Crystallography of Lanthanide Pyrohafnates
ICDD
T.J. Anderson,a R. Ubic,a and D. Goutb
aDepartment of Materials Science and Engineering, Boise State University
bJülich Center for Neutron Science–SNS and Oak Ridge National Laboratory
Abstract
Neutron Diffraction
The purpose of this study is to investigate the structure of Ln2Hf2O7 pyrohafnates
and to determine the degree of disorder on the cation and anion sublattices.
Ln2Hf2O7 compounds (Ln = La Lu) were synthesized by solid-state reaction
and analyzed by diffraction of x-rays, neutrons, and electrons. While La Tb
yielded pyrochlore compounds, Dy Lu compounds had fluoritic structures,
although short-range pyrochloric ordering was observed in some grains of
Tb2Hf2O7 and Dy2Hf2O7. The unit-cell volume decreases as Z increases through
the series.
111
111
P
Lattice constants and atomic coordinates were determined by neutron
diffraction. (Fig. 6).
* - Pyrochlore
La2Hf2O7
V – Vanadium L – Lu4Hf3O12
*
*
V*
Pr2Hf2O7
P
Introduction
The nature and degree of disorder in the Ln2Hf2O7 (Ln = La → Lu)
series has never been fully quantified. Oxide ion conductivity in
pyrochlores (Fig. 1), a crucial parameter for electrolytes in
SOFCs, occurs due to the presence of intrinsic anion disordered
in the lattice. Ordered oxygen vacancies provide a low energy
pathway but also cause low oxide mobility due to the
discontinuous nature of the pathways. As the structure becomes
more flouritic (Fig. 2), disordered vacancies provide more mobile
oxide ions and a higher-energy continuous pathway. A pyrochlore
(Fd3m) becomes fluoritic ( Fm3m) as the formation energy of
cation antisites decreases and oxygen ions are randomly
distributed on 48f (x=0.375 with Hf at the origin), 8b, and 8a sites.
Pr2Hf2O7
*
*
*
Tb2Hf2O7
P
*
*
Nd2Hf2O7
*
Fig. 1 Ln2Hf2O7 as a
pyrochlore
Nd2Hf2O7
V
Yb2Hf2O7
L
Lu2Hf2O7
Fig. 2 Ln2Hf2O7 as a flourite
V
L
L
L
P
Tb2Hf2O7
Results
Intensity (arbitrary units)
Low-Z lanthanides result in pyrochloric compounds whereas high-Z lanthanides
form fully fluoritic ones (Fig. 3), as predicted by the phase diagrams (Fig. 4),
Intermediate lanthanides form partially disordered pyrochlores, yet evidence of
short-range order has been observed in both Tb2Hf2O7 and Dy2Hf2O7 (Fig. 5).
Conclusions
Fig. 3 XRD traces of Ln2Hf2O7
Lu
Dy2Hf2O7
Yb
Dy
Tb
Nd
Pr
La
0
20
40
60
Diffraction Angle (2θ)
As expected, the unit-cell volume was
found to decrease as Z increases
(Table
1).
Crystallographic
x
parameters for the 48f oxygen
position
increase
through
the
pyrochlore series. While some
Tb2Hf2O7 grains exhibit only shortrange order, others show clearly the
pyrochlore
superlattice.
Both
Yb2Hf2O7 and Lu2Hf2O7 are fluoritic.
80
100
120
Table 1: Crystallographic information
Compound a (Å) structure 48f x
La2Hf2O7 10.7688 Pyrochlore 0.33027
Pr2Hf2O7
d spacing (Å)
Fig 6: Neutron diffraction of Ln2Hf2O7. Some of the peaks unique to the pyrochlore phase have been marked.
Yb2Hf2O7
10.6965 Pyrochlore 0.33275
While compounds for which Ln = La, Pr, Nd, or Tb have been
established as ordered (or partially ordered) pyrochlores, no
superlattice could be found in the case of Ln = Yb or Lu, which can both
be described as fluoritic. In the case of Tb2Hf2O7, some grains showed
evidence of a superlattice with only short-range order while others were
clearly pyrochloric. The Dy2Hf2O7 compound showed only very faint
evidence of short-range order and so has been deemed fluoritic,
although no refinement has yet been possible to confirm either the
distribution of oxygen or the x parameter of its 48f oxygen site. It is still
possible that disorder exists, especially in the higher-Z pyrohafnates,
and the distribution of oxygen ions may not be uniform.
Nd2Hf2O7 10.6469 Pyrochlore 0.33334
Tb2Hf2O7
10.4716 Pyrochlore 0.34973
Acknowledgements
Dy2Hf2O7 10.4813 Pyrochlore 0.3687
Yb2Hf2O7
5.1611
Fluorite
0.375
Lu2Hf2O7
5.1478
Fluorite
0.375
Lu2Hf2O7
Fig. 4 Phase diagrams of Ln2O3-HfO2 systems
Fig. 5 Electron diffraction patterns of
Ln2Hf2O7 compounds parallel to [110]
This work has been supported by NASA ISGC, the ICDD, NSF MRI grants
(0521315 and 0619795), and the BSU Center for Materials
Characterization.