Probing cation antisite disorder in Gd
2
Ti
2
O
7
pyrochlore by site-specific near-edge
x-ray-absorption fine structure and x-ray photoelectron spectroscopy
P. Nachimuthu,
1,2,
* S. Thevuthasan,
3
M. H. Engelhard,
3
W. J. Weber,
3
D. K. Shuh,
2
N. M. Hamdan,
2
B. S. Mun,
2
E. M. Adams,
3
D. E. McCready,
3
V. Shutthanandan,
3
D. W. Lindle,
1
G. Balakrishnan,
4
D. M. Paul,
4
E. M. Gullikson,
2
R. C. C. Perera,
2
J. Lian,
5
L. M. Wang,
5
and R. C. Ewing
5
1
Department of Chemistry, University of Nevada, Las Vegas, Nevada 89154, USA
2
Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3
Pacific Northwest National Laboratory, Richland, Washington 99352, USA
4
Department of Physics, University of Warwick, Coventry, United Kingdom
5
Department of Nuclear Engineering & Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
(Received 7 May 2004; published 16 September 2004)
Disorder in Gd
2
Ti
2
O
7
is investigated by near-edge x-ray-absorption fine structure (NEXAFS) and x-ray
photoelectron spectroscopy (XPS). NEXAFS shows Ti
4+
ions occupy octahedral sites with a tetragonal distor-
tion induced by vacant oxygen sites. O 1s XPS spectra obtained with a charge neutralization system from
Gd
2
Ti
2
O
7
100 and the Gd
2
Ti
2
O
7
pyrochlore used by Chen et al. [Phys. Rev. Lett. 88, 105901 (2002)], both
yielded a single peak, unlike the previous result on the latter that found two peaks. The current results give no
evidence for an anisotropic distribution of Ti and O. The extra features reported in the aforementioned com-
munication resulted from charging effects and incomplete surface cleaning. Thus, a result confirming the direct
observation of simultaneous cation-anion antisite disordering and lending credence to the split vacancy model
has been clarified.
DOI: 10.1103/PhysRevB.70.100101 PACS number(s): 66.30.Hs, 61.10.Ht, 72.80.Ng, 79.60.Ht
Pyrochlore materials are potentially useful for a range of
technological applications.
1–4
In particular, their use in solid
oxide fuel cells and as host matrices for actinide wastes are
receiving increasing attention because of the recent discov-
eries showing that the isovalent substitution of Zr for Ti in
Gd
2
Ti
2
O
7
results in a four to five orders-of-magnitude in-
crease in the oxygen ion conductivity at 875 K and in resis-
tance to energetic particle irradiation.
2,5
The mechanisms re-
sponsible for the large increase in these properties have been
investigated by several experimental and theoretical
methods.
3,4,6–10
These studies show that the increase in the
ionic conductivity in pyrochlore is most likely due to the
increased oxygen vacancies at the 48 f site as a result of
cation and anion disordering, which are responsible for the
increased ionic conductivity.
3–5,8,9
The increased radiation
tolerance is attributed to the ease of rearrangement and re-
laxation of Gd, Zr, and O ions/defects within the crystal
structure, which inhibits amorphization by causing the
irradiation-induced defects to relax and form cation antisite
defects and anion Frenkel defects.
2
However, there is limited
direct evidence for the presence of cation antisite disorder in
a highly ordered pyrochlore structure.
Recently, Chen et al.
11,12
reported on the disorder in
Gd
2
Ti
1-y
Zr
y
2
O
7
pyrochlores measured by laboratory x-ray
photoelectron spectroscopy (XPS), and the results provided
direct evidence that cation antisite disorder occurs simulta-
neously with anion disorder. A key piece of information in
these studies is the O 1s XPS spectra for Gd
2
Ti
2
O
7
, which
exhibits a broad feature with two components centered at
binding energies (BEs) of 526 and 531 eV. These compo-
nent peaks were attributed to oxygen ions coordinated solely
to Gd
3+
, or to both Gd
3+
and Ti
4+
ions, respectively. The BE
difference of 5 eV between the two oxygen species is large
for a single-phase compound. To obtain XPS spectra, Chen et
al.
11,12
Ar
+
-sputtered the surface, which removes contami-
nants but can preferentially deplete lighter elements from the
surface region and induce defects/disorder. Although anneal-
ing under an oxygen partial pressure recovers the full oxygen
stoichiometry and surface order in pyrochlore materials, the
O1s XPS feature has been shown to be insensitive to sput-
tering and subsequent annealing.
13
In light of the XPS results from Refs. 11 and 12, the
nature of disorder in Gd
2
Ti
2
O
7
pyrochlore has been re-
examined using a multi-technique approach investigating the
Ti 2p and O 1s of Gd
2
Ti
2
O
7
100 by site-specific near-edge
x-ray-absorption fine structure (NEXAFS) and XPS, as well
as XPS of the same Gd
2
Ti
2
O
7
(and other pyrochlore compo-
sitions) employed in Refs. 11 and 12. NEXAFS involves
electronic transitions originating from a selected atomic core
level to unoccupied electronic states, which permits the de-
termination of site-specific unoccupied electronic structure
and site symmetry by use of linearly polarized x rays.
13
Laboratory XPS is used to provide information pertaining to
the occupied electronic density of states from both pyro-
chlore materials that can be compared to the results from
Refs. 11 and 12. The NEXAFS herein have about the same
surface sensitivity as the XPS measurements.
Single crystals of Gd
2
Ti
2
O
7
were grown by the floating
zone technique using an infrared image furnace.
14
The pyro-
chlore structure of the Gd
2
Ti
2
O
7
single crystal was con-
firmed by x-ray diffraction (XRD) and a series of pole figure
measurements. Part of the crystal was powdered to measure
the bulk lattice parameter a = 1.01857 nm and confirmed
the absence of secondary or minor phases.
14
The Gd
2
Ti
2
O
7
single crystal was cut and polished to obtain a (100) surface
to better than 1°. The preparation and characterization of
PHYSICAL REVIEW B 70, 100101(R)(2004)
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