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Single crystal and HREM study of the “Bi-Sr” stabilized BaMnO3 9R polytype
Materials Research Bulletin, Vol. 32, No. 1. pp. 35-42,1997 Copyright 0 1996 Elswis Science Ltd Rind in the USA. All rights resewed 0025-5408/97 S17.00 +.OO
PI1 SOO25-5408(96)00169-9
SINGLE
CRYSTAL
AND HREM STUDY OF THE “Bi-!W BaMnOs 9R POLYTYPE
STABILIZED
Ph. Boullay, M. Hervieu, Ph. Labbe and B. Raveau Laboratoire CRISMAT-ISMRA, URA 13 18 associe au CNRS, ISMRA and Universite de Caen, Bd Marechal Juin, 14050 Caen Cedex, France (Refereed) (Received August 30, 1997; Accepted October 7, 1996)
ABSTRACT Single crystals of the BaMnOs 9R polytype were grown at normal pressure. Their EDS analysis shows that they are bismuth doped (less than 3% per Mn atom), which is in agreement with their black color. The accurate structure was determined from a single crystal X-ray diffraction study. The rhombohedral lattice, referred to as the related hexagonal axes (a = 5.663(l) A and c = 20.995(3) A) and the space group R3m are consistent with the (hhc), close packed layers stacking sequence. No evidence of oxygen vacancies, with respect to the “03” stoichiometry, was detected, and the stacking defects checked from IIRBM investigations revealed that the 9R stacking is established very regularly over wide areas. KEYWORDS: A. oxides, C. X-ray diffraction
B. crystal
growth,
C. electron
microscopy,
INTRODUCTION The perovskite is one of the most attractive structures. By a number of original mechanisms, its framework can adapt to accommodate ions substitution and other non-stoichiometric features. The phenomenal range of catalytic, electronic, and magnetic properties exhibited by transition metal perovskite undoubtedly accounts for the extensive work devoted to these materials. Recently, the discovery of giant magnetoresistance in manganites with the perovskite structure [l] has attracted considerable attention to these Mn-based oxides Ln,_xA.MnOp (A = Ca, Sr, Ba, Pb). The size of the interpolated cation and the hole carrier density were demonstrated as important parameters for the latter properties. To understand 35
PH. BOUL.LAY et al.
36
Vol. 32, No. 1
the structural effects, such as lattice distortion, it is essential to get accurate information on the atom positions and bonds, especially in the Ln-A-Mn-0 systems, where numerous perovskite variants have been characterized. Among these variants, the “hexagonal” perovskites (see ref. 2 for a review) exhibit a structure characterized by a close packing of AOs layers forming octahedral cavities, but differ from the “classical” perovskite by the distribution of the M ions in the octahedral holes. When M is able to take several oxidation states and coordinations, such as Mn, oxygen deficient AMOS-~oxides are synthesized. This is the case of the BaMn0~ system, where several polytypes were characterized in the early 197Os, corresponding to the socalled 2H, 4H, 6H, 8H, lOH, and 15R (see ref. 3 as an overview). An uncommon 9R form of BaMnOs, was stabilized at normal pressure by either introducing 5% of ruthenium on the Mn sites [4] or using high pressure techniques [5-6]. Although crystals could be obtained at high pressure, no structure determination was performed tirn single cystals. In the course of the investigation of the Bi-Ba-Mn-0 system at normal pressure, we could isolate single crystals of this 9R form, which is doped with strontium and bismuth. We report herein on the single crystal X-ray structure determination and HREM study of this 9R BaMn0~_~form.
TABLE 1 Summary of Crystallographic Data A. Crystal Data Chemicalformula Z
BaMnOJ 9
Pcalc
RTm (No. 166) a = 5.663(l) A c =20.955(3) A 582.0(2) A3 6.17 g/cm3
p (MoKa) Morphology Crystal size
thin hexagonal-like plate 75X70X 10,Um
B. Data Collection Diffractometer Wavelength
MoKa 0.71073 A
Monochromator Scan mode
graphite W-8
Scan width (“) Slit aperture (mm) Theta range
1.1 +0.35tane 1.05+tane 2O-45” (0) 3 measured every 50 mn 650 hmax 11 kmsx 11 I,, 41 366
Space group Cell dimensions Volume
240 cm-’
CAD4 Enraf Nonius
Standard reflections Measured reflections kin
0
kin
0
kin
Reflections
0
with I > 3cr(l)
Vol. 32, No. 1
BARIUM MANGANESE OXIDE
37
EXPERIMENTAL Synthesis. Black lamellar single crystals of BaMnOs, 9R were grown as by-products of the following described preparation. The starting composition consisted of MnOz (Aldrich 99%), BaC03 (RP Prolabo 99.5%), and, as the melting agent, B&O3 (RP Prolabo 99%), in the molar ratio 24/6/l. The mixture was placed in an alumina crucible and heated in air at 900°C for 24 h, for decarbonation, and then heated at 126O’C for 2 h. After slow cooling at a rate of 1Wh for 260 h, the sample was cooled down to room temperature in 20 h. _ Single Crystal X-ray Diffraction. A crystal of 75 x 70 x 10 pm3 was selected for the structure determination (Table 1). The cell parameters were refined from 25 reflections with 18” I 8 I 25”. The data collection was performed on a CAD4 Enraf Nonius diffractometer using MoKa radiation. The intensities were corrected for Lorentz and polarization effects. The faces of the crystal were clearly defined and because of the strong linear absorption factor, absorption corrections were applied using the Gaussian method. The refinements of the atomic coordinates and the anisotropic thermal factors were achieved using the SDS Program 171. Electron Microscopy. Crystals were collected and ground in an agate mortar in n-butanol and deposited on a holey carbon-coated copper grid. The electron diffraction (ED) study was performed with a JEOL 200CX electron microscope fitted with an eucentric goniometer (f60”) and the high resolution electron microscopy (HREM) study was performed with a TOPCON 002B microscope having a point resolution of 1.8 A. EDS analyses were
[OlO] ED pattern showing reflection R? m space group.
FIG. 1 conditions hkl : 41 + k + 1 = 3n compatible
with the
38
PH. BOULLAY et al.
Vol. 32, No. 1
TABLE 2 Atomic Parameters for BaMn0~ element Bal
Baz Mni Mn2 Gl 02
X
Y
z
V,,
site
0
0
0
0.0070(2) 0.0074(2) 0.0042(4) 0.0037(3) 0.009( 1) 0.008(Z)
3a 6c 3b 6c 18h 9e
0 0 0 0.1489(4) 0.5
0 0.21859(3) 0 0.5 0 0.38145(7) 0.85 1l(4) 0.5584(2) 0 0 SG: RTm, a = 5.663(l) A,c= 20.955(3) A
occupancy
1 1 1 1 1 1
performed with Kevex analyzers mounted on both microscopes. Simulated calculated using the multislice method of the NCEMSS program [8]. RESULTS
images were
AND DISCUSSION
The electron difiaction investigation provided evidence that all the selected microcrystals, obtained by crushing a few single crystals of the preparation, exhibit a rhombohedral lattice.
(b)
(4
h C
I!-.-.- Mn(2)06
h C
------
Mn(l)O,
i ----.- Mn(2)06 h
b
1
I-
a FIG. 2 BalMnO, 9R structure: (a) schematic [OlO] projection and (b) perspective
view.
Vol. 32, No.. 1
39
BARIUM MANGANESE OXIDE
In regard to the related hexagonal
non-primitive
cell with c z 2 1 A, the conditions
limiting
the reflection are hkl : -h + k + 1= 3n compatible with the space group Rjm. These data are consistent with a nine-layer structure. A typical [0 lo] ED pattern is shown in Figure 1. The HREM study confirmed the (hhc)~ layer stacking mode (see the last section). The parameters of the hexagonal non-primitive cell were refined to a = 5.663(l) A and c = 20.995(3) A (Table 1). The EDS analyses were performed on numerous microcrystals. Results show that the cationic c’omposition mainly corresponds to a l/l ratio of Ba and Mn, but traces of impurities, namely, Bi and Sr, in very small amounts (always less than 3%) are also present. The presence of Bi can be attributed to the melting agent, and Sr may be an impurity of the barium carbonate which is concentrated in the crystals. Both cations are undetectable by Xray diffraction (XRD), but we cannot rule out the eventual role of such cations in the stabilization of this phase at normal pressure; therefore, this phase should be formulated as Ba,_ 6 (Bi,Sr)&InO, (8 I 0.06). The single crystal structure refinement confirms the structure consists of nine close packed (BaOs) layers stacked along the hexagonal axis according to the sequence (hhch. The refined atomic parameters (Table 2) are close to those obtained from powder XRD on high pressure synthesis samples [5-6]. The atomic positions are projected in Figure 2a. The Ba atoms are 12-fold coordinated and the Mn cations occupy the interlayer octahedral sites, forming [MnsOlz] groups of three face-sharing octahedra; these groups are linked by the comer sharing of the extreme octahedra (Fig. 2b). The calculated interatomic distances (Table 3) are significantly different from those obtained from X-ray powder data, because the oxygen positions could not be determined with accuracy in the previous work, as pointed out by the authors [6]. The Ba-0 distances range between 2.832 and 2.935 A. The Mn(1) cation, which is located on an inversion center, is thereby centered in the Mn(l)-06 octahedron and exhibits six equivalent Mn( l)--O( 1) distances of 1.906 A. This Mn( 1)-06 octahedron is located in the middle of the [Mn3012] groups (Fig. 2b) and shares faces with the two adjacents Mn(2w6 octahedra. Considering a perfect octahedron with an Mn-0 distance equal to 1.9 1 A, the calculated distance between two opposite faces is 2.2 1 A. Here this distance is close to 2.45 A, which corresponds to an elongation of the octahedron along
BaMnOJ Interatomic Ba,-02
2.902(4) 2.832( 1)
(x6) (x6)
BarO, Ba2-0, Bar01
2.840(2) 2.935(4) 2.908( 1)
(x6) (x3) (x3)
l%-Q
Mn,-Mn2 hh-0,
Mnl-Mnz Mnl-Mn I Mn*-0, Mn2-Ol
TABLE 3 Distances (A) and Angles (“)
- 2.484(2) 1.906(4)
(x2) (x6)
O,-Mn,-0, O,_Mn,-0, O,-Mn,-0,
180.0°(0) 96.8”(2) 83.2”(2)
(x3) (x6) (x6)
3.842(2) 2.484(2) 1.930(4) 1.921(l)
(x3) (xl) (x3) (x3)
0 ,-Mn2-O2 O,-MnTO, O,-Mnz-02 02-Mn2-O2
170.9”(2) 8 1.9”(2) 91.2”(l) 95.oy 1)
(x3) (x3) (x6) (x3)
40
PH. BOULLAY et al.
Vol. 32, No. 1
the c axis (O(l)-Mn(l)-O( 1) angle of 97”). The Mn(2) cations exhibit three Mn(2)-0(1) distances of 1.930 A and three others, Mn(2)-0(2), of 1.92 1 A. The Mn(2)-06 octahedra are located at the extremities of the [MnsOlt] groups and share one face with the Mn(l)-Q octahedron and one comer with an equivalent Mn(2)-06 octahedron. Again, considering a perfect octahedron with an average Mn-0 distance equal to 1.925, the distance between two opposite faces is 2.22 A. The observed distance is close to 2.27 A, which indicates that the Mn(2)-0~ octahedra are only slightly distorted. In these octahedra, the Mn(2) cations are shifted along the c axis from the center position, which should corresponds to a z coordinate close to 0.387, resulting in an elongation of the Mn( l)-Mn(2) distances. Thus, the distances between Mn( 1) and Mn(2) should be close to 2.35 A for Mn cations located in the center of undistorted octahedra, whereas inside the [MnsOlz] groups the two Mn(l)-Mn(2) distances are 2.484 A. The Mn(2)-Mn(2) distances between two groups are 3.842 A with an Mn(2)O(2)-Mn(2) angle of 180”. From the single crystal X-ray diffraction refinements, no evidence for anionic vacancies was detected, suggesting a fully oxygenated stoichiometric structure BaMn03 for the 9R form prepared under normal pressure. The existence of stacking defects is a rather usual characteristic of these phases because polytypes that only differ by the stacking mode of the (AO,) layers are easily stabilized. Electron microscopy is a powerful tool for checking the microstructural state of these layered structures, and it was established a long time ago [9] that the complex c and h stacking modes can easily be identified from EM images. The HREM investigation carried out on the crushed single crystals showed that the 9R stacking is established very regularly over wide areas, as shown from the overall image displayed in Figure 3a. In the enlarged HREM image presented in Figure 3b, high electron density zones, especially the barium positions, are highlighted. For that focus value, evaluated close to -500 A, the Ba(2) atoms located within the h layers are imaged as bright spots, whereas the Ba(1) atoms of the c layers appear as greyer ones. The sequence bright-grey-bright is correlated to the hch sequence. This interpretation is confirmed by image simulations (Fig. 3b); the simulatedthrough-focus series were calculated with the positional parameters displayed in Table 2, by varying the crystal thickness. A second type of information is provided by the HREM, dealing with the oxygen stoichiometry. Because of the weak scattering factor of oxygen, a slight variation of the oxygen content is impossible to detect if the oxygen atoms and vacancies are statistically distributed within the layers. However, a systematical existence of anionic vacancies over one specific atomic site would involve a local modification of the environment and, likely, a small displacement of the Mn cations, the coordination of which decreases. Such a feature was observed in the 6L BaMri_,Fe,O,_, [lo]. In the 9R form of BaMnOx, the contrast appears very regular whatever the focus value and the crystal thickness. This even contrast allows us to rule out the existence of short range oxygen/vacancies ordering. This is in agreement with the XRD results, in favor of an oxygen content close to 3. In that way, considering the different BaMnOs, polytypes, 2H [ 111, 9R (this work), 8H and 15R [3] appear as nearly stoichiometric. Referring to Negas and Roth’s work [3], in these cases the variation of the ratio beetween the c parameter and the number N of close packed layers (c/N) is linear versus the percentage of cubic stacking. This ratio increases with the number of cubic layers. It is interesting to note that the present 9R form lies on this line.
Vol. 32, No. 1
BARIUM MANGANESE OXIDE
41
(a)
(W FIG. 3 Typical [lOlO] HREM images. (a) A wide area: the 9R stacking is established very regularly. (b) An enlarged view: the theoretical image calculated for a focus value close to -500 A and a crystal thickness of 3 nm is superimposed. A unit cell is also represented.
42
PH. BOULLAY et al.
Vol. 32, No. 1
In conclusion, a “Bi-Sr” doped 9R BaMnO~, phase has been synthesized for the first time at normal pressure in the form of single crystals, allowing an accurate determination of its crystal structure. The main remarkable features of this compound, deals with the absence of stacking defects involving the polytypes and the quasi “03” stoichiometry. The presence of bismuth suggests that manganese exhibits a mean valency Mn(III)/Mn(IV), in agreement with the black color of the crystals, but, in any case, the Mn(II1) content is smaller than 3% of the total manganese species. REFERENCES 1. R.M. Kusters, J. Singleton, D.A. Keen, R. McGreevy and W. Hayes, Physica B 155,362 (1989). 2. 3. 4. 5. 6. 7. 8.
C.N.R Rao and B. Raveau, Transition Metal Oxides, VCH, New York (1995). T. Negas and R.S. Roth, J. Solid State Chem. 3,323 (1971). P.C. Donohue, L. Katz and R. Ward, Inorg. Chem. 5,339 (1966). Y. Syono, S. Akimoto and K. Kohn, J. Phys. Sot. Jup. 26,993 (1969). B.L. Chamberland, A.W. Sleight and J.F. Weiher, J. Solid State Gem. 1,506 (1970). V. Petricek, SDS94, Institute of Physics, Czech. Academy of Sciences, P&a. NCEMSS Program, National Center for Electron Microscopy, Materials and Chemical Science Division, Lawrence Berkeley Laboratory, Berkeley, CA (1989). 9. J.L. Hutchinson and A.J. Jacobson, .I Solid State Chem. 20,417 (1977). 10. V. Caignaert, M. Hervieu, B. Domenges, N. Nguyen, J. Pat-metier and B. Raveau, J. Solid State Chem. 73, 107 (1988). 11. A. Hardy, Actu Ctystallogr. 15, 179 (1962).
PI1 SOO25-5408(96)00169-9
SINGLE
CRYSTAL
AND HREM STUDY OF THE “Bi-!W BaMnOs 9R POLYTYPE
STABILIZED
Ph. Boullay, M. Hervieu, Ph. Labbe and B. Raveau Laboratoire CRISMAT-ISMRA, URA 13 18 associe au CNRS, ISMRA and Universite de Caen, Bd Marechal Juin, 14050 Caen Cedex, France (Refereed) (Received August 30, 1997; Accepted October 7, 1996)
ABSTRACT Single crystals of the BaMnOs 9R polytype were grown at normal pressure. Their EDS analysis shows that they are bismuth doped (less than 3% per Mn atom), which is in agreement with their black color. The accurate structure was determined from a single crystal X-ray diffraction study. The rhombohedral lattice, referred to as the related hexagonal axes (a = 5.663(l) A and c = 20.995(3) A) and the space group R3m are consistent with the (hhc), close packed layers stacking sequence. No evidence of oxygen vacancies, with respect to the “03” stoichiometry, was detected, and the stacking defects checked from IIRBM investigations revealed that the 9R stacking is established very regularly over wide areas. KEYWORDS: A. oxides, C. X-ray diffraction
B. crystal
growth,
C. electron
microscopy,
INTRODUCTION The perovskite is one of the most attractive structures. By a number of original mechanisms, its framework can adapt to accommodate ions substitution and other non-stoichiometric features. The phenomenal range of catalytic, electronic, and magnetic properties exhibited by transition metal perovskite undoubtedly accounts for the extensive work devoted to these materials. Recently, the discovery of giant magnetoresistance in manganites with the perovskite structure [l] has attracted considerable attention to these Mn-based oxides Ln,_xA.MnOp (A = Ca, Sr, Ba, Pb). The size of the interpolated cation and the hole carrier density were demonstrated as important parameters for the latter properties. To understand 35
PH. BOUL.LAY et al.
36
Vol. 32, No. 1
the structural effects, such as lattice distortion, it is essential to get accurate information on the atom positions and bonds, especially in the Ln-A-Mn-0 systems, where numerous perovskite variants have been characterized. Among these variants, the “hexagonal” perovskites (see ref. 2 for a review) exhibit a structure characterized by a close packing of AOs layers forming octahedral cavities, but differ from the “classical” perovskite by the distribution of the M ions in the octahedral holes. When M is able to take several oxidation states and coordinations, such as Mn, oxygen deficient AMOS-~oxides are synthesized. This is the case of the BaMn0~ system, where several polytypes were characterized in the early 197Os, corresponding to the socalled 2H, 4H, 6H, 8H, lOH, and 15R (see ref. 3 as an overview). An uncommon 9R form of BaMnOs, was stabilized at normal pressure by either introducing 5% of ruthenium on the Mn sites [4] or using high pressure techniques [5-6]. Although crystals could be obtained at high pressure, no structure determination was performed tirn single cystals. In the course of the investigation of the Bi-Ba-Mn-0 system at normal pressure, we could isolate single crystals of this 9R form, which is doped with strontium and bismuth. We report herein on the single crystal X-ray structure determination and HREM study of this 9R BaMn0~_~form.
TABLE 1 Summary of Crystallographic Data A. Crystal Data Chemicalformula Z
BaMnOJ 9
Pcalc
RTm (No. 166) a = 5.663(l) A c =20.955(3) A 582.0(2) A3 6.17 g/cm3
p (MoKa) Morphology Crystal size
thin hexagonal-like plate 75X70X 10,Um
B. Data Collection Diffractometer Wavelength
MoKa 0.71073 A
Monochromator Scan mode
graphite W-8
Scan width (“) Slit aperture (mm) Theta range
1.1 +0.35tane 1.05+tane 2O-45” (0) 3 measured every 50 mn 650 hmax 11 kmsx 11 I,, 41 366
Space group Cell dimensions Volume
240 cm-’
CAD4 Enraf Nonius
Standard reflections Measured reflections kin
0
kin
0
kin
Reflections
0
with I > 3cr(l)
Vol. 32, No. 1
BARIUM MANGANESE OXIDE
37
EXPERIMENTAL Synthesis. Black lamellar single crystals of BaMnOs, 9R were grown as by-products of the following described preparation. The starting composition consisted of MnOz (Aldrich 99%), BaC03 (RP Prolabo 99.5%), and, as the melting agent, B&O3 (RP Prolabo 99%), in the molar ratio 24/6/l. The mixture was placed in an alumina crucible and heated in air at 900°C for 24 h, for decarbonation, and then heated at 126O’C for 2 h. After slow cooling at a rate of 1Wh for 260 h, the sample was cooled down to room temperature in 20 h. _ Single Crystal X-ray Diffraction. A crystal of 75 x 70 x 10 pm3 was selected for the structure determination (Table 1). The cell parameters were refined from 25 reflections with 18” I 8 I 25”. The data collection was performed on a CAD4 Enraf Nonius diffractometer using MoKa radiation. The intensities were corrected for Lorentz and polarization effects. The faces of the crystal were clearly defined and because of the strong linear absorption factor, absorption corrections were applied using the Gaussian method. The refinements of the atomic coordinates and the anisotropic thermal factors were achieved using the SDS Program 171. Electron Microscopy. Crystals were collected and ground in an agate mortar in n-butanol and deposited on a holey carbon-coated copper grid. The electron diffraction (ED) study was performed with a JEOL 200CX electron microscope fitted with an eucentric goniometer (f60”) and the high resolution electron microscopy (HREM) study was performed with a TOPCON 002B microscope having a point resolution of 1.8 A. EDS analyses were
[OlO] ED pattern showing reflection R? m space group.
FIG. 1 conditions hkl : 41 + k + 1 = 3n compatible
with the
38
PH. BOULLAY et al.
Vol. 32, No. 1
TABLE 2 Atomic Parameters for BaMn0~ element Bal
Baz Mni Mn2 Gl 02
X
Y
z
V,,
site
0
0
0
0.0070(2) 0.0074(2) 0.0042(4) 0.0037(3) 0.009( 1) 0.008(Z)
3a 6c 3b 6c 18h 9e
0 0 0 0.1489(4) 0.5
0 0.21859(3) 0 0.5 0 0.38145(7) 0.85 1l(4) 0.5584(2) 0 0 SG: RTm, a = 5.663(l) A,c= 20.955(3) A
occupancy
1 1 1 1 1 1
performed with Kevex analyzers mounted on both microscopes. Simulated calculated using the multislice method of the NCEMSS program [8]. RESULTS
images were
AND DISCUSSION
The electron difiaction investigation provided evidence that all the selected microcrystals, obtained by crushing a few single crystals of the preparation, exhibit a rhombohedral lattice.
(b)
(4
h C
I!-.-.- Mn(2)06
h C
------
Mn(l)O,
i ----.- Mn(2)06 h
b
1
I-
a FIG. 2 BalMnO, 9R structure: (a) schematic [OlO] projection and (b) perspective
view.
Vol. 32, No.. 1
39
BARIUM MANGANESE OXIDE
In regard to the related hexagonal
non-primitive
cell with c z 2 1 A, the conditions
limiting
the reflection are hkl : -h + k + 1= 3n compatible with the space group Rjm. These data are consistent with a nine-layer structure. A typical [0 lo] ED pattern is shown in Figure 1. The HREM study confirmed the (hhc)~ layer stacking mode (see the last section). The parameters of the hexagonal non-primitive cell were refined to a = 5.663(l) A and c = 20.995(3) A (Table 1). The EDS analyses were performed on numerous microcrystals. Results show that the cationic c’omposition mainly corresponds to a l/l ratio of Ba and Mn, but traces of impurities, namely, Bi and Sr, in very small amounts (always less than 3%) are also present. The presence of Bi can be attributed to the melting agent, and Sr may be an impurity of the barium carbonate which is concentrated in the crystals. Both cations are undetectable by Xray diffraction (XRD), but we cannot rule out the eventual role of such cations in the stabilization of this phase at normal pressure; therefore, this phase should be formulated as Ba,_ 6 (Bi,Sr)&InO, (8 I 0.06). The single crystal structure refinement confirms the structure consists of nine close packed (BaOs) layers stacked along the hexagonal axis according to the sequence (hhch. The refined atomic parameters (Table 2) are close to those obtained from powder XRD on high pressure synthesis samples [5-6]. The atomic positions are projected in Figure 2a. The Ba atoms are 12-fold coordinated and the Mn cations occupy the interlayer octahedral sites, forming [MnsOlz] groups of three face-sharing octahedra; these groups are linked by the comer sharing of the extreme octahedra (Fig. 2b). The calculated interatomic distances (Table 3) are significantly different from those obtained from X-ray powder data, because the oxygen positions could not be determined with accuracy in the previous work, as pointed out by the authors [6]. The Ba-0 distances range between 2.832 and 2.935 A. The Mn(1) cation, which is located on an inversion center, is thereby centered in the Mn(l)-06 octahedron and exhibits six equivalent Mn( l)--O( 1) distances of 1.906 A. This Mn( 1)-06 octahedron is located in the middle of the [Mn3012] groups (Fig. 2b) and shares faces with the two adjacents Mn(2w6 octahedra. Considering a perfect octahedron with an Mn-0 distance equal to 1.9 1 A, the calculated distance between two opposite faces is 2.2 1 A. Here this distance is close to 2.45 A, which corresponds to an elongation of the octahedron along
BaMnOJ Interatomic Ba,-02
2.902(4) 2.832( 1)
(x6) (x6)
BarO, Ba2-0, Bar01
2.840(2) 2.935(4) 2.908( 1)
(x6) (x3) (x3)
l%-Q
Mn,-Mn2 hh-0,
Mnl-Mnz Mnl-Mn I Mn*-0, Mn2-Ol
TABLE 3 Distances (A) and Angles (“)
- 2.484(2) 1.906(4)
(x2) (x6)
O,-Mn,-0, O,_Mn,-0, O,-Mn,-0,
180.0°(0) 96.8”(2) 83.2”(2)
(x3) (x6) (x6)
3.842(2) 2.484(2) 1.930(4) 1.921(l)
(x3) (xl) (x3) (x3)
0 ,-Mn2-O2 O,-MnTO, O,-Mnz-02 02-Mn2-O2
170.9”(2) 8 1.9”(2) 91.2”(l) 95.oy 1)
(x3) (x3) (x6) (x3)
40
PH. BOULLAY et al.
Vol. 32, No. 1
the c axis (O(l)-Mn(l)-O( 1) angle of 97”). The Mn(2) cations exhibit three Mn(2)-0(1) distances of 1.930 A and three others, Mn(2)-0(2), of 1.92 1 A. The Mn(2)-06 octahedra are located at the extremities of the [MnsOlt] groups and share one face with the Mn(l)-Q octahedron and one comer with an equivalent Mn(2)-06 octahedron. Again, considering a perfect octahedron with an average Mn-0 distance equal to 1.925, the distance between two opposite faces is 2.22 A. The observed distance is close to 2.27 A, which indicates that the Mn(2)-0~ octahedra are only slightly distorted. In these octahedra, the Mn(2) cations are shifted along the c axis from the center position, which should corresponds to a z coordinate close to 0.387, resulting in an elongation of the Mn( l)-Mn(2) distances. Thus, the distances between Mn( 1) and Mn(2) should be close to 2.35 A for Mn cations located in the center of undistorted octahedra, whereas inside the [MnsOlz] groups the two Mn(l)-Mn(2) distances are 2.484 A. The Mn(2)-Mn(2) distances between two groups are 3.842 A with an Mn(2)O(2)-Mn(2) angle of 180”. From the single crystal X-ray diffraction refinements, no evidence for anionic vacancies was detected, suggesting a fully oxygenated stoichiometric structure BaMn03 for the 9R form prepared under normal pressure. The existence of stacking defects is a rather usual characteristic of these phases because polytypes that only differ by the stacking mode of the (AO,) layers are easily stabilized. Electron microscopy is a powerful tool for checking the microstructural state of these layered structures, and it was established a long time ago [9] that the complex c and h stacking modes can easily be identified from EM images. The HREM investigation carried out on the crushed single crystals showed that the 9R stacking is established very regularly over wide areas, as shown from the overall image displayed in Figure 3a. In the enlarged HREM image presented in Figure 3b, high electron density zones, especially the barium positions, are highlighted. For that focus value, evaluated close to -500 A, the Ba(2) atoms located within the h layers are imaged as bright spots, whereas the Ba(1) atoms of the c layers appear as greyer ones. The sequence bright-grey-bright is correlated to the hch sequence. This interpretation is confirmed by image simulations (Fig. 3b); the simulatedthrough-focus series were calculated with the positional parameters displayed in Table 2, by varying the crystal thickness. A second type of information is provided by the HREM, dealing with the oxygen stoichiometry. Because of the weak scattering factor of oxygen, a slight variation of the oxygen content is impossible to detect if the oxygen atoms and vacancies are statistically distributed within the layers. However, a systematical existence of anionic vacancies over one specific atomic site would involve a local modification of the environment and, likely, a small displacement of the Mn cations, the coordination of which decreases. Such a feature was observed in the 6L BaMri_,Fe,O,_, [lo]. In the 9R form of BaMnOx, the contrast appears very regular whatever the focus value and the crystal thickness. This even contrast allows us to rule out the existence of short range oxygen/vacancies ordering. This is in agreement with the XRD results, in favor of an oxygen content close to 3. In that way, considering the different BaMnOs, polytypes, 2H [ 111, 9R (this work), 8H and 15R [3] appear as nearly stoichiometric. Referring to Negas and Roth’s work [3], in these cases the variation of the ratio beetween the c parameter and the number N of close packed layers (c/N) is linear versus the percentage of cubic stacking. This ratio increases with the number of cubic layers. It is interesting to note that the present 9R form lies on this line.
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(a)
(W FIG. 3 Typical [lOlO] HREM images. (a) A wide area: the 9R stacking is established very regularly. (b) An enlarged view: the theoretical image calculated for a focus value close to -500 A and a crystal thickness of 3 nm is superimposed. A unit cell is also represented.
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In conclusion, a “Bi-Sr” doped 9R BaMnO~, phase has been synthesized for the first time at normal pressure in the form of single crystals, allowing an accurate determination of its crystal structure. The main remarkable features of this compound, deals with the absence of stacking defects involving the polytypes and the quasi “03” stoichiometry. The presence of bismuth suggests that manganese exhibits a mean valency Mn(III)/Mn(IV), in agreement with the black color of the crystals, but, in any case, the Mn(II1) content is smaller than 3% of the total manganese species. REFERENCES 1. R.M. Kusters, J. Singleton, D.A. Keen, R. McGreevy and W. Hayes, Physica B 155,362 (1989). 2. 3. 4. 5. 6. 7. 8.
C.N.R Rao and B. Raveau, Transition Metal Oxides, VCH, New York (1995). T. Negas and R.S. Roth, J. Solid State Chem. 3,323 (1971). P.C. Donohue, L. Katz and R. Ward, Inorg. Chem. 5,339 (1966). Y. Syono, S. Akimoto and K. Kohn, J. Phys. Sot. Jup. 26,993 (1969). B.L. Chamberland, A.W. Sleight and J.F. Weiher, J. Solid State Gem. 1,506 (1970). V. Petricek, SDS94, Institute of Physics, Czech. Academy of Sciences, P&a. NCEMSS Program, National Center for Electron Microscopy, Materials and Chemical Science Division, Lawrence Berkeley Laboratory, Berkeley, CA (1989). 9. J.L. Hutchinson and A.J. Jacobson, .I Solid State Chem. 20,417 (1977). 10. V. Caignaert, M. Hervieu, B. Domenges, N. Nguyen, J. Pat-metier and B. Raveau, J. Solid State Chem. 73, 107 (1988). 11. A. Hardy, Actu Ctystallogr. 15, 179 (1962).