCTION
The pyrochlore oxides, A
2
B
2
O
7
, can be described as an
ordered superlattice of the simple, isometric fluorite structure
type, AX
2
.
1,2
In the pyrochlore structure type, the A- and
B-site 16c and 16d cations alternate on the fcc sublattice in
rows along the 110 directions. The anion sublattice is com-
prised of three different oxygen sites, two of which are oc-
cupied 8a and 48 f , and the third site 8b is vacant. Hence,
the anion vacancies are ordered. The A-site cations are in
eightfold coordination, and the B-site cations are in sixfold
coordination. All anion sites are tetrahedrally coordinated:
the 8a site is surrounded by four A-site cations. The 48 f site
O2 is surrounded by two A- and two B-site cations that are
slightly displaced from the center of the tetrahedral site to-
ward the unoccupied 8b site. The magnitude of the displace-
ment is represented by the positional parameter x of oxygen
at the 48 f site, which is 0.375 for the ideal fluorite structure.
The tetrahedral site 8b formed by four B-site cations is
vacant. The anion vacancies in pyrochlore A
2
B
2
O
7
are ar-
ranged around the smaller B-site cations at distances
1
2
111
fluorite
and form a tetrahedral network. The structural
transition from the pyrochlore P-type to the defect-fluorite
F-type is an order-disorder transition, which requires disor-
dering between A- and B-site cations, as well as disordering
of the anion vacancies. During the order-disorder phase tran-
sition, changes in symmetry, lattice parameters, and coordi-
nation number of cations are observed.
1–6
Pyrochlore oxides are one of the most promising candi-
date materials as ionic and electronic conductors.
1,7–9
Their
structure and electronic properties depend on the disordering
of cations and anions. The stability of the disordered
structure is generally related to the ratio of ionic radii of
the cations in the A and B sites.
1
Cation disordering
greatly enhances the formation of anion Frenkel defects.
10–12
At normal pressure, the ordered pyrochlore structure can
form for r
A
/ r
B
=1.46 to 1.78 Refs. 1, 6, and 13 and high-
pressure or high-temperature synthesis can greatly extend
this stability range.
14,15
Usually, smaller values of r
A
/ r
B
favor the formation of disordered F-type structure.
11
The
order-disorder transition from the pyrochlore to defect
fluorite structure type may be induced by adjusting the
composition,
1
increasing the temperature,
3
or by applying
high-energy irradiations.
16
The disordering of cation and an-
ion sublattices can occur independently.
17
Neutron scattering
results in Y
2
Ti
1-x
Zr
x
2
O
7
reveal that the anion disordering
precedes cation disordering,
18
a result which is confirmed by
in situ TEM observations.
19
The P-type Gd
2
Zr
2
O
7
, which has the smallest value of
r
Gd3+
/ r
Zr4+
= 1.46, is one of the most interesting pyrochlores
with A
2
B
2
O
7
stoichiometry and because oxygen conductivity
increases with the degree of lattice disorder.
20
Its structure
and degree of order in the binary system can be tuned by
changing the stoichiometry and temperature. According to
the phase diagram,
3
ordered pyrochlore can be formed in a
ZrO
2
-Gd
2
O
3
system with a Gd content of 32–62 wt % at
temperatures below 1500 ° C. The P-type to F-type phase
transition temperature for Gd
2
Zr
2
O
7
is about 1550 °C.
3
Re-
cently, it was found that Gd
2
Zr
2
O
7
and other pyrochlore ox-
ides are radiation-resistant ceramics that disorder to a defect-
fluorite structure that does not become amorphous.
10,16,21–27
Sickafus et al.
11
have proposed that oxygen-deficient F-type
A
2
B
2
O
7
structure with a random arrangement of the cations
should be more stable upon irradiation. The lower the cation
antisite defect energy such as for Gd
2
Zr
2
O
7
, the more
readily will the radiation-induced transition to the disordered
fluorite structure occur, thus leading to a material that does
not amorphize. This has been demonstrated by irradiations
with 2 MeV Au
2+
and 1 MeV Kr
+
of Gd
2
Zr
2
O
7
that trans-
forms the P-type to F-type structure at room temperature,
27
but unlike other pyrochlore oxides, this zirconate pyrochlore
cannot be amorphized even at doses as high as 100 dis-
placements per atom dpa.
Heavy ion irradiation of pyrochlore oxides leads to the
distortion of the unit cell and the order-disorder transition
occurs during the relaxation and recovery phase of the dam-
age cascade. At high pressures, the unit cell is compressed
and the order-disorder transition and even amorphization
have been observed in Ti-based pyrochlore oxides.
28,29
The
mechanism of pressure-induced phase transition in pyro-
chlore is not yet clear. In this paper, we describe the struc-
tural transitions of Gd
2
Zr
2
O
7
up to pressures of 43 GPa as
monitored in situ by XRD and Raman measurements.
PHYSICAL REVIEW B 76, 214104 2007
1098-0121/2007/7621/2141045 ©2007 The American Physical Society 214104-1