Journal of Alloys and Compounds 307 (2000) 149–156
L
www.elsevier.com / locate / jallcom
The A-site deficient ordered perovskite Th 0.25 h 0.75 NbO 3 : a re-investigation
a,
a
b
Anton R. Chakhmouradian *, Roger H. Mitchell , Peter C. Burns
a
b
Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada P7 B 5 E1
Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, IN 46556 -0767, USA
Received 22 June 1999; received in revised form 25 February 2000; accepted 20 March 2000
Abstract
The crystal structures of the non-stoichiometric perovskite Th 0.25 h 0.75 NbO 3 prepared by quenching from a melt and by solid-state
reaction are determined to be identical on the basis of single crystal X-ray diffractometry using a CCD area detector (final R52.4%) and
Rietveld refinement of the XRD powder pattern (R F 57.1%), respectively. The overall tetragonal symmetry of the structure [P4 /mmm;
˚ c57.8448(2) and 7.8502(11) A,
˚ Z52; where the smaller cell dimensions correspond to the powder data],
a53.8956(1) and 3.8994(6) A,
is derived from the cubic perovskite aristotype by long range ordering of Th 41 along the c-axis into 1b sites. In response to this ordering,
˚ along the c-axis towards the layers with vacant A-sites (1a). The TEM data obtained in this study indicate
Nb atoms are displaced 0.08 A
that long-range ordering also resulted in the formation of antiphase domains oriented parallel to [100] p and [010] p , and having an average
periodicity (M) of 6a p . In contrast to some previous studies of this compound, neither long-range ordering of Th 41 within the layers, nor
tilting of NbO 6 polyhedra was observed. This study presents a statistically superior refinement of the structure of Th 0.25 h 0.75 NbO 3.
 2000 Elsevier Science S.A. All rights reserved.
Keywords: Th 0.25 h 0.75 NbO 3 ; Perovskite; Structure; Cation ordering; Antiphase domains
1. Introduction
Complex perovskite-type oxides with the general for41
51
mula A 0.25 h 0.75 B O 3 (A5Ce, Th, U; B5Nb, Ta; h5
vacancy) were first described as products of ceramic
synthesis by Kovba and Trunov [1]. X-ray diffraction
patterns of these compounds, with the exception of orthorhombic Ce 0.25 h 0.75 NbO 3 were indexed on a tetragonal
cell. Trunov and Kovba [1] proposed that, in contrast to
]
the perovskite aristotype (Pm3 m), the studied compounds
show an ordered arrangement of the A41 cations which
results in a doubling of the periodicity along c (niobates) or
a and b (tantalates). Subsequently, Trunov and Kovba [2]
refined the structure of Th 0.25 h 0.75 NbO 3 in the space
˚ Z52), and showed
group P4 /mmm (a53.889; c57.825 A,
that in this compound, a framework of NbO 6 octahedra
hosts ‘empty’ layers and Th-bearing layers alternating
along c. As the stoichiometry of Th 0.25 h 0.75 NbO 3 does not
allow for complete occupancy of the Th-bearing layers, a
statistical distribution of 50% Th 41 and 50% vacancies
among the 1b sites was proposed [1,2]. An independent
study of Th 0.25 h 0.75 NbO 3 by Keller [3] suggested that the
*Corresponding author. Fax: 11-807-346-7853.
E-mail address: [email protected] (A.R. Chakhmouradian)
Th 41 cations are ordered not only between the successive
layers parallel to (001), but also within the individual
layers. Given that in the simple P4 /mmm model with a¯a p
and c¯2a p , there is only one Th-site per layer, further
ordering of Th 41 within the layers would inevitably result
in increasing periodicity along the a and b axes. Consequently, a new tetragonal cell with doubled periodicities
˚ Z58)
along all three axes (a5b57.783(8); c57.837(8) A,
was proposed, but no possible structural models were
discussed [3]. A single-crystal study of Th niobate by
Labeau and Joubert [4] gave results consistent with the
˚
original model of Trunov and Kovba [2], i.e. a53.898 A
˚ Finally, Alario-Franco et al. [5] have
and c57.852 A.
demonstrated that the degree of ordering of Th 41 cations
and, thus, structural characteristics of Th 0.25 h 0.75 NbO 3
depend on the rate of cooling, and have meticulously
described the orthorhombic polymorph of this compound
(P2 mm) obtained by slow cooling (2 K h 21 ) of the melt.
With regard to the quenched tetragonal polymorph, AlarioFranco et al. [5] have noted that ‘‘thorium / vacancy
ordering results in a doubling of all three perovskite
subcell axes, as reported by Keller (1965)’’ (p. 178).
Hence, contradictory interpretations of the crystal structure
and degree of ordering of Th 41 in the quenched form of
Th 0.25 h 0.75 NbO 3 have been thus far provided by different
0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved.
PII: S0925-8388( 00 )00830-6
150
A.R. Chakhmouradian et al. / Journal of Alloys and Compounds 307 (2000) 149 – 156
workers. The present study was initiated to determine the
crystal structure of this compound, and possibly resolve
the existing contradiction in the experimental data. For this
purpose, we employed transmission-electron microscopy
(TEM), as well as methods that have not been previously
applied to Th niobate, i.e. single-crystal X-ray diffraction
(XRD) using a charge-coupled device area detector and
Rietveld profile refinement of XRD powder data.
2. Experimental
Th 0.25 h 0.75 NbO 3 was prepared from stoichiometric
quantities of ThO 2 and Nb 2 O 5 (high purity grade). The
reagents were dried at 1508C for 24 h, and then mixed and
ground in an agate mortar. Sample TN1 was prepared by
reacting the starting mixture for 24 h at 14008C, i.e. well
above the liquidus, and subsequent rapid cooling of the
melt in air. We used air, not liquid N 2 , as a cooling
medium in order that crystals of Th niobate could reach the
size sufficient for single-crystal X-ray examination. Sample TN1 consisted of intimately intergrown transparent
crystals up to a few millimeters across. The crystals are
optically uniaxial and invariably exhibit multi-domain
twinning indicative of a phase transition from a higher
]
symmetry, most probably, Pm3 m. A few relatively large
crystals were crushed, and several optically homogeneous
fragments were hand-picked under the microscope. Finally,
a single-domain fragment measuring 0.1630.1630.06 mm
was chosen from this batch for single-crystal studies. The
absence of twinning in the selected crystal was later
confirmed by single-crystal XRD. Sample TN2 was prepared using the ceramic technique by heating the reagents
first for 24 h at 11008C and, after regrinding, for 48 h at
13008C. The composition of samples TN1 and TN2 was
determined using energy-dispersive X-ray spectrometry
(EDS) on a Hitachi 570 scanning electron microscope
equipped with a LINK ISIS analytical system. The EDS
examination confirmed that both samples had the desired
stoichiometry Th 0.25 h 0.75 NbO 3 .
Single-crystal diffraction studies of TN1 were carried
out using a Bruker PLATFORM four-circle diffractometer
equipped with a SMART CCD (charge-coupled device)
detector with a crystal-to-detector distance of 5 cm. In
contrast to the conventional scintillation detectors, the
CCD area detector allows simultaneous detection of X-ray
intensities over slices of reciprocal space [6]. This facilitates examination of superstructures, twinning, exsolution
and defects in crystal structures. Other advantages of the
CCD detector relative to scintillation detectors include
improved sensitivity to weak reflections, higher resolution,
and reduced data-collection times [6].
The X-ray diffraction data were collected using Mo Ka
X-radiation and v scans, with frame-widths of 0.038 and
10 s spent counting for each frame. More than a hemisphere of three-dimensional data were collected for 38#
2u #568. The data were integrated using the Bruker
program SAINT, and corrections for Lorentz, polarization,
and background effects were applied. An empirical correction for absorption was done on the basis of the intensities
of equivalent reflections with the crystal modeled as an
ellipse. The unit-cell parameters refined by least-squares
˚ are in reasontechniques (a53.8994(6); c57.8502(11) A),
able agreement with those determined by Trunov and
Kovba from electron diffraction data [2]. A total of 1366
reflections were collected, of which 122 reflections were
unique with 121 classed as observed (uFo u$4sF ).
XRD powder patterns of sample TN2 were obtained on
a Philips 3710 diffractometer (T5208C; radiation, Cu Ka;
2u range, 208–1408; step D2u, 0.028; time per step, 2 s).
The XRD patterns were analyzed by the Rietveld method
using the FULLPROF program [7], as it provides an
option to refine patterns composed of diffraction lines with
differing line-width parameters. The difference in linewidth parameters of ‘structure’ and superstructure lines is
related to the presence of antiphase domains, and commonly accompanies A- and B-site ordering phenomena in
perovskites [8–10]. Sample TN2 was also examined using
a Jeol 2010F field emission STEM operated at 200 kV.
Specimens for the TEM experiments were prepared by
placing powder suspended in ethanol on a holey carbon
grid.
3. Structure solution and refinement
3.1. Sample TN1
Scattering curves for neutral atoms, as well as anomalous dispersion corrections were taken from ‘International
Tables for X-ray Crystallography’ [11]. The Bruker
SHELXTL Version 5 system of programs was used for the
determination and refinement of the crystal structure.
Reflection statistics were consistent with the space group
P4 /mmm, in agreement with the earlier findings of Trunov
and Kovba [2]. A model that included refined positional
parameters and anisotropic displacement parameters gave a
final R value of 2.4% for 121 observed reflections (uFo u$
4sF ), and an s (goodness-of-fit) of 1.16. In the final
refinement cycle, the maximum peaks in the difference˚ 3 . The final atomic
Fourier maps were below 0.75 e / A
coordinates, anisotropic displacement parameters, selected
interatomic distances and angles for TN1 are given in
Table 1.
The structure of Th niobate obtained from the quenched
melt consists of corner-linked NbO 6 octahedra arranged in
a three-dimensional framework typical of perovskite-type
compounds. This framework hosts two types of comparatively larger cationic sites, 1a and 1b. The Th 41 cations are
statistically distributed only over one half of the 1b sites
with the other half remaining vacant to maintain the 1:4
A.R. Chakhmouradian et al. / Journal of Alloys and Compounds 307 (2000) 149 – 156
151
Table 1
Th 0.25 h 0.75 NbO 3 (melt synthesis): final positional and thermal parameters a
Atom
Position
Site occupancy
x
y
z
U11
U22
U33
Ueq b
Th
Nb
O1
O2
O3
13dNb–O1
13dNb–O2
43dNb–O3
1b
2h
1c
1d
4i
˚
1.880(1) A
˚
2.045(1) A
˚
1.9612(8) A
0.50
1.00
1.00
1.00
1.00
O3–Nb–O3
43dTh–O2
83dTh–O3
0
1/2
1/2
1/2
0
167.6(4)8
˚
2.7573(4) A
˚
2.676(5) A
0
1/2
1/2
1/2
1/2
1/2
0.2395(1)
0
1/2
0.2665(9)
0.0182(5)
0.0226(5)
0.057(6)
0.087(8)
0.045(4)
0.0182(5)
0.0226(5)
0.057(6)
0.087(8)
0.019(3)
0.0154(5)
0.0280(6)
0.012(4)
0.011(4)
0.057(4)
0.0173(5)
0.0244(5)
0.042(4)
0.061(5)
0.040(2)
a
b
˚ V5119.36(5) A
˚ 3.
R5(SuuF0 u2uFC uu) /(SuF0 u)52.4%, wR55.6%, s51.16. Unit-cell parameters (P4 /mmm): a53.8994(6), c57.8502(11) A,
Ueq 51 / 3(S i S jUij a* i a* j a i a j ).
cationic ratio of this compound (Fig. 1). Thus, the present
study essentially confirms the original structural model
suggested for Th 0.25 h 0.75 NbO 3 by Trunov and Kovba [2],
but does provide a statistically better refinement (R I 5
2.4%, as compared to R I 57.8% of Trunov and Kovba) and
significantly improved structural parameters.
Importantly, the single-crystal data for sample TN1 were
carefully checked for any reflections that would necessitate
a doubling of the a dimension, as required by a structural
model that allows for Th ordering within the h001j planes.
This model, here termed the ‘8Z’ model, is based on a
tetragonal cell (space group P4 /mmm) with all three
dimensions doubled relative to the pseudocubic cell parameters, and hence Z58. Our X-ray diffraction data did not
contain any observed reflections that required a larger unit
cell. However, as a further test for the presence of the very
Fig. 1. Crystal structure of Th 0.25 h 0.75 NbO 3 . Displacement of Nb atoms
is exaggerated for clarity. Note that only 50% of the Th sites are
statistically occupied.
weak reflections indicating doubling of the a dimension,
the data were re-integrated with the larger cell imposed.
There were 2359 reflections that would violate the small
cell if observed. Only 83 reflections had an intensity of
more than 3s, and inspection of these reflections revealed
that all were at most 4 or 5s. Given the relatively large
size of the crystal and the sensitivity of the CCD detector,
violation of the small cell should have been found if the a
dimension were doubled.
3.2. Sample TN2
Initially, the XRD pattern of sample TN2 was refined
using the ‘simple’ P4 /mmm model with a¯a p , c¯2a p and
Z52, i.e. strictly analogous to that determined by the
single-crystal study (see above). In the raw structural
model, we used the unit-cell dimensions determined by
Trunov and Kovba [2], and the ‘ideal’ atomic parameters z
(1 / 4) for Nb and O3. During the final cycles, 20 independent variables, including the instrumental parameters,
structural parameters and asymmetry-correction coefficients were refined simultaneously, and the refinement
converged at R F 57.1% and R B 57.2% (Fig. 2). To test the
assumption that Th in sample TN2 may have been partially
disordered into the 1a sites, site occupancies at 1a and 1b
were introduced into the refinement as an additional
variable. However, this refinement converged with all Th
occupying exclusively the 1b sites. Further, the two-phase
option of the FULLPROF program was used to refine the
‘structure’ and superstructure peaks separately, as some
antiphase domains may be present in the sample. This
refinement did not result in any significant improvement of
the agreement factors, nor standard deviations pertaining to
the structural parameters. Finally, new XRD patterns were
measured with higher counting times, first at 4 s and then
at 6 s per step. The refinement of these patterns following
the same procedure as described above, gave virtually no
improvement of R F or R B . The crystallographic characteristics of TN2 refined from its XRD powder pattern are
given in Table 2. Importantly, the results obtained by the
single-crystal and Rietveld profile-refinement methods are
in remarkable agreement with each other both with respect
A.R. Chakhmouradian et al. / Journal of Alloys and Compounds 307 (2000) 149 – 156
152
Fig. 2. Calculated (line), observed (dots) XRD patterns and difference spectrum for Th 0.25 h 0.75 NbO 3 . For agreement factors see Table 2.
to atomic coordinates and interatomic distances (cf. Tables
1 and 2).
Given that Th 0.25 h 0.75 NbO 3 produced by solid-state
reaction could differ structurally from that obtained from
the melt, the alternative ‘8Z’ structural model was tested
for sample TN2. In this model, the Th 41 cations are
allowed to order over three different sites (1a, 1c and 2f )
within the xy plane at z50. The Nb atoms are assigned to
the 8r, and oxygens to the 4j, 4k, 8s and 8t sites. The
‘ideal’ x and z coordinates for the Nb and oxygen atoms,
and the unit-cell parameters proposed by Keller [3], were
used during the initial stages of refinement (Table 3).
During the final cycles, 28 variables, including the site
occupancies for the three independent Th sites, were
refined simultaneously. This refinement converged with
reasonable atomic coordinates, site occupancies and isotropic displacement parameters, but gave a poorer fit
between the observed and calculated data (R B 59.6%), in
comparison with the ‘simple’ P4 /mmm model previously
tested.
3.3. TEM study
Selected-area electron diffraction (SAED) patterns obtained in the present study are shown in Fig. 3. Strong
reflections observed on the patterns from the [010] p ,
Table 2
˚ 2 )a
Th 0.25 h 0.75 NbO 3 (ceramic synthesis): final positional and isotropic thermal parameters (A
Atom
Position
Site occupancy
x
y
z
Bb
Th
Nb
O1
O2
O3
13dNb–O1
13dNb–O2
43dNb–O3
1b
2h
1c
1d
4i
˚
1.885(3) A
˚
2.037(3) A
˚
1.966(3) A
0.50
1.00
1.00
1.00
1.00
O3–Nb–O3
43dTh–O2
83dTh–O3
0
1/2
1/2
1/2
0
164(1)8
˚
2.755(1) A
˚
2.63(1) A
0
1/2
1/2
1/2
1/2
1/2
0.2403(4)
0
1/2
0.275(2)
0.63(8)
1.00(7)
3.1(4)
3.1(4)
3.1(4)
a
Final agreement factors and cell parameters (P4 /mmm): R exp 59.6%, R p 510.1%, R wp 513.2%, R F 57.1%, RB 57.2%, x 2 51.90, s51.38, a53.89556(7),
˚ V5119.048(7) A
˚ 3.
c57.8448(2) A,
b
B factors were kept at the same values for all oxygen atoms.
A.R. Chakhmouradian et al. / Journal of Alloys and Compounds 307 (2000) 149 – 156
153
Table 3
Th 0.25 h 0.75 NbO 3 (ceramic synthesis): alternative trial models a
Atom
Position
Site occupancy
˚
Unit-cell parameters (A)
Atomic coordinates
x
y
z
Intralayer ordering, no tilting (‘8Z’ model)
P4 /mmm
Th1
1a
1.00
Th2
1c
0.50
Th3
2f
0.25
Nb
8r
1.00
O1
4j
1.00
O2
4k
1.00
O3
8s
1.00
O4
8t
1.00
0
1/2
1/2
|1 / 4
|1 / 4
|1 / 4
|1 / 4
|1 / 4
0
1/2
0
|1 / 4
|1 / 4
|1 / 4
0
1/2
0
0
0
|1 / 4
0
1/2
|1 / 4
|1 / 4
a 2 a 2 c 1 tilting, no intralayer ordering
Pmc21
Th
2a
0.50
Nb
4c
1.00
O1
2a
1.00
O2
2b
1.00
O3
4c
1.00
O4
4c
1.00
0
|1 / 4
0
1/2
|1 / 4
|1 / 4
|1 / 4
|3 / 4
|1 / 4
|1 / 4
|1 / 2
|0
|0
|0
|1 / 2
|1 / 2
|1 / 4
|1 / 4
|1 / 4
|3 / 4
|3 / 4
|3 / 4
1/2
0
0
|1 / 4
0
1/2
|1 / 4
|1 / 4
a
b
7.791
a 2 a 2 c8 tilting, no intralayer ordering, minor displacement of Th b
Pmam
Th
2e
0.50
1/4
Nb
4k
1.00
1/4
O1
2e
1.00
1/4
O2
2f
1.00
1/4
O3
4g
1.00
0
O4
4h
1.00
0
c
7.845
7.845
5.509
5.509
5.509
5.509
7.845
a
For detailed discussion see text.
Based on the structural model suggested by Alario-Franco et al. [5]; these authors obtained slightly different unit-cell parameters, i.e. a5b55.517(1),
˚
c57.858(2) A.
b
]
[001] p and [110] p zone axes (Fig. 3a–c) are consistent
with the structural model used in the refinements of singlecrystal and powder data. Superlattice reflections clearly
indicating a doubled periodicity relative to a p are observed
at (0 0 1 / 2) p , whereas reflections at (1 / 2 0 0) p and (0 1 / 2
0) p are not present on our patterns (cf. data of Keller [3]).
]
On some images with [110] orientation, we resolved very
weak satellite peaks whose position is consistent with
short-range ordering of Th described in a slowly cooled
sample of Th 0.25 h 0.75 NbO 3 [5] (Fig. 3d,e). However, in
our case, the sharpness and intensity of the satellite
reflections is distinctly lower than in the slowly cooled
]
sample. The absence of such reflections in other [110] p
zone-axis patterns (Fig. 3c) suggests the existence of
domains with a slightly different degree of order (see
below).
The SAED pattern from the [001] p zone axis (Fig. 3b)
also contains weak split superlattice reflections that can be
indexed only on a doubled (face-centered) perovskite cell.
Identical reflections were observed in a slowly cooled
sample of Th niobate by Alario-Franco et al. [5]. These
authors attributed the presence of reflections at (1 / 2 1 / 2
0) p to tilting of NbO 6 octahedra in the crystal structure,
and their splitting to the presence of microdomains. From
Fig. 3, it is evident that superlattice reflections with h and
k odd, and l both odd and even are present (indexing on a
doubled cell). The odd–odd–even reflections are comparatively fainter, and were ignored from consideration by
Alario-Franco et al. [5]. We considered an alternative
structural model in which both types of reflections were
accounted for. From the equality of the two parameters a p
and b p in the structure of Th 0.25 h 0.75 NbO 3 , we infer that
tilting, if present, must be of the type aac. According to
Glazer [12], the observed superlattice reflections could
originate from a combination of anti-phase and in-phase
tilting of the NbO 6 polyhedra (tilt system a 2 a 2 c 1 ). For a
disordered structure, this type of tilting results in an overall
orthorhombic symmetry and space group Pnma [13].
Ordering of Th perpendicular to the longest axis would
result in a loss of the diagonal glide plane and transition
]
]
from Pnma to Pmc21 (a¯2a p , b¯Œ2a p , c¯Œ2a p , Z54).
‘Ideal’ atomic positions for a perovskite structure with
planar ordering of A-site cations and a 2 a 2 c 1 octahedral
tilting are listed in Table 3. Refinement of the XRD
powder data using the Pmc21 structural model gave
significantly poorer agreement between the observed and
calculated intensities (R F 512.1% and R B 511.8%), in
comparison with the simple tetragonal model. Analysis of
154
A.R. Chakhmouradian et al. / Journal of Alloys and Compounds 307 (2000) 149 – 156
]
Fig. 3. Representative SAED patterns of Th 0.25 h 0.75 NbO 3 . (a–d) Observed patterns with the electron beam parallel to [010] p (a), [001] p (b), and [110] p
]
(c,d); (e) calculated [110] p zone-axis pattern resulting from short-range ordering of Th within the (001) p planes, after Alario-Franco et al. [5].
the diffraction pattern shows that superlattice reflections
indicative of lower-than-tetragonal symmetry are indistinguishable from the background (Fig. 4). As the weak
odd–odd–even reflections could result from secondary
diffraction of electrons, we finally attempted to refine the
XRD pattern of TN2 in the two-tilt structural model
Fig. 4. Calculated (line) and observed (dots) XRD patterns for Th 0.25 h 0.75 NbO 3 , based on an alternative structural model incorporating octahedral tilting
and long-range ordering of Th. For details see Table 3. Tick marks in the upper row are indexed on a Pmc21 cell; indexing on a P4 /mmm cell (lower row)
is given for comparison.
A.R. Chakhmouradian et al. / Journal of Alloys and Compounds 307 (2000) 149 – 156
proposed by Alario-Franco et al. [5] (Table 3). However,
this refinement also proved unsuccessful.
Split superlattice reflections similar to those described
above were also observed in partially ordered Cu-, Ag- and
Au-based alloys, and attributed to the presence of regularly
spaced antiphase domains in the structure [14–18]. Size
and orientation of such domains can be estimated from the
character and magnitude of splitting [18]. The available
TEM data indicate that domains in the structure of
Th 0.25 h 0.75 NbO 3 are essentially parallel to [100] p and
[010] p , and average 6a p 36a p in size (M1 ¯M2 ¯6). In
contrast to the structurally similar Cu-alloy systems, the Th
atoms in Th niobate are not ordered within the (001) p
planes, i.e. superlattice reflections at (1 / 2 0 0) p and (0 1 / 2
155
0) p are lacking (cf. Cu 0.75 Pd 0.25 and CuAu [14,15]).
Consequently, stacking of antiphase domains in
Th 0.25 h 0.75 NbO 3 occurs essentially along [001] p (along
the direction of Th ordering), and boundaries between
individual domains are best resolved in lattice images
parallel to the z axis (Fig. 5).
High-temperature TEM studies of partially ordered
alloys demonstrate that a degree of ordering may differ
from one domain to another (e.g. Ref. [14]). It has also
been shown that antiphase domains are stable significantly
(50–1208C) above the order–disorder transition point
[14,15]. We believe that Th niobate may possess similar
characteristics, as indicated by the existence of short-range
order in some domains (Fig. 3d), and obvious structural
similarity between specimens with different cooling history
(cf. this study and Ref. [5]).
4. Discussion and conclusions
˚
Fig. 5. Lattice-resolution images of Th 0.25 h 0.75 NbO 3 . Scale bar is 50 A
for both images. (a) (100) p and (001) p lattice fringes; (b) anti-phase
boundaries viewed approximately along [110] p .
Since the pioneering works of Trunov and Kovba [2],
and Iyer and Smith [19], a number of perovskite-type
compounds have been found to exhibit cation ordering at
the A-site (e.g. Refs. [8,10]). The degree of ordering and of
structural distortion relative to the cubic aristotype in these
compounds depends on the difference in size and charge of
the A-site cations. The highest degree of ordering is
observed in non-stoichiometric perovskites of the general
formula A x h 12x BO 3 (A5Th 41 , U 41 , Ce 41 , Ln 31 ; B5
41
51
51
Ti , Nb , Ta ) incorporating high-valence cations and
a proportional amount of vacancies at the A-site [19] (this
work). Perovskites with a complete or nearly complete
occupancy at the A-site, and incorporating alkali metals as
well as the high-valence cations, may exhibit incomplete
ordering or lack ordering at all [8,10].
Most of A-site ordering phenomena described thus far in
perovskites involve systematic arrangement of cations on
alternating h001j planes, with relatively few compounds
showing alternative ordering mechanisms, e.g. on h111j in
Pb 12x Ca x TiO 3 : [20]. Among the former, the majority are
tetragonal and crystallize with the space group P4 /mmm
(a¯a p , c¯2a p and Z52). This type of structure not only
allows ordered arrangement of the A-site cations, but also
some adjustment of BO 6 polyhedra to charge imbalances
created by the presence of high-valence cations in one
layer, and vacancies in the successive layer. The most
common response to this charge imbalance and underbonding of the oxygen atoms O1 in the ‘empty’ layer is
displacement of the B-site cations from their ideal positions along c. According to the data available in the
literature [8,10,19], this displacement is invariably toward
the O1 atoms, and its magnitude ranges from 0.07 to 0.13
Å. As illustrated in Fig. 1, the Nb atoms in the structure of
Th 0.25 h 0.75 NbO 3 also exhibit a displacement of similar
˚
sign and magnitude (0.08 A).
156
A.R. Chakhmouradian et al. / Journal of Alloys and Compounds 307 (2000) 149 – 156
As shown by Trunov and Kovba [2], and in the present
study, the tetragonal polymorph of Th 0.25 h 0.75 NbO 3 exhibits incomplete ordering of Th 41 over the 12 coordinated
A-sites, retaining a generally statistical distribution of
Th 41 with respect to the 1b sites. It is noteworthy that the
orthorhombic polymorph obtained by slow cooling (2 K
h 21 ) still lacks complete long-range ordering [5]. A
complete ordering of the Th 41 cations within the populated
layers h001j would result in a P4 /mmm cell with doubled
periodicities along all three axes (‘8Z’ model, see above)
or less symmetrical structures (e.g. Pmmm) if octahedral
tilting is involved in the distortion. An alternative mechanism that may ultimately result in fully ordered distribution
of the A-site cations is a three-component tilting of BO 6
octahedra denoted as a 1 a 1 a 1 [13]. This tilting creates two
A-sites of differing size and geometry, and results in the
]
overall symmetry Im3 [13]. In contrast to the P4 /mmm
structures and their derivatives, A-site cation ordering
associated with the a 1 a 1 a 1 tilting is ‘three-dimensional’,
rather than confined to the planes perpendicular to one of
the 4-fold axes. This type of ordering is commonly
observed in complex perovskites of the general formula
99 BO 3 [21], but, to our knowledge, has not been
A 90.25 A 0.75
observed in non-stoichiometric perovskites with 75%
vacancies at the A-site. Undoubtedly, the reason why
Th 0.25 h 0.75 NbO 3 and isostructural phases fail to develop a
complete long-range ordering is a low diffusion rate of the
large, highly charged A-site cations. Consequently, higher
degrees of ordering may be expected in perovskites with
relatively ‘mobile’ low-valence cations such as Li 11 .
The results obtained in the present study essentially
confirm the original structural model suggested for
Th 0.25 h 0.75 NbO 3 by Trunov and Kovba [2]. This structure
exhibits a long-range cation ordering along c, but a
disordered distribution of the Th 41 cations within h001j.
The structural response to this type of ordering is: (i) a
distortion of the NbO 6 octahedra resulting in an offset of
the Nb atoms along c, toward the severely underbonded O1
atoms; and (ii) formation of anti-phase domains with a
superperiod of 6a p oriented parallel to [100] p and [010] p .
The single-crystal and Rietveld X-ray diffraction study
indicate that long-range ordering within the (001) planes is
not present in our samples of Th 0.25 h 0.75 NbO 3 . This
conclusion is not in contradiction with the TEM studies
that resolve some local short-range order (see Fig. 3d,e). In
our samples, such ordered domains are insufficiently well-
organized to impose their structural characteristics on the
average structure of the compound.
Acknowledgements
This work was supported by the Natural Sciences and
Engineering Research Council of Canada and Lakehead
University, Ontario, Canada. Fred Pearson of the Brockhouse Institute for Materials Research (McMaster University, Ontario) is thanked for assistance with the TEM
studies. We are also grateful to an anonymous referee for
constructive comments on the earlier version of the
manuscript.
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