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Structure of perovskite type compounds, ALaMnTaO6 (A=Sr, Ba)
Journal of Alloys and Compounds 274 (1998) 122–127
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Structure of perovskite type compounds, ALaMnTaO 6 (A5Sr, Ba) Takayoshi Horikubi*, Hayato Watanabe, Naoki Kamegashira Department of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi, 441 -8580 Japan Received 10 January 1998; received in revised form 14 March 1998
Abstract The crystal structure of ALaMnTaO 6 (A5Sr, Ba) have been determined from the powder X-ray diffraction data. The Rietveld refinement confirms that SrLaMnTaO 6 is a monoclinic with the space group P2 1 /n, a50.56939(1), b50.57561(2), c50.80750(2) nm and b 589.99(1) deg. The deformation of oxygen octahedron was derived and the oxygen octahedron tilts against c-axis. The profile ¯ and the lattice parameter refinement of BaLaMnTaO 6 shows that it has a structure of the (NH 4 ) 3 FeF 6 -type, with the space group Fm3m a50.81752(2) nm. The ordered distribution of Mn and Ta ions over the B sites was derived in both compounds. 1998 Elsevier Science S.A. Keywords: Perovskite type compounds
1. Introduction The compounds with the structure derived from the double perovskite of the general formula A 2 BB9O 6 and AA9BB9O 6 have been widely studied. A typical feature of the crystal structure of these oxides is the presence of a superlattice owing to the ordered arrangement of the cations in the oxygen octahedral nodes (B sites). The superlattice formation due to displacement of the anions from their ideal sites may be also considered as another cause. In general, the size of the cation at the A site influences a larger effect on the crystal symmetry significantly, while that of the cation at the B site does not change the symmetry but changes the lattice volume proportionally. For compounds with the formula ALaMnMO 6 or ALaTaMO 6 (A5Sr, Ba, M5transition metal), several papers have been reported such as SrLaMnTiO 6 SrLaMnIrO 6 , SrLaCoTaO 6 , SrLaNiTaO 6 , SrLaCuTaO 6 [1], SrLaFeTaO 6 [2], SrLaMgWO 6 , SrLaMnWO 6 [3], BaLaMnMoO 6 [4], SrLaMnMoO 6 , SrLaMgTaO 6 , BaLaFeTaO 6 , BaLaMgTaO 6 [5]. SrLaMnTaO 6 and BaLaMnTaO 6 have been firstly synthesized by Nakamura and Gohshi [4], and Nakamura and Choy [5]. According to them, these compounds show pseudo-cubic symmetry, which suggests the perovskite structure with superlattice lines caused by the rock salt arrangement (1:1 ordering) of Mn and Ta ions in the B *Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00514-3
sites. The magnetic susceptibilities of SrLaMnTaO 6 and BaLaMnTaO 6 obey the Curie-Weiss law above room temperature. Any other reports on these compounds have not been known. The present paper describes the detailed crystal structure of SrLaMnTaO 6 and BaLaMnTaO 6 using Rietveld analysis with powder X-ray diffraction patterns. The formation of these perovskite compounds needs the existence of divalent manganese ions so that the comparatively low oxygen partial pressure such as in the wet hydrogen atmosphere [4] is necessary during the preparation at high temperature. The phase behavior under various, preparative conditions is also studied with X-ray diffractometry.
2. Experimental The polycrystalline sample of ALaMnTaO 6 (A5Sr, Ba) was prepared from high-purity ACO 3 (A5Sr, Ba) (99.9%), La 2 O 3 (99.99%), MnO (99.9%), and Ta 2 O 5 (99.9%). These starting materials were treated by the following method before use. ACO 3 was heated at 673 K in CO 2 flow for 24 h. La 2 O 3 was heated at 1273 K in Ar flow for 24 h. MnO was heated at 1273 K in H 2 flow for 24 h and quenched. Ta 2 O 5 was heated at 1273 K in air for 24 h. Mixture of these powders with the ratio 2:1:2:1 of the starting materials was pressed into pellet and calcined at 1523 K in Ar gas containing 1% hydrogen for 48 h. Then the sample was slowly cooled to room temperature in the
T. Horikubi et al. / Journal of Alloys and Compounds 274 (1998) 122 – 127
furnace. The formation of the single phase of each ALaMnTaO 6 was identified by powder X-ray diffractometry. The powder X-ray diffraction data of SrLaMnTaO 6 and BaLaMnTaO 6 were collected with Cu–Ka radiation using 18 MAC MXP powder X-ray diffractometer equipped with a single-crystal graphite monochromator at room temperature. The conditions for data collection were as follows: 2u range, 10,2u ,100 deg.; step width (2u ), 0.04 deg.; counting time, 4 s; number of data, 2250. The powder X-ray diffraction patterns obtained were analyzed by Rietveld method using the program RIETAN [6,7]. The pseudo-Voigt peak-shape function without preferred crystallite orientation was used. The refined parameters were the 28 zero-point shift, scale factor, background parameters, profile half-width parameters (U, V, W ), asymmetry correction factor (A), lattice constants, atomic fractional coordinates and isotropic thermal parameters. The excluded region is between 108 and 258 for the both refinements.
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Table 1 Crystallographic data for ALaMnTaO 6 from Rietveld refinement at room temperature, the standard deviation of last digit is given in parenthesis SrLaMnTaO 6
BaLaMnTaO 6
Space group a / nm b / nm c / nm b / degree Volume / nm 3 Z Calculated density / g cm 23 No. of reflections No. of profile parameters refined No. of structure parameters refined No. atoms / asymmetric unit
P2 1 /n 0.56940 (1) 0.57561 (1) 0.80751 (2) 89.99(1) 0.2647 2 7.0071 542 14 18 6
¯ Fm3m 0.81752(2) – – – 0.5464 4 7.3926 54 14 6 4
Reliability factor R wp Rp RE RI RF
8.91% 6.82% 5.18% 2.39% 2.13%
9.49% 7.29% 4.21% 2.80% 2.02%
R wp 5ho w i [y obs -y cal ] 2 / o w i [y obs ] 2 j 1 / 2 , R p 5(o uy obs -y cal u) / o y obs , R I 5(o uIobs 2Ical u) / o Iobs , R F 5[o u(Iobs )1 / 2 2(Ical )1 / 2 u] /(Iobs )1 / 2 .
3. Results and discussion
3.1. SrLaMnTaO6 The powder X-ray diffraction pattern of SrLaMnTaO 6 obtained in this experiment showed that this compound has an orthorhombic or monoclinic cell. The most probable space group for SrLaMnTaO 6 estimated by the CELL program [8,9] belonged to monoclinic symmetry. Reflection conditions of h1l52n for h0l, k52n for 0kl, h52n for h00 and l52n for 00l, correspond to the P2 1 /n (No. 14) space group. There are several perovskite type compounds with P2 1 /n space group and the model of the structure for the Rietveld refmement was built by reference to that of CaPrLiTeO 6 [10] with a distorted perovskite structure. In this refinement both models for a random and an ordered distribution of Mn and Ta ions in the B-sites (octahedral sites) were carried out, while a random distribution of Sr and La ions over A-sites is assumed. In the refined structure the assigned occupation sites for an ordered model are 4e for Sr / La, O1, O2 and O3, 2d for Mn and 2c for Ta, respectively. In the disordered model various occupation ratio of Mn / Ta was assumed over 2d and 2c site. The crystallographic data are given in Table 1 and a plot of the observed and calculated profiles are shown in Fig. 1. The positional parameters obtained in this refinement are listed in Table 2 for the ordered model. Since the angle b (89.99(1) deg.) is nearly equal to 90 deg., the possibility of orthorhombic symmetry was also evaluated with the Pmmn space group (No. 59) which was most probable estimated from the CELL program [8,9] and the extinction law of diffraction patterns. Comparing both data, the R wp value for P2 1 /n is much better than that for
Fig. 1. The observed and calculated profiles of SrLaMnTaO 6 . (P2 1 /n and B-site ordering are assumed. Experimental points are shown by circles and the calculated profile by a solid line. The bottom curve is the difference pattern, y obs 2y calc and the small bars indicate the angular positions of the allowed Bragg reflections). Table 2 Fractional atomic coordinates of SrLaMnTaO 6 at room temperature, the standard deviation of last digit is given in parentheses Atom
Position a
gb
x
y
z
Sr / La Mn Ta O1 O2 O3
4e 2d 2c 4e 4e 4e
1.0 1.0 1.0 1.0 1.0 1.0
0.508(2) 1/2 0 0.212(7) 20.276(8) 0.392(7)
0.5357(4) 0 1/2 0.228(7) 0.317(8) 20.015(5)
0.2485(7) 0 0 20.054(6) 0.021(7) 0.262(6)
a
, Multiplicity and Wyckoff notation. , Occupancy.
b
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Fig. 2. Difference of calculated profile of SrLaMnTaO 6 between Pmmn (a) and P2 1 /n (b). (Experimental points are shown by circle and the calculated profile by a solid line. The bottom curve is the difference pattern, y obs 2y calc and the small bars indicate the angular positions of the allowed Bragg reflections).
Pmmn (R wp 515.47%). One of characteristic difference of the diffraction patterns between these two space groups is shown in Fig. 2, where fitting to the detailed diffraction patterns are compared for both space groups. It is clear from this figure that the degree of fitting for P2 1 /n is better than that for Pmmn. For P2 1 /n space group, the distribution of Mn and Ta over B sites was also studied for various occupation ratio of Mn / Ta, whose results are shown in Fig. 3. The extra superlattice lines by the occupation of Mn and Ta ions in 2c and 2d sites,
Fig. 4. X-ray profile refinement of SrLaMnTaO 6 based on ordered (NaCl-type) (a) and disordered model (b).
respectively, are observed as shown in Fig. 4. It is straightly concluded that the possibility of random distribution of Mn and Ta is excluded from these results. The bond distances and bond angles between atoms are given in Table 3. A slightly distorted octahedral arrangement around the Mn and Ta atoms exists in this compound.
Table 3 Interatomic distances (nm) and bond. angles (deg.) of SrLaMnTaO 6 , the standard deviation of last digit is given in parentheses Sr / La-O1
Ta-O1 -O2 -O3 Mean length: Shannon (sum): Mn-O1 -O2 -O3 Mean length: Shannon (sum):
0.203 (3)32 0.190 (4)32 0.202 (5)32 0.198 0.204 0.214 (3)32 0.223 (4)32 0.220 (5)32 0.219 0.223
Mean length: Shannon (sum):
0.249 (5) 0.262 (5) 0.298 (5) 0.345 (5) 0.254 (7) 0.268 (5) 0.290 (5) 0.341 (6) 0.230 (4) 0.267 (3) 0.324 (3) 0.343 (4) 0.289 0.280
O1-O2 (around Mn) O1-O3 (around Mn) O2-O3 (around Mn)
0.302 (7) 0.319 (7) 0.295 (7)
O1-O2 (around Ta) O1-O3 (around Ta) O2-O3 (around Ta)
0.289 (7) 0.284 (6) 0.295 (7)
Sr / La-O2
Sr / La-03
Fig. 3. R wp vs. degree of Mn occupancy on 2d site of SrLaMnTaO 6 (S.G. P2 1 /n).
/Mn-O-Ta (along [001] direction) /Mn-O-Ta (along [110] direction) /O1-Mn-O2 /O1-Mn-O3 /O2-Mn-O3 /O1-Ta-O2 /O1-Ta-O3 /O2-Ta-O3
152(3) deg. 146(2) deg. 93(2) deg. 90(2) deg. 83(2) deg. 95(3) deg. 89(2) deg. 98(2) deg.
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Fig. 5. Distorted octahedron of SrLaMnTaO 6 (S.G. P2 1 /n) around Mn and Ta ions.
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angle among Mn, O and Ta ions are different along each direction. There are usually twelve oxygen ions around the A-site, but Sr and La on the A-site are surrounded by eight oxygens on this compound and four oxygens are further away (above 0.3 nm) from the atoms on the A-site as is shown in Fig. 6. The overview of SrLaMnTaO 6 is shown in Fig. 7. The oxygen octahedron tilts against c-axis with an angle of 148. The effect of ambient oxygen partial pressure on the formation of SrLaMnTaO 6 phase was also studied. This phase could be prepared by heating under refined Ar atmosphere, although the degree of split in some diffraction peaks became ambiguous as is shown in Fig. 8 near 2u 5568, probably because of a little change of oxygen nonstoichiometry. The similar effect was observed for high temperature X-ray diffractometry as shown Fig. 9. Broadening is seen in several peaks as temperature increases, corresponding to liberation from a higher degree of distortion by thermal energy. Therefore, the complete formation of the highly distorted phase of SrLaMnTaO 6 is rather subtle depending upon preparative conditions.
There are three different Mn–O distances, 0.214(3), 0.223(4) and 0.220(5) nm (mean length: 0.219 nm) and also three different Ta–O distances, 0.203(3), 0. 190(4) and 0.202(5) nm (mean length: 0.198 nm). These bond distances could be well compared with the value of 0.223 nm for Mn–O and of 0.204 nm for Ta–O distances calculated from the ionic radii of Shannon [11]. The coordination around Mn and Ta ions is illustrated in Fig. 5. The MnO 6 and TaO 6 octahedra in SrLaMnTaO 6 are thus slightly deformed and connected each other with zigzag in turn along three directions. The tilting angle of octahedra is about 148 against c-axis. There are three different lengths between Mn and O ions in an octahedron consisting of six oxygen surrounding one manganese ion. The similar situation exists in an oxygen octahedra surrounding one Ta ion, although the deformation is a little different ways. The
3.2. BaLaMnTaO6
Fig. 6. Environment around the Sr / La site (projection along [110] direction).
Fig. 7. Overview structure of SrLaMnTaO 6 . The TaO 6 octahedra are shaded, and the MnO 6 one are unshaded. Spheres show Sr / La site.
The powder X-ray diffraction patterns of BaLaMnTaO 6 obtained in this experiment showed that the compound has a cubic cell. The space group for BaLaMnTaO 6 estimated by the CELL program [8,9] was one of cubic symmetry and a reflection condition is as follows: h1k, h1l, k1l5 2n for hkl, k, l52n for 0kl, h1l52n for hhl and h52n for ¯ (No. 225) space group. h00, corresponding to the Fm3m ¯ space group is shown as Superlattice lines with Fm3m asterisk in Fig. 10. Since there are no diffraction lines in
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¯ Fig. 10. The observed and calculated profiles of BaLaMnTaO 6 . (Fm3m and B-site ordering are assumed. Experimental points are shown by circle and the calculated profile by a solid line. The bottom curve is the difference pattern, y obs 2y calc and the small bars indicate the angular positions of the allowed Bragg reflections).
Fig. 8. Powder X-ray diffraction patterns of SrLaMnTaO 6 prepared in 1%H 2 –Ar flow (a) and in Ar flow (b).
¯ near these superlattice lines, it is obvious that fitting Pm3m ¯ ¯ by Fm3m is better than by Pm3m. The model of the structure for the Rietveld refinement was built by reference to that of BaPrLiTeO 6 [10]. In this model, the two kinds of atoms on B sites (Mn and Ta) are considered to be either random or ordered distribution, respectively. In this compound, the ordered distribution was also preferred, judging from both of the values of R wp and the superlattice lines.
In Table 1 the crystallographic data of BaLaMnTaO 6 are also given. In the refined structure, the assigned occupation sites are 8c for Ba / La, 4b for Mn, 4a for Ta and 24e for O, respectively. Positional parameters obtained are listed in Table 4. The degree of agreement between calculated and observed X-ray patterns are also shown in Fig. 10. The bond distances and bond angles between atoms in BaLaMnTaO 6 are given in Table 5. The bond distances of 0.211(9) nm for Mn–O and 0.197(9) nm for Ta–O are also comparable with the values of 0.223 nm for Mn–O and 0.204 nm for Ta–O distances calculated from the ionic radii of Shannon [11].
Table 4 Fractional atomic coordinates of BaLaMnTaO 6 at room temperature, the standard deviation of last digit is given in parentheses Atom
Position a
gb
x
y
z
Ba / La Mn Ta O
8c 4b 4a 24e
1.0 1.0 1.0 1.0
1/4 1/2 0 0.24(1)
1/4 1/2 0 0
1/4 1/2 0 0
a
, Multiplity and Wyckoff notation. , Occupancy.
b
Table 5 Interatomic distances (nm) of BaLaMnTaO 6 , the standard deviation of last digit is given in parentheses
Fig. 9. High temperature powder X-ray diffraction patterns of monoclinic SrLaMnTaO 6 at 713K (a), 603K (b), 493K (c), 383K (d) and room temperature (e).
Ba / La–O Ta–O Mn–O
Calculated
Shannon
0.28912(7) 0.197(9) 0.211(9)
0.301 0.204 0.223
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4. Conclusions The crystal structure of SrLaMnTaO 6 is slightly distorted with monoclinic symmetry, while that of BaLaMnTaO 6 is cubic with a double cell size of normal perovskite phase.
References [1] G. Blasse, J. Inorg. Nucl. Chem. 27 (1965) 993. [2] T. Nakamura, T. Sata, J. Phys. Soc. Jpn. 30 (1971) 1501.
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[3] M. Yoshimura, K. Kamata, T. Nakamura, Chem. Lett. (1972) 737– 740. [4] T. Nakamura, Y. Gohshi, Chem. Lett. (1975) 1171–1176. [5] T. Nakamura, Jin-Ho Choy, J. Solid State Chem. 20 (1977) 233. [6] F. Izumi, J. Crystallogr. Jpn. 27 (1985) 23. [7] F. Izumi, J. Minerallogr. Soc. Jpn. 17 (1985) 37. [8] Y. Takaki, T. Taniguchi, K. Nakata, H. Yamaguchi, J. Ceram. Soc. Jpn. 97(7) (1989) 763. [9] Y. Takaki, T. Taniguchi, K. Hori, J. Ceram. Soc. Jpn. 101(3) (1993) 373. [10] M.L. Lopez, A. Jerez, C. Pico, R. Saez-Puche, M.L. Veiga, J. Solid State Chem. 105 (1993) 19. [11] R.D. Shannon, C.T. Prewitt, Acta Cryst. B25 (1969) 925.
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Structure of perovskite type compounds, ALaMnTaO 6 (A5Sr, Ba) Takayoshi Horikubi*, Hayato Watanabe, Naoki Kamegashira Department of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi, 441 -8580 Japan Received 10 January 1998; received in revised form 14 March 1998
Abstract The crystal structure of ALaMnTaO 6 (A5Sr, Ba) have been determined from the powder X-ray diffraction data. The Rietveld refinement confirms that SrLaMnTaO 6 is a monoclinic with the space group P2 1 /n, a50.56939(1), b50.57561(2), c50.80750(2) nm and b 589.99(1) deg. The deformation of oxygen octahedron was derived and the oxygen octahedron tilts against c-axis. The profile ¯ and the lattice parameter refinement of BaLaMnTaO 6 shows that it has a structure of the (NH 4 ) 3 FeF 6 -type, with the space group Fm3m a50.81752(2) nm. The ordered distribution of Mn and Ta ions over the B sites was derived in both compounds. 1998 Elsevier Science S.A. Keywords: Perovskite type compounds
1. Introduction The compounds with the structure derived from the double perovskite of the general formula A 2 BB9O 6 and AA9BB9O 6 have been widely studied. A typical feature of the crystal structure of these oxides is the presence of a superlattice owing to the ordered arrangement of the cations in the oxygen octahedral nodes (B sites). The superlattice formation due to displacement of the anions from their ideal sites may be also considered as another cause. In general, the size of the cation at the A site influences a larger effect on the crystal symmetry significantly, while that of the cation at the B site does not change the symmetry but changes the lattice volume proportionally. For compounds with the formula ALaMnMO 6 or ALaTaMO 6 (A5Sr, Ba, M5transition metal), several papers have been reported such as SrLaMnTiO 6 SrLaMnIrO 6 , SrLaCoTaO 6 , SrLaNiTaO 6 , SrLaCuTaO 6 [1], SrLaFeTaO 6 [2], SrLaMgWO 6 , SrLaMnWO 6 [3], BaLaMnMoO 6 [4], SrLaMnMoO 6 , SrLaMgTaO 6 , BaLaFeTaO 6 , BaLaMgTaO 6 [5]. SrLaMnTaO 6 and BaLaMnTaO 6 have been firstly synthesized by Nakamura and Gohshi [4], and Nakamura and Choy [5]. According to them, these compounds show pseudo-cubic symmetry, which suggests the perovskite structure with superlattice lines caused by the rock salt arrangement (1:1 ordering) of Mn and Ta ions in the B *Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00514-3
sites. The magnetic susceptibilities of SrLaMnTaO 6 and BaLaMnTaO 6 obey the Curie-Weiss law above room temperature. Any other reports on these compounds have not been known. The present paper describes the detailed crystal structure of SrLaMnTaO 6 and BaLaMnTaO 6 using Rietveld analysis with powder X-ray diffraction patterns. The formation of these perovskite compounds needs the existence of divalent manganese ions so that the comparatively low oxygen partial pressure such as in the wet hydrogen atmosphere [4] is necessary during the preparation at high temperature. The phase behavior under various, preparative conditions is also studied with X-ray diffractometry.
2. Experimental The polycrystalline sample of ALaMnTaO 6 (A5Sr, Ba) was prepared from high-purity ACO 3 (A5Sr, Ba) (99.9%), La 2 O 3 (99.99%), MnO (99.9%), and Ta 2 O 5 (99.9%). These starting materials were treated by the following method before use. ACO 3 was heated at 673 K in CO 2 flow for 24 h. La 2 O 3 was heated at 1273 K in Ar flow for 24 h. MnO was heated at 1273 K in H 2 flow for 24 h and quenched. Ta 2 O 5 was heated at 1273 K in air for 24 h. Mixture of these powders with the ratio 2:1:2:1 of the starting materials was pressed into pellet and calcined at 1523 K in Ar gas containing 1% hydrogen for 48 h. Then the sample was slowly cooled to room temperature in the
T. Horikubi et al. / Journal of Alloys and Compounds 274 (1998) 122 – 127
furnace. The formation of the single phase of each ALaMnTaO 6 was identified by powder X-ray diffractometry. The powder X-ray diffraction data of SrLaMnTaO 6 and BaLaMnTaO 6 were collected with Cu–Ka radiation using 18 MAC MXP powder X-ray diffractometer equipped with a single-crystal graphite monochromator at room temperature. The conditions for data collection were as follows: 2u range, 10,2u ,100 deg.; step width (2u ), 0.04 deg.; counting time, 4 s; number of data, 2250. The powder X-ray diffraction patterns obtained were analyzed by Rietveld method using the program RIETAN [6,7]. The pseudo-Voigt peak-shape function without preferred crystallite orientation was used. The refined parameters were the 28 zero-point shift, scale factor, background parameters, profile half-width parameters (U, V, W ), asymmetry correction factor (A), lattice constants, atomic fractional coordinates and isotropic thermal parameters. The excluded region is between 108 and 258 for the both refinements.
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Table 1 Crystallographic data for ALaMnTaO 6 from Rietveld refinement at room temperature, the standard deviation of last digit is given in parenthesis SrLaMnTaO 6
BaLaMnTaO 6
Space group a / nm b / nm c / nm b / degree Volume / nm 3 Z Calculated density / g cm 23 No. of reflections No. of profile parameters refined No. of structure parameters refined No. atoms / asymmetric unit
P2 1 /n 0.56940 (1) 0.57561 (1) 0.80751 (2) 89.99(1) 0.2647 2 7.0071 542 14 18 6
¯ Fm3m 0.81752(2) – – – 0.5464 4 7.3926 54 14 6 4
Reliability factor R wp Rp RE RI RF
8.91% 6.82% 5.18% 2.39% 2.13%
9.49% 7.29% 4.21% 2.80% 2.02%
R wp 5ho w i [y obs -y cal ] 2 / o w i [y obs ] 2 j 1 / 2 , R p 5(o uy obs -y cal u) / o y obs , R I 5(o uIobs 2Ical u) / o Iobs , R F 5[o u(Iobs )1 / 2 2(Ical )1 / 2 u] /(Iobs )1 / 2 .
3. Results and discussion
3.1. SrLaMnTaO6 The powder X-ray diffraction pattern of SrLaMnTaO 6 obtained in this experiment showed that this compound has an orthorhombic or monoclinic cell. The most probable space group for SrLaMnTaO 6 estimated by the CELL program [8,9] belonged to monoclinic symmetry. Reflection conditions of h1l52n for h0l, k52n for 0kl, h52n for h00 and l52n for 00l, correspond to the P2 1 /n (No. 14) space group. There are several perovskite type compounds with P2 1 /n space group and the model of the structure for the Rietveld refmement was built by reference to that of CaPrLiTeO 6 [10] with a distorted perovskite structure. In this refinement both models for a random and an ordered distribution of Mn and Ta ions in the B-sites (octahedral sites) were carried out, while a random distribution of Sr and La ions over A-sites is assumed. In the refined structure the assigned occupation sites for an ordered model are 4e for Sr / La, O1, O2 and O3, 2d for Mn and 2c for Ta, respectively. In the disordered model various occupation ratio of Mn / Ta was assumed over 2d and 2c site. The crystallographic data are given in Table 1 and a plot of the observed and calculated profiles are shown in Fig. 1. The positional parameters obtained in this refinement are listed in Table 2 for the ordered model. Since the angle b (89.99(1) deg.) is nearly equal to 90 deg., the possibility of orthorhombic symmetry was also evaluated with the Pmmn space group (No. 59) which was most probable estimated from the CELL program [8,9] and the extinction law of diffraction patterns. Comparing both data, the R wp value for P2 1 /n is much better than that for
Fig. 1. The observed and calculated profiles of SrLaMnTaO 6 . (P2 1 /n and B-site ordering are assumed. Experimental points are shown by circles and the calculated profile by a solid line. The bottom curve is the difference pattern, y obs 2y calc and the small bars indicate the angular positions of the allowed Bragg reflections). Table 2 Fractional atomic coordinates of SrLaMnTaO 6 at room temperature, the standard deviation of last digit is given in parentheses Atom
Position a
gb
x
y
z
Sr / La Mn Ta O1 O2 O3
4e 2d 2c 4e 4e 4e
1.0 1.0 1.0 1.0 1.0 1.0
0.508(2) 1/2 0 0.212(7) 20.276(8) 0.392(7)
0.5357(4) 0 1/2 0.228(7) 0.317(8) 20.015(5)
0.2485(7) 0 0 20.054(6) 0.021(7) 0.262(6)
a
, Multiplicity and Wyckoff notation. , Occupancy.
b
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Fig. 2. Difference of calculated profile of SrLaMnTaO 6 between Pmmn (a) and P2 1 /n (b). (Experimental points are shown by circle and the calculated profile by a solid line. The bottom curve is the difference pattern, y obs 2y calc and the small bars indicate the angular positions of the allowed Bragg reflections).
Pmmn (R wp 515.47%). One of characteristic difference of the diffraction patterns between these two space groups is shown in Fig. 2, where fitting to the detailed diffraction patterns are compared for both space groups. It is clear from this figure that the degree of fitting for P2 1 /n is better than that for Pmmn. For P2 1 /n space group, the distribution of Mn and Ta over B sites was also studied for various occupation ratio of Mn / Ta, whose results are shown in Fig. 3. The extra superlattice lines by the occupation of Mn and Ta ions in 2c and 2d sites,
Fig. 4. X-ray profile refinement of SrLaMnTaO 6 based on ordered (NaCl-type) (a) and disordered model (b).
respectively, are observed as shown in Fig. 4. It is straightly concluded that the possibility of random distribution of Mn and Ta is excluded from these results. The bond distances and bond angles between atoms are given in Table 3. A slightly distorted octahedral arrangement around the Mn and Ta atoms exists in this compound.
Table 3 Interatomic distances (nm) and bond. angles (deg.) of SrLaMnTaO 6 , the standard deviation of last digit is given in parentheses Sr / La-O1
Ta-O1 -O2 -O3 Mean length: Shannon (sum): Mn-O1 -O2 -O3 Mean length: Shannon (sum):
0.203 (3)32 0.190 (4)32 0.202 (5)32 0.198 0.204 0.214 (3)32 0.223 (4)32 0.220 (5)32 0.219 0.223
Mean length: Shannon (sum):
0.249 (5) 0.262 (5) 0.298 (5) 0.345 (5) 0.254 (7) 0.268 (5) 0.290 (5) 0.341 (6) 0.230 (4) 0.267 (3) 0.324 (3) 0.343 (4) 0.289 0.280
O1-O2 (around Mn) O1-O3 (around Mn) O2-O3 (around Mn)
0.302 (7) 0.319 (7) 0.295 (7)
O1-O2 (around Ta) O1-O3 (around Ta) O2-O3 (around Ta)
0.289 (7) 0.284 (6) 0.295 (7)
Sr / La-O2
Sr / La-03
Fig. 3. R wp vs. degree of Mn occupancy on 2d site of SrLaMnTaO 6 (S.G. P2 1 /n).
/Mn-O-Ta (along [001] direction) /Mn-O-Ta (along [110] direction) /O1-Mn-O2 /O1-Mn-O3 /O2-Mn-O3 /O1-Ta-O2 /O1-Ta-O3 /O2-Ta-O3
152(3) deg. 146(2) deg. 93(2) deg. 90(2) deg. 83(2) deg. 95(3) deg. 89(2) deg. 98(2) deg.
T. Horikubi et al. / Journal of Alloys and Compounds 274 (1998) 122 – 127
Fig. 5. Distorted octahedron of SrLaMnTaO 6 (S.G. P2 1 /n) around Mn and Ta ions.
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angle among Mn, O and Ta ions are different along each direction. There are usually twelve oxygen ions around the A-site, but Sr and La on the A-site are surrounded by eight oxygens on this compound and four oxygens are further away (above 0.3 nm) from the atoms on the A-site as is shown in Fig. 6. The overview of SrLaMnTaO 6 is shown in Fig. 7. The oxygen octahedron tilts against c-axis with an angle of 148. The effect of ambient oxygen partial pressure on the formation of SrLaMnTaO 6 phase was also studied. This phase could be prepared by heating under refined Ar atmosphere, although the degree of split in some diffraction peaks became ambiguous as is shown in Fig. 8 near 2u 5568, probably because of a little change of oxygen nonstoichiometry. The similar effect was observed for high temperature X-ray diffractometry as shown Fig. 9. Broadening is seen in several peaks as temperature increases, corresponding to liberation from a higher degree of distortion by thermal energy. Therefore, the complete formation of the highly distorted phase of SrLaMnTaO 6 is rather subtle depending upon preparative conditions.
There are three different Mn–O distances, 0.214(3), 0.223(4) and 0.220(5) nm (mean length: 0.219 nm) and also three different Ta–O distances, 0.203(3), 0. 190(4) and 0.202(5) nm (mean length: 0.198 nm). These bond distances could be well compared with the value of 0.223 nm for Mn–O and of 0.204 nm for Ta–O distances calculated from the ionic radii of Shannon [11]. The coordination around Mn and Ta ions is illustrated in Fig. 5. The MnO 6 and TaO 6 octahedra in SrLaMnTaO 6 are thus slightly deformed and connected each other with zigzag in turn along three directions. The tilting angle of octahedra is about 148 against c-axis. There are three different lengths between Mn and O ions in an octahedron consisting of six oxygen surrounding one manganese ion. The similar situation exists in an oxygen octahedra surrounding one Ta ion, although the deformation is a little different ways. The
3.2. BaLaMnTaO6
Fig. 6. Environment around the Sr / La site (projection along [110] direction).
Fig. 7. Overview structure of SrLaMnTaO 6 . The TaO 6 octahedra are shaded, and the MnO 6 one are unshaded. Spheres show Sr / La site.
The powder X-ray diffraction patterns of BaLaMnTaO 6 obtained in this experiment showed that the compound has a cubic cell. The space group for BaLaMnTaO 6 estimated by the CELL program [8,9] was one of cubic symmetry and a reflection condition is as follows: h1k, h1l, k1l5 2n for hkl, k, l52n for 0kl, h1l52n for hhl and h52n for ¯ (No. 225) space group. h00, corresponding to the Fm3m ¯ space group is shown as Superlattice lines with Fm3m asterisk in Fig. 10. Since there are no diffraction lines in
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T. Horikubi et al. / Journal of Alloys and Compounds 274 (1998) 122 – 127
¯ Fig. 10. The observed and calculated profiles of BaLaMnTaO 6 . (Fm3m and B-site ordering are assumed. Experimental points are shown by circle and the calculated profile by a solid line. The bottom curve is the difference pattern, y obs 2y calc and the small bars indicate the angular positions of the allowed Bragg reflections).
Fig. 8. Powder X-ray diffraction patterns of SrLaMnTaO 6 prepared in 1%H 2 –Ar flow (a) and in Ar flow (b).
¯ near these superlattice lines, it is obvious that fitting Pm3m ¯ ¯ by Fm3m is better than by Pm3m. The model of the structure for the Rietveld refinement was built by reference to that of BaPrLiTeO 6 [10]. In this model, the two kinds of atoms on B sites (Mn and Ta) are considered to be either random or ordered distribution, respectively. In this compound, the ordered distribution was also preferred, judging from both of the values of R wp and the superlattice lines.
In Table 1 the crystallographic data of BaLaMnTaO 6 are also given. In the refined structure, the assigned occupation sites are 8c for Ba / La, 4b for Mn, 4a for Ta and 24e for O, respectively. Positional parameters obtained are listed in Table 4. The degree of agreement between calculated and observed X-ray patterns are also shown in Fig. 10. The bond distances and bond angles between atoms in BaLaMnTaO 6 are given in Table 5. The bond distances of 0.211(9) nm for Mn–O and 0.197(9) nm for Ta–O are also comparable with the values of 0.223 nm for Mn–O and 0.204 nm for Ta–O distances calculated from the ionic radii of Shannon [11].
Table 4 Fractional atomic coordinates of BaLaMnTaO 6 at room temperature, the standard deviation of last digit is given in parentheses Atom
Position a
gb
x
y
z
Ba / La Mn Ta O
8c 4b 4a 24e
1.0 1.0 1.0 1.0
1/4 1/2 0 0.24(1)
1/4 1/2 0 0
1/4 1/2 0 0
a
, Multiplity and Wyckoff notation. , Occupancy.
b
Table 5 Interatomic distances (nm) of BaLaMnTaO 6 , the standard deviation of last digit is given in parentheses
Fig. 9. High temperature powder X-ray diffraction patterns of monoclinic SrLaMnTaO 6 at 713K (a), 603K (b), 493K (c), 383K (d) and room temperature (e).
Ba / La–O Ta–O Mn–O
Calculated
Shannon
0.28912(7) 0.197(9) 0.211(9)
0.301 0.204 0.223
T. Horikubi et al. / Journal of Alloys and Compounds 274 (1998) 122 – 127
4. Conclusions The crystal structure of SrLaMnTaO 6 is slightly distorted with monoclinic symmetry, while that of BaLaMnTaO 6 is cubic with a double cell size of normal perovskite phase.
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[3] M. Yoshimura, K. Kamata, T. Nakamura, Chem. Lett. (1972) 737– 740. [4] T. Nakamura, Y. Gohshi, Chem. Lett. (1975) 1171–1176. [5] T. Nakamura, Jin-Ho Choy, J. Solid State Chem. 20 (1977) 233. [6] F. Izumi, J. Crystallogr. Jpn. 27 (1985) 23. [7] F. Izumi, J. Minerallogr. Soc. Jpn. 17 (1985) 37. [8] Y. Takaki, T. Taniguchi, K. Nakata, H. Yamaguchi, J. Ceram. Soc. Jpn. 97(7) (1989) 763. [9] Y. Takaki, T. Taniguchi, K. Hori, J. Ceram. Soc. Jpn. 101(3) (1993) 373. [10] M.L. Lopez, A. Jerez, C. Pico, R. Saez-Puche, M.L. Veiga, J. Solid State Chem. 105 (1993) 19. [11] R.D. Shannon, C.T. Prewitt, Acta Cryst. B25 (1969) 925.