A Structural Study of the Perovskite Series CaTi1-2xFexNbxO3

Article abstract of DOI:10.1006/jssc.1998.7803

An X-ray powder diffraction study of the series CaTi_1-2xFe_xNb_xO₃is presented. The series comprises orthorhombic perovskites (Pbnm,a≈b≈√2a_p,c≈2a_p,Z=4) in the range 0≤x≤0.3, and monoclinic perovskites (P2₁/n,a≈b≈√2a_p,c≈2a _p,β≠90°,Z=4) in the range 0.4≤x≤0.5. The structure of the orthorhombic members is derived from the cubic aristotype by octahedral rotationa^-a^-c⁺. The structural distortion in the monoclinic members involves octahedral rotation and short-range cation ordering at theB-site (4cand 4d). In the series CaTi_1-2xFe_xNb_xO₃, the unit-cell parameters and degree of octahedral rotation increase withx. The [111]_ptilt angle increases from 16.1° in CaTiO₃to 17.6-18.9° in CaFe_1/2Nb_1/2O₃(for the NbO₆and FeO₆octahedra, respectively). In contrast to previous studies, here the diffraction pattern of the end-member CaFe_1/2Nb_1/2O₃is interpreted to exhibit splitting of thehklandh0llines indicative of a monoclinic derivative of the CaTiO₃-type structure.

Full text of DOI:10.1006/jssc.1998.7803

JOURNAL OF SOLID STATE CHEMISTRY 138, 272—277 (1998)
ARTICLE NO. SC987803
A Structural Study of the Perovskite Series CaTi1؊2xFexNbxO3
Anton R. Chakhmouradian and Roger H. Mitchell
Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada P7B 5E1
Received November 12, 1997; in revised form February 9, 1998; accepted February 17, 1998
the octahedral tilting exhibited by CaTiO is written as
ꢀ
\ \ >
a a c .
An X-ray powder diffraction study of the series CaTi1؊2xFex
Nbx O3 is presented. The series comprises orthorhombic perov-
skites (Pbnm, a+b+'2ap, c+2ap, Z ؍ 4) in the range
The compound CaFe Nb O has not been encoun-
tered in nature, but has been synthesized and shown to have
ꢁꢂꢃ ꢁꢂꢃ ꢀ
04x40.3, and monoclinic perovskites (P21/n, a+b+'2ap, the perovskite-type structure (11). Filip’ev and Fesenko (11)
c+2ap, bO90°, Z ؍ 4) in the range 0.44x40.5. The struc-
ture of the orthorhombic members is derived from the cubic
aristotype by octahedral rotation a؊a؊c؉. The structural distor-
tion in the monoclinic members involves octahedral rotation and
short-range cation ordering at the B-site (4c and 4d). In the series
CaTi1؊2xFexNbxO3, the unit-cell parameters and degree of oc-
tahedral rotation increase with x. The [111]p tilt angle increases
claim that CaFe Nb O has a monoclinically deformed
ꢁꢂꢃ ꢁꢂꢃ ꢀ
perovskite cell, and exhibited no cation ordering at the
B-site. Therefore, the actual symmetry of this perovskite was
suggested to be orthorhombic with a+b+'2a , c+a .
The proposed monoclinic deformation of the perovskite cell
N
N
implies a shear distortion such that the angle b between
°
a
and c is no longer 90 . The aluminous analogue of
°
from 16.1° in CaTiO3 to 17.6–18.9 in CaFe1/2Nb1/2O3 (for the
N
N
CaFe Nb
O
with the formula CaAl Nb
O was
NbO6 and FeO6 octahedra, respectively). In contrast to previous
studies, here the diffraction pattern of the end-member
CaFe1/2Nb1/2O3 is interpreted to exhibit splitting of the hkl and
h0l lines indicative of a monoclinic derivative of the CaTiO3-type that among the complex oxides A
structure. ( 1998 Academic Press
ꢁꢂꢃ ꢁꢂꢃ ꢀ
ꢁꢂꢃ ꢁꢂꢃ ꢀ
suggested to have 1 : 1 cation ordering at the B-site, resulting
in a doubling of the periodicity along c (11). It is noteworthy
ꢃ> ꢀ>
ꢄ>
B
B
O
ꢁꢂꢃ
ꢁꢂꢃ ꢀ
ꢃ>
ꢀ>
(A "Ca, Sr, Ba, Pb; B "Al, In, Ga, Mn, Cr, Fe, Y, Bi,
ꢄ>
¸n; B "Nb, Ta, Sb), the majority exhibit partial or com-
plete ordering of cations at the B-site (11—15). According to
Fesenko et al. (12), the ordering is controlled by the differ-
INTRODUCTION
ence in charge (*q) and radius (*R ""R
!R
"/
(ꢄ>)
(ꢀ>)
Among naturally occurring perovskite-type compounds,
R
) of the cations. For the complex perovskites
(ꢄ>)
ꢀ>
ꢄ>
the mineral perovskite (ideally CaTiO ) is the most abun- AB
B
O , *R should exceed 0.09 for ordering to
ꢀ
ꢁꢂꢃ
ꢁꢂꢃ ꢀ
ꢀ>
dant in the Earth’s crust. Compositional variations of nat- occur (12). High *R (when B approaches or exceeds A in
urally occurring perovskite-group minerals are important radius) may result in the ‘‘inverse’’ perovskite structure (by
ꢀ>
indicators of geochemical evolution in geological systems. analogy with the inverse spinel structure (11, 13)). For Fe
-
Perovskite commonly contains significant amounts of Fe bearing perovskites, *R and thus the probability of cation
and Nb, thus forming a solid solution series with ordering at the B-site, would depend on the spin state of
ꢀ>
CaFe Nb
O
(1, 2). The maximum content of the Fe (16). *R calculated for CaFe Nb O using Shan-
ꢁꢂꢃ ꢁꢂꢃ ꢀ
ꢁꢂꢃ ꢁꢂꢃ ꢀ
ꢁꢂꢃ ꢁꢂꢃ ꢀ
ꢀ>
CaFe Nb O end-member (up to 23.5 mol.%) calculated non’s ionic radii (16), is 0.14 for the low-spin Fe
and
ꢀ>
from the electron-microprobe analyses, is found in the min- a mere 0.01 for the high-spin Fe . This suggests that the
eral latrappite (2). The crystal structure of latrappite has ordering is more likely to occur when the Fe cations are
been refined using the Rietveld method and shown to be in the low-spin state. In the absence of long-range cation
ꢀ>
orthorhombic and similar to that of CaTiO perovskite (2). ordering, the perovskite structure may still exhibit reduction
ꢀ
The structure of CaTiO perovskite has been studied in symmetry due to short-range ordering, as has been dem-
ꢀ
extensively over the past five decades (3—9). This structure is onstrated by Woodward (15) for CaFe Ta O (15).
ꢁꢂꢃ ꢁꢂꢃ ꢀ
derived from the aristotype by octahedral tilting about three
The present work is a part of the comprehensive study of
four fold axes of the cubic subcell, and represents one of the naturally occurring perovskite-type compounds and their
fundamental perovskite hettotypes. The tilting results in synthetic analogues. The objective of the present work was
orthorhombic symmetry of CaTiO (space group Pnma or to determine the structural characteristics of the series
ꢀ
Pbnm). Most authors use the unconventional setting Pbnm CaTi
Fe Nb O , with particular emphasis on the crys-
ꢁ\ꢃV
V
V ꢀ
with a+b+'2a , c+2a (5—8). In Glazer’s (10) notation, tal chemistry of the end-member CaFe Nb O (x"0.5).
N
N
ꢁꢂꢃ ꢁꢂꢃ ꢀ
272
0022-4596/98 $25.00
Copyright ( 1998 by Academic Press
All rights of reproduction in any form reserved.

STRUCTURAL STUDY OF CaTi
Fe Nb O
273
ꢁ\ꢃV
V
V
ꢀ
EXPERIMENTAL
TABLE 1
CaTiO3 (x ؍ 0): Comparison with Previously Published
Structural Data
Compositions corresponding to the series CaTi
Fe
ꢁ\ꢃV
V
Nb O were synthesized from stoichiometric amounts of
V ꢀ
Buttner and
Liu and
Lieberman (7)
CaCO , TiO , Fe O , and Nb O (high purity grade). The
ꢀ
ꢃ
ꢃ ꢀ
ꢃ ꢄ
Characteristics This study Sasaki et al. (5) Maslen (6)
°
reagents dried at 150 C were mixed, ground in an agate
°
mortar, and heated in air for 24 h at 1100 C. After regrind-
a, A
5.3814(1)
5.4418(1)
7.6409(2)
5.3796(1)
5.4423(3)
7.6401(5)
5.388(1)
5.447(1)
7.654(1)
5.3785(2)
5.4419(2)
7.6400(3)
s
°
ing, the samples were heated in air for 48 h at 1300 C and
then rapidly cooled in air to room temperature. X-ray
diffraction (XRD) powder patterns of the synthesis products
were obtained on a Philips 3710 diffractometer (¹"20 C;
radiation CuKa; 2h range 10 —145 ; *2h step 0.02 ; time per
step 2 s). The XRD patterns were analyzed by the Rietveld
method using the FULLPROF program (17).
Compositions of samples were determined by X-ray en-
ergy-dispersion spectrometry (EDS) using a Hitachi 570
scanning electron microscope equipped with a LINK ISIS
analytical system incorporating a Super ATW Light Ele-
ment Detector (133 eV FwHm MnK). EDS spectra were
aquired for 100 seconds (live time) with an accelerating
voltage of 20 kV and beam current of 0.86 nA. The spectra
were collected and processed with the LINK ISIS-SEM-
QUANT software package. Well-characterized mineral
standards were employed for the determination of Ca, Ti, Fe
(perovskite), and Nb [loparite-(Ce)]. The accuracy of this
method has been checked previously by wavelength-disper-
sion electron-microprobe analysis using an automated
CAMECA SX-50 microprobe, following procedures de-
scribed by Mitchell and Vladykin (18).
b, A
s
c, A
s
Ca
x
0.9937(7)
0.0344(3)
1/4
0.99324(7)
0.03602(6)
1/4
0.99371(7)
0.03393(6)
1/4
0.993(2)
0.033(1)
1/4
°
y
z
Ti
x
y
z
O1
x
y
z
°
°
°
0
1/2
0
0
1/2
0
0
1/2
0
0
1/2
0
0.068(1)
0.485(1)
1/4
0.0714(3)
0.4838(2)
1/4
0.0707(3)
0.4842(2)
1/4
0.072(3)
0.489(3)
1/4
O2
x
y
0.712(1)
0.2892(9)
0.0402(8)
0.7108(2)
0.2888(2)
0.0371(1)
0.7109(1)
0.2884(1)
0.0370(1)
0.717(2)
0.284(2)
0.035(2)
z
orthorhombic members of the series (x"0.1, 0.3) are given
in Table 2, and selected interatomic distances and bond
angles in Table 3. As in CaTiO , the actual coordination
number of the A-site cation for x"0.1—0.3, is eight
(2;O1#6;O2), and the Ca polyhedron is a four-fold
antiprism rather than a cubo-octahedron.
From Table 2 and Fig. 1, it is seen that the unit-cell
parameters of perovskite increase with x. Given the signifi-
cant difference between the ionic radii of Ti , or Nb and
ꢀ
RESULTS AND DISCUSSION
Complete solubility was observed between the end-mem-
ꢅ>
ꢄ>
bers of the series CaTi
Fe Nb O . In most cases, the
ꢁ\ꢃV
V
V ꢀ
ꢀ>
Fe (16), it is expected that the distortion of the structure
XRD patterns comprise only the diffraction lines of perov-
skite-type phases. For x"0.1 and 0.2, the XRD patterns
also include a weak diffraction line with d"3.25 A, which
should also increase with x. As demonstrated by Zhao et al.
corresponds to the strongest reflection of rutile TiO . The
ꢃ
TABLE 2
intensity of this line does not exceed 1.6% of the strongest
CaTi1–2xFexNbxO3: Crystallographic Characteristics
for x ؍ 0.1 and 0.3 (in italic)
perovskite line (112). The SEM/EDS analysis also indicates
the presence of minor rutile in the samples with x"0.1
and 0.2.
The crystal structure of CaTiO at room temperature has
been studied extensively and is well characterized (5—9). For Ca
Atom
Position
x
y
z
B(Aꢃ)
ꢀ
4c
4b
4c
8d
0.994(1)
0.0340(4)
1/4
—
0
—
1/4
1.17(4)
1.49(6)
0.92(4)
0.92(3)
0.5(2)
0.5(2)
1.6(2)
1.4(2)
0.992(1) 0.0386(5)
0
—
the Rietveld refinement, we used data obtained by Sasaki
Ti (Fe, Nb)
O1
1/2
—
et al. (5) as a starting structural model. In the present study,
the unconventional space Pbnm was adopted instead of
Pnma, as the former is more commonly applied to this
perovskite hettotype by materials scientists. The results of O2
the refinement for x"0 compare well with the previously
published single-crystal and Rietveld structural data (Table
Fe Nb O , the orthorhombic dis-
tortion of the perovskite structure persists to the composi-
tion x"0.3. The crystallographic characteristics of two 5.5079(5), c"7.7077(7) A.
0.071(1)
0.480(1)
0.075(1) 0.481(1)
0.711(1) 0.291(1)
0.706(1) 0.290(1)
—
0.042(1)
0.047(1)
Note. Final agreement factors and cell parameters for x"0.1: R "15.4,
ꢀ
1). In the series CaTi
R
"22.2, R "6.6, sꢃ"1.62, a"5.3939(3), b"5.4647(3), c"7.6610(4) A;
ꢁ\ꢃV
V
V ꢀ
ꢁꢀ
'
for x"0.3: R "14.3, R "19.1, R "6.6, sꢃ"1.44, a"5.4217(5), b"
ꢀ
ꢁꢀ
'

274
CHAKHMOURADIAN AND MITCHELL
TABLE 3
CaTi1؊2xFexNbxO3: Selected Interatomic Distances (A)
and Angles (°) for x ؍ 0.1 and 0.3
0.1
0.3
0.1
0.3
Ca—O1
Ca—O1
Ca—O1
Ca—O1
2;d Ca—O2 2.351
2;d Ca—O2 2.612
2;d Ca—O2 2.704
2;d Ca—O2 3.270
2.361
2.475
3.053
3.065
2.372
2.477
3.087
3.105
2.337
2.598
2.738
3.349
1.972
1.981
2.005
O1—B—O1
2;O1—B—O2
2;O1—B—O2
2;O1—B—O2
2;O1—B—O2
2;O2—B—O2
2;O2—B—O2
2;O2—B—O2
2;O2—B—O2
B—O1—B
180.0
87.8
89.8
90.2
92.2
89.0
180.0
91.0
180.0
156.5
153.8
180.0
87.4
88.8
91.2
92.6
89.1
180.0
90.9
180.0
155.5
151.6
2;d B—O1
2;d B—O2
2;d B—O2
1.956
1.959
1.982
B—O2—B
Note. B"Ti, Fe, Nb.
(19), the distortion of the orthorhombic Pbnm perovskites
can be adequately described in terms of tilting of the BX
ꢆ
octahedra about the [001] and [110] axes. In the litera-
ꢀ
ꢀ
ture, different symbols are used to denote the above oc-
tahedral rotations. This may result in confusion when the
same symbol is applied to two different types of rotation
(19, 20). In the present work, we use the symbols suggested
by Zhao et al. (19). The tilt ꢀ about [001] corresponds to
ꢀ
a single in-phase rotation of the octahedra about the c axis
of the orthorhombic (Pbnm) cell. The tilt h about [110]
ꢀ
combines two anti-phase rotations about [100] and [010]
ꢀ
ꢀ
and can be viewed as an independent tilt about the b axis of
the orthorhombic cell. The ꢀ and h rotations can be com-
FIG. 2. CaTi
Fe Nb O : crystal structures and unit-cell relation-
ꢁ\ꢃV
V
V
ꢀ
ships. (a) Orthorhombic structure of CaTiO ; ꢀ, h, and ' denote octahed-
ral tilting about [001] , [110] , and [111] , respectively. (b) Monoclinic
ꢀ
bined into a single tilt ' about the [111] axis (Fig. 2a). The
ꢀ
ꢀ
ꢀ
ꢀ
ordered structure of CaFe Nb O . Open circles represent Caꢃ>, shaded
ꢁꢂꢃ ꢁꢂꢃ
ꢀ
polyhedra are TiO (a) and FeO and NbO (b). Crystallographic axes a, b,
ꢆ
ꢆ
ꢆ
c correspond to the Pbnm or P2 /n setting; aꢀ, bꢀ, and cꢀ to the P2 /c setting.
(c) Relationships between the pseudocubic subcell a ;a ;a correspond-
ing to the aristotype Pm3m (double line), cubic cell 2a ;2a ;2a corres-
ponding to the ordered untilted structure Fm3m (thin solid line), and
monoclinic cells a;b;c (P2 /n, dashed line) and aꢀ;bꢀ;cꢀ (P2 /c, thick
solid line) corresponding to the ordered tilted structure. Two types of
ꢁ
ꢁ
N
N
N
N
N
N
ꢁ
ꢁ
B-cations are shown by open and solid circles.
tilt angles ꢀ, h, and ' can be calculated using only the
unit-cell parameters (19, 20) or both unit-cell parameters
and mean interatomic distance B—O (21). It is noteworthy
that the tilt angles calculated using the latter method are
consistently larger than those derived from the unit-cell
dimensions alone (Fig. 3). However, regardless of the calcu-
lation method, in the compositional range 04x40.3, the
octahedral rotations smoothly increase with x (Fig. 3).
Peak splitting on the XRD pattern of CaFe Nb
O
ꢁꢂꢃ ꢁꢂꢃ ꢀ
(x"0.5) is essentially similar to that observed on the XRD
FIG. 1. CaTi
(reduced to pseudocubic) with the composition.
Fe Nb O : variation of the unit-cell parameters
ꢁ\ꢃV
V
V
ꢀ
pattern of CaTiO . For x"0.5, most multiplets show better
ꢀ

STRUCTURAL STUDY OF CaTi
Fe Nb O
275
ꢁ\ꢃV
V
V
ꢀ
ꢀ>
ꢄ>
long-range ordering between Fe and Nb . However, the
hkl and h0l lines are broadened, reduced in intensity, and
flattened at the tip. This indicates further splitting of these
lines into hkl#hk-l and h0l#h0-l components, and sug-
gests a monoclinic distortion of the crystal structure. An
orthorhombic-to-monoclinic reduction in symmetry with-
out long-range ordering of the B-site cations has been de-
scribed for CaFe Ta
O
and CaMn Ta
O
by
ꢁꢂꢃ ꢁꢂꢃ ꢀ
ꢁꢂꢃ ꢁꢂꢃ ꢀ
Woodward (15). In this study, we tried both possible struc-
tural models, orthorhombic and monoclinic. A monoclinic
unit-cell derived from the parental orthorhombic cell (Pbnm
or Pnma) by 1 : 1 cation ordering at the B-site corresponds
°
to the space group P2 /n (a+b+'2a , c+2a , bO90 )
ꢁ
N
N
(22, 23).
As in the case of CaFe Ta O and CaMn Ta
O
ꢁꢂꢃ ꢁꢂꢃ ꢀ
ꢁꢂꢃ ꢁꢂꢃ ꢀ
(15), the refinement using the space group P2 /n achieves
ꢁ
FIG. 3. CaTi
Fe Nb O : variation of the tilt angles with the com-
better fit between the calculated and observed patterns than
ꢁ\ꢃV
V
V
ꢀ
position. Solid symbols are tilt angles derived directly form the unit-cell
parameters (19, 20), empty symbols are angles calculated form the unit-cell
parameters and mean B—O distances (21).
the refinement based on the orthorhombic Pbnm model. The
reduction in symmetry may result from short-range order-
ꢀ>
ing at the B-site due to the charge difference between Fe
ꢄ>
and Nb , rather than difference in ionic radii. The Mo
¨
s-
sbauer data on some synthetic (24) and naturally occurring
Fe-bearing perovskites (C. McCammon, personal commun-
ication, 1997) are consistent with the high-spin state of
resolution between the component peaks, indicating greater
structural distortion compared to CaTiO (Table 4). On the
ꢀ
XRD pattern of CaFe Nb O , the superstructure peaks
ꢁꢂꢃ ꢁꢂꢃ ꢀ
ꢀ>
ꢀ>
ꢄ>
Fe . As noted above, high-spin Fe and Nb are similar
in ionic radii (16).
h0l (h,l"odd) are not observed, suggesting that there is no
Figure 4 illustrates good agreement between the cal-
culated and observed patterns for x"0.5 and the space
TABLE 4
group P2 /n. The crystallographic characteristics of
X-ray Diffraction Data for CaFe1/2Nb1/2O3
ꢁ
ꢁꢂꢃ ꢁꢂꢃ ꢀ
CaFe Nb O are given in Table 5. The site occupancy
h
k
l
d (A)
I
I
h
k
l
d (A)
I I
ꢂ"ꢃ #!ꢄ#
refined by the Rietveld method supports the chosen struc-
ꢂ"ꢃ
#!ꢄ#
tural model and indicates some degree of ordering between
1
0
1
1
0
1
1
2
0
1
2
1
1
2
2
1
1
1
1
2
2
0
0
2
2
1
0
1
0
2
1
3.89
3.88
3.48
54
23
2
3
48
87
53
23
2
2
50
87
1
2
1
2
2
1
1
3
1
1
1
1
0
2
2
3
3
1
1
0
2
2
0
4
3
2
3
1.790
1
2
10
5
7
6
6
5
3
3
17
16
23
13
10
22
24
1
1
8
16
14
1
1
ꢀ>
ꢄ>
Fe and Nb at the 4c and 4d sites (Table 5). Selected
interatomic distances and bond angles for x"0.5 (Table 6)
demonstrate that the two types of octahedra are slightly
different in size, and that the true coordination number of
Ca is eight.
1 !3 1.779
3
2
1
11
4
6
6
5
5
3
3
16
15
22
13
10
21
23
1
1
7
16
14
1
0
2
1.754
1.740
1 !1 3.48
2
1
1 !2 2.750 100 100
0
2
2
1
2
0
2
2.778
2.750
2 !2 1.740
1.738
1
4
1 !4 1.738
1
3
The structure of the ordered perovskite CaFe Nb
O
0
1
0
0
1
2.727
2.616
2.475
2.448
2.358
46
1
1
2
2
6
3
1
3
1
1
2
1
70
35
3
1
2
48
1
1
2
2
6
3
1
3
1
1
1
1
67
34
2
1
1
0
1
1.727
1.711
ꢁꢂꢃ ꢁꢂꢃ ꢀ
is compared with that of CaTiO in Figs. 2a and 2b. In
ꢀ
3 !1 1.711
1.598
CaFe Nb O , the FeO and NbO octahedra alternate
ꢁꢂꢃ ꢁꢂꢃ ꢀ
ꢆ
ꢆ
3
2
along the x, y, and z axes of the pseudocubic subcell. In the
absence of octahedral rotation, the space group of such an
ordered structure would be Fm3m (Fig. 2c). As in the case of
3 !2 1.598
2
0
0 !3 2.339
2.334
1 !1 2.334
2.156
1 !3 2.156
2.087
2 !2 2.087
4
4
1.592
1.582
1
1
0 !4 1.582
1.579
1 !2 1.578
1.452
the orthorhombic CaTiO -type perovskites, the tilt angles
ꢀ
1
3
1
2
ꢀ, h, and ' can be used to characterize the distortion of the
structure in CaFe Nb O . In contrast to the orthorhom-
ꢁꢂꢃ ꢁꢂꢃ ꢀ
2
2
3
3
bic perovskites, the tilt angles cannot be derived directly
3 !3 1.452
4
2
2 !4 1.374
4
0
from the unit-cell dimensions because the b angle is no
1
2
0
2
2
2
0
4
3
1
2.070
1.946
1.942
1.894
1.887
0
4
1.389
1.375
°
longer 90 . Therefore, for CaFe Nb
O we used the
ꢁꢂꢃ ꢁꢂꢃ ꢀ
equations given by Groen et al. (21). The tilt angles were
calculated from the unit-cell parameters assuming regularity
of the octahedra, and compared to the tilt angles derived
from the atomic coordinates. The values obtained by two
different methods are in a good agreement with each other
1
0
1.367
1.363
8
1
7
1
2 !1 1.887
2 !3 1.311
Note. Diffraction lines with I (1 are omitted.
ꢂ"ꢃ

276
CHAKHMOURADIAN AND MITCHELL
FIG. 4. Calculated (line) and observed (dots) XRD patterns and difference spectrum for x"0.5. For the agreement factors see Table 5.
TABLE 6
(Table 6). The XRD powder pattern of the perovskite with
x "0.4 can be successfully refined using both orthorhom-
bic and monoclinic structural models. Because the hkl and
h0l diffraction lines on the XRD pattern appear split, we
have favored the monoclinic model.
Selected Interatomic Distances (A), Bond Angles (°),
and Tilt Angles (°) for CaFe1/2Nb1/2O3 (x ؍ 0.5)
Interatomic Distances and Bond Angles
The unit-cell parameters of the monoclinic members of
Ca—O1 2.336
Ca—O1 2.560
Ca—O1 2.797
2;d Fe—O1
2;d Fe—O2
2;d Fe—O3
2;d Nb—O1
2;d Nb—O2
2;d Nb—O3
O1—Fe—O1
O1—Fe—O2
O1—Fe—O2
O1—Fe—O3
O1—Fe—O3
O2—Fe—O2
1.934
1.910
1.940
2.099
2.102
2.030
180.0
85.7
94.3
O2—Fe—O3
O2—Fe—O3
O3—Fe—O3
90.2
89.8
180.0
the series CaTi
the orthorhombic members in Table 7 and Fig. 1. Variation
of the tilt angles with composition is shown on Fig. 3. (Note Ca—O1 3.431
Fe Nb O are compared with those of
ꢁ\ꢃV
V
V ꢀ
O1—Nb—O1 180.0
Ca—O2 2.396
Ca—O2 2.634
Ca—O2 2.721
Ca—O2 3.369
Ca—O3 2.389
Ca—O3 2.451
Ca—O3 3.110
Ca—O3 3.170
O1—Nb—O2
O1—Nb—O2
O1—Nb—O3
O1—Nb—O3
O2—Nb—O2 180.0
O2—Nb—O3
O2—Nb—O3
87.3
92.7
85.0
95.0
TABLE 5
CaTi1؊2xFexNbxO3: Crystallographic Characteristics for
x ؍ 0.5
86.5
93.5
180.0
87.9
92.1
Site
O3—Nb—O3 180.0
Atom Position occupancy
x
y
z
B (Aꢃ)
Ca
4e
4c
4d
4d
4c
4e
4e
4e
1
0.507(1) 0.5438(6)
0.254(1) 1.26(6)
Tilt Angles?
Fe1
Fe2
Nb2
Nb1
O1
0.33(7)
0.17(7)
0.33(7)
0.17(7)
1
0
1/2
1/2
0
0
0
0
0
0.75(4)
0.75(4)
0.75(4)
0.75(4)
Derived from unit-cell parameters Derived from atomic coordinates
1/2
0
ꢀ (Fe)
ꢀ (Nb)
h (Fe)
h (Nb)
' (Fe)
' (Nb)
11.9
11.0
14.9
13.8
18.9
17.6
ꢀ (Fe)
ꢀ (Nb)
h (Fe)
h (Nb)
' (Fe)
' (Nb)
10.0
10.9
12.4
11.9
15.9
16.1
0
1/2
0.200(4)
0.292(4)
0.427(2)
0.224(4) !0.057(3) 0.7(6)
0.684(4) !0.033(3) 1.0(7)
0.978(2)
O2
O3
1
1
0.256(2) 0.6(3)
Note. Final agreement factors: R "14.6, R "19.0; R "6.4; sꢃ"
ꢀ
ꢁꢀ
'
1.63; cell parameters in the space group P2 /n: a"5.4480(4), b"5.5517(4),
ꢁ
? Tilt angles calculated from the equations given by Groen et al. (21). The
c"7.7612(5) A, b"90.11(2)°; cell parameters in the space group P2 /c:
ꢁ
tilt ꢀ corresponds to rotation of the octahedra about [0 01] , h to rotation
a"7.7612(5), b"5.5517(4), c"9.474(3) A, b"144.2(3)°. B factors are
ꢀ
kept at the same values for all B-site cations.
about [1 1 0] , and ' to octahedral tilting about [1 1 1] .
ꢀ
ꢀ

STRUCTURAL STUDY OF CaTi
Fe Nb O
277
ꢁ\ꢃV
V
V
ꢀ
TABLE 7
CaTi1؊2xFexNbxO: Variation of Unit-cell Parameters (A)
and Angles (°)
Doctors Patrick Woodward, Jeffrey Foley, and Andrew McDonald and
two reviewers for the Journal of Solid State Chemistry provided many
constructive comments on the early version of the manuscript and on use of
conventional and unconventional settings in the monoclinic system. Dr. J.
Rodriguez-Carvajal is thanked for providing access to the FULLPROF
program.
x
a
b
c
b
Orthorhombic (Pbnm)
0
5.3814(1)
5.3939(3)
5.4066(4)
5.4217(5)
5.4418(1)
5.4647(3)
5.4871(5)
5.5079(5)
7.6409(2)
90
90
90
90
REFERENCES
0.1
0.2
0.3
7.6610(4)
7.6814(6)
7.7077(7)
1. A. R. Chakhmouradian and R. H. Mitchell, Can. Mineral. 35, 1293
(1997).
Monoclinic (P2 /n)
2. R. H. Mitchell, J. B. Choi, F. C. Hawthorne, C. A. McCammon, and
P. C. Burns, Can. Mineral. 35, in press (1997).
3. St. v. Naray-Szabo´ , Naturwissenschaften 41, 202 (1943).
ꢁ
0.4
0.5
5.4358(5)
5.4480(4)
5.5342(5)
5.5517(4)
7.7414(5)
7.7612(5)
90.20(3)
90.11(2)
4. H. F. Kay and P. C. Bailey, Acta Crystallogr. 10, 219 (1957).
5. S. Sasaki, C. T. Prewitt, J. D. Bass, and W. A. Schulze, Acta Crystallogr.
Sect. C 43, 1668 (1987).
that the monoclinic members have different tilt angles for
the two types of octahedra). From Figs. 1 and 3, it is obvious
that the unit-cell dimensions and tilting of the octahedra
correlate positively with x. The increase in tilt angles with
increasing Fe and Nb contents presumably results from the
structural tension created by difference in charge and radii
between these two elements and Ti.
6. R. H. Buttner and E. N. Maslen, Acta Crystallogr. Sect. B 48, 644
(1992).
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(1965).
CONCLUSIONS
12. E. G. Fesenko, V. S. Filip’ev, and M. F. Kupriyanov, Izv. Akad. Nauk.
SSSR Ser. Fiz. 28, 669 (1964). [In Russian]
The present work demonstrates complete solubility in the 13. H. Brusset, H. Gillier-Pandraud, and M. Pax Rajaonera, C. R. Seances
Acad. Sci. Ser. C 271, 810 (1970).
perovskite series CaTi
range 04x40.3, the XRD powder patterns show splitting
of diffraction lines typical of the orthorhombic CaTiO -type
Fe Nb O . In the compositional
ꢁ\ꢃV
V
V ꢀ
14. P. M. Woodward, R.-D. Hoffman, and A. W. Sleight J. Mater. Res. 9,
2118 (1994).
15. P. M. Woodward, Ph.D. Thesis. Oregon State University, 1996.
16. R. D. Shannon, Acta Crystallogr. Sect. A 32, 751 (1976).
ꢀ
perovskites. In the range 0.44x40.5, the hkl and h0l
diffraction lines are broadened and reduced in intensity, 17. J. Rodriguez-Carvajal, ‘‘FULLPROF program: Rietveld Pattern
Matching Analysis of Powder Patterns.’’ Institut Leue—Longevin,
Grenoble, 1990.
18. R. H. Mitchell and N. V. Vladykin, Mineral. Mag. 57, 651 (1993).
19. Yu. Zhao, D. J. Weidner, J. B. Parise, and D. E. Cox, Phys. Earth Planet.
Inter. 76, 1 (1993).
indicating a further reduction of symmetry to monoclinic.
This distortion results from short-range ordering of Fe
ꢀ>
ꢄ>
and Nb
CaTi
between the 4c and 4d sites. In the series
Fe Nb O , the unit-cell parameters and degree of
ꢁ\ꢃV
V
V ꢀ
octahedral tilting (measured in rotation angles ꢀ, h, and ') 20. H. D. Megaw, ‘‘Crystal Structures: A Working Approach.’’ Saunders,
increase with x.
Philadelphia, 1973.
21. W. A. Groen, F. P. F. van Berkel, and D. J. W. Ijdo, Acta Crystallogr.
Sect. C 42, 1472 (1986).
ACKNOWLEDGMENTS
22. M. T. Anderson, K. B. Greenwood, G. A. Taylor, and K. R. Poep-
pelmeier, Prog. Solid State Chem. 22, 197 (1993).
This work was supported by the Natural Sciences and Engineering 23. P. M. Woodward, Acta Crystallogr. Sect. B 53, 32 (1997).
Research Council of Canada and Lakehead University, Ontario, Canada. 24. S. Nomura and T. Nakagawa, J. Phys. Soc. Jpn. 30, 491 (1971).