Mn5+ 2H-perovskite-type Ba4Mn2NaO9 oxide

Solid State Sciences 6 (2004) 931–938 www.elsevier.com/locate/ssscie

Crystal structure of the mixed Mn4+ /Mn5+ 2H-perovskite-type Ba4Mn2 NaO9 oxide Eric Quarez a , Pascal Roussel a , Olivier Pérez b , Henri Leligny b , Abdelaziz Bendraoua a,c , Olivier Mentré a,∗ a Laboratoire de cristallochimie et physicochimie du solide, UMR CNRS 8012, ENSCL, université des sciences et technologies de Lille,

B.P. 108, 59652 Villeneuve d’Ascq cedex, France b Laboratoire CRISMAT/ISMRA, UMR CNRS 6008, 6, bd du Maréchal Juin, 14050 Caen cedex, France c Laboratoire de physico-chimie des matériaux, université des sciences et technologies d’Oran, Algeria

Received 3 February 2004; received in revised form 7 May 2004; accepted 11 May 2004

Abstract Single crystals of the new Ba4 Mn2 NaO9 have been prepared by electrosynthesis in molten NaOH. Its crystal structure has been solved from XRD data (a = 10.006(2), c = 8.210(3), space group P 321, Z = 3, R = 3.21%, wR = 3.52%). It belongs to the wide family of 2H-related perovskite materials and shows columns of face-sharing MnO6 octahedra and NaO6 prisms according to the –(oct–oct–prism)– sequence isolated by Ba2+ cations. The main characteristic of this new oxide is its mixed Mn4+ /Mn5+ valence rarely reported up today. Help to the superspace formalism, Ba4 Mn2 NaO9 (or Ba1+x Nax Mn1−x O3 with x = 1/3) can be regarded as composed of two interpenetrating sublattices [Ba1+x ] and [Na1/3 Mn2/3 O3 ] with their own period along the c axis. The ratio γ of these two periods is rational γ = c1 /c2 = 2/3. This composite structure approach which allows us, help to established rules, to predict the crystal structure from the γ -only knowledge has also been pointed out in this work.  2004 Elsevier SAS. All rights reserved. Keywords: Electrosynthesis; Hexagonal perovskite; Layers stacking; Manganese oxides; Composite structure

1. Introduction The perovskite family is the most spread out series of oxides because of its ability to modify its crystal structure, dimensionality and properties from minor to drastic layers stacking modifications (for a review, see reference therein [1]). Iso-formulated ABO3 compounds can thus adopt the tridimensional ideal 3C cubic perovskite type (ABC stacking of [A3 O9 ] layers), the bidimensional hexagonal 2H- form (AB stacking) and an infinity of intermediary forms (various stacking sequences) leading to the wide family of hexagonal perovskites. It is noteworthy that within the possible anionic lattice, the B cations always occupy the available octahedral sites. This ability to expend towards new periodic arrangement can be illustrated by the BaMnO3−δ case, which adopts a number of polymorphous structures depending on the oxy* Corresponding author.

E-mail address: [email protected] (O. Mentré). 1293-2558/$ – see front matter  2004 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.05.005

gen content and of the subsequent manganese valence. It has been shown that the 2H-BaMnO3 is modified during the reduction process by means of the introduction of BaO2.50 cubic layers yielding the 15-R BaMnO2.90, 8-H BaMnO2.875, 6H-BaMnO2.833, 10H-BaMnO2.80, 4-H BaMnO2.75 [2–4], 21-R BaMnO2.92–2.88 [5]. More recently a special interest has been focused on 2H variant compounds, which result from the substitution of [A3 O9 ] layers of the AB arrangement for [A3 A O6 ] layers yielding A3n+3m An B3m+n O9m+6n (also written A1+x Ax B1−x O3 with x = n/(3m+2n)), where m and n stand for the number of each kinds of layers, see references herein for review and theory [6–8]. Then, the infinite [BO3 ]∞ columns of face-sharing octahedra of the 2H structure are defected by the creation of A O6 trigonal prisms centered on the [A3 A O6 ] layer instead of two octahedra. The two end members are given by m = 1 and n = 0 (. . . –oct–oct–oct–. . . sequence, 2H-BaNiO3 type [9]) and m = 0, n = 1 (. . . –oct–prism–oct–prism–. . . sequence, Sr4 PtO6 type [10]) and, once again, all possible interme-

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diate arrangements are viewable since trigonal prisms remain isolated. The unfavorable 2:1 height ratio between prisms (= Dp ) and octahedra (= Do ) is responsible for a structural relaxation within the polyhedral chains which results in atomic displacement towards a more reasonable Dp ∼ (1.6–1.7) × Do depending on the A nature. Consequently, the crystal can be regarded as two modulated interpenetrating sublattices (commensurately or not), [A1+x ] and [Ax B1−x O3 ] [7]. The recent renew of interest for this family is mostly due to the progress in super-space formalism particularly well suited for the investigation of most of such composite-structures. From the chemical point of view, after successful investigation of the Ba–Ni/Cu–Na–O [11] and Ba–Ru–Na–O [12–14] systems by electrosynthesis in molten alkali hydroxide, our group was interested in the promising Ba–Mn–Na–O system [15]; the aim is to obtain high mixed valence manganese oxides. This leads to the characterization of new Ba3n+3m Nan Mn3m+n O9m+6n compounds. This paper is devoted to the crystal structure determination of the first term of these series: the new comNaO9 compound, which corresponds mensurate Ba4 Mn4.5+ 2 to the m = 1, n = 3 case (x = 1/3 with respect to the Ba1+x Nax Mn1−x O3 terminology). Special efforts have been provided to characterize the title compound using both the classical 3D approach and the 4D formalism in order to generalize the description of Ba1+x Nax Mn1−x O3 materials in further works to come.

2. Experimental 2.1. Synthesis Single crystals of Ba1+x Nax Mn1−x O3 were grown by electrosynthesis using the electrochemical system previously described [11–15]. NaOH was used as an oxidizing electrolyte particularly well suited because of its low melting point (320 ◦ C). NaOH (pellets), Ba(OH)2 ·3H2 O, V2 O5 , Bi2 O3 and MnO2 in 0.5–0.5–0.5–0.5 proportions (g) have been mixed and molten in an alumina crucible batch, set in quartz cell heated at 800 ◦ C in a vertical tubular furnace under flowing air. The temperature was controlled at the melt level using a standard K-type thermocouple. In the liquid, a constant potential (= 2.2 V) has been applied between a nickel and a zirconium foils playing the anode and cathode role, respectively. The current decreased from 20 to almost 5 mA during the reaction. After a 24 h reaction, black single crystals have been extracted and water-washed from the deposited material at the cathode. Only few single crystals corresponding to the title compound could be isolated from the melt mixture. An EDS (energy dispersive spectroscopy) analysis was performed on the selected single crystal but did not show any trace of vanadium nor bismuth. One must also notice the electrosynthesis permitting this preparation is hardly reproducible. Moreover, none of the numerous attempts performed to prepare the title com-

pound by solid state reaction under different experimental conditions such as heating in air, flowing N2 , or NaOH flux from BaCO3 /Ba(OH)2 –3H2O, MnO2 /Mn2 O3 , Na2 CO3 lead to the expected material. 2.2. Data collection Preliminary studies were undertaken for Ba4 Mn2 NaO9 on a three circles Bruker SMART CCD 1K, leading to cell parameters c1 = 2.758(9) Å, c2 = 4.129(4) Å, then to γ = c1 /c2 = 0.668(3) equal, within standard deviation, to the rational value γ = 2/3. So the mean c parameter is given by 2 · c1 = 3 · c2 = 8.266(9) Å. In order to ensure the commensurate character of Ba4 NaMn2 O9 , supplementary tests were performed on a Kappa CCD (Bruker–Nonius) four circles diffractometer and Mo-Kα X-ray radiation. First, frames collected with large Ω and Φ scanning angles allowed us to control the crystalline quality and to determine the cell parameters. Second, accurate precession pictures were calculated from frames collected up to θ = 25◦ with a small scan angle (Φ–Ω scans of 0.3◦ ) and a short sample-detector distance (Dx = 34 mm); they are enough precise to obtain an overall view of the reciprocal space. Two sets of reflections exhibiting weak and strong intensities could clearly be identified allowing us to describe the diffraction pattern using either a supercell, either a composite description. A full overlap of satellite reflections is, indeed, clearly evidenced outlining the commensurate character of the compound. Using these observations, a suitable data collection strategy has been defined. Owing to the cell parameters and the relatively large spot size (probably due to a strong mosaicity of the crystal), a scanning angle of 1◦ and a Dx value of 34 mm have been chosen; Φ–Ω scans were used. To collect a great number of weak reflections but avoiding any detector saturation by reflections of strong intensity, two different exposure times (180 and 18 s per degree) have been used. The diffracted intensities were collected up to θ = 45◦ . A redundancy of 2 for 90% of the reflections was chosen. The EvalCCD software [16] was used to extract reflections from the collected frames; reflections were merged and rescaled as function of the exposure time. Data were corrected from absorption using Jana2000 program [17] within the analytical option based on the crystal morphology. The diffraction pattern is consistent with a trigonal symmetry. Results for both data recording (SMART 1K and KAPPA CCD) are similar, but only the crystal structure resulting from the more accurate latest dataset will be presented in this work. The crystal and refinement data are gathered in Table 1.

3. Crystal structure 3.1. 3D versus 4D As reported in Section 1, by analogy with other members of this structural family arising from the [Ba3 O9 ] lay-

E. Quarez et al. / Solid State Sciences 6 (2004) 931–938

Table 1 Crystal data, data collection and structure refinement parameters for Ba4 Mn2 NaO9 Crystal data

Compound II

Crystal symmetry Space group Unit cell (Å)

Trigonal P 321 (No. 150) a = 10.0060(4) c = 8.2290(8) 713.5(1) 3 5.768, 18.94

Volume (Å3 ) Z Calculated density (g cm−3 ), µ (mm−1 ) for Mo-Kα Data collection Diffractometer

Number of measured reflections Number of independent reflections [I > 4σ (I )] Crystal size Transmission factor range R merging factor (%) Weighting scheme Twin ratio (%)

Kappa CCD (Bruker–Nonius) 0.71073 Φ–Ω scans 4–40 −14
h
14 −18
k
15 0
l
9 2977 865 50 × 70 × 40 0.31/0.54 5.8 Unit 74.3(6)/25.7(3)

Refinement parameters Refinement method Number of refined parameters R, wR [I > 4σ (I )] (%) Isotropic secondary extinction Max/min ρ (e Å−3 )

Least squares on F 51 3.21; 3.52 None 1.88/−1.68

Radiation Mo-Kα (Å) Scan mode Recorded angular range θ (deg) Recording reciprocal space

ers substitution for [Ba3 NaO6 ] layers, the refinement of the crystal structure using a 4D treatment with a q = γ c1∗ modulation wave vector (commensurate or not) is the common alternative to the use of a conventional 3D model [6]. The Ba1+x Nax Mn1−x O3 crystal structure can be, indeed, depicted as two interpenetrating sublattices, [Nax Mn1−x O3 ] and [Ba1+x ] with their own periodicities along c, c1 and c2 , respectively. c1 is related to the octahedra/prism framework and corresponds to the average height of polyhedra within a column. Typical values observed for c1 are about 2.7 Å. Note that this value is slightly higher than the expected one (c2H /2) since, as already pointed out, Dp > Do . The c2 parameter corresponds to the average distance between two superimposed Ba in the AB stacking. Typical values for c2 are about 4.8 Å, which corresponds to the c value of the 2H structure. The component γ of the modulation wave vector can thus be calculated using the formula γ = c1 /c2 . c1 and c2 are directly accessible on the diffraction patterns where reflections corresponding to the contribution of the two sublattices can be identified; plus some extra satellite spots with weaker intensities due to the interaction between the two sublattices are observable. So, spots can be considered as resulting from:

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(a) the [Nax Mn1−x O3 ] lattice, rhombohedral within the a, b, c1 , (b) the [Ba1+x ] sublattice (primitive within the a, b, c2 unit cell), (c) satellite spots resulting from both sublattices interaction (only indexable in the a, b, c unit cell). Fig. 1 shows the hk0 layer that displays reflections arising from the two sublattices. First order satellite are located on the upper layer. 3.2. Chemical composition It is noteworthy that according to the intensive investigation of this family of materials, the composition and crystal structure can be directly predicted from the γ value. It can be shown that, within the A1+x Ax B1−x O3 formula, the x value can be deduced from the relation γ = (1 + x)/2 [6,7]. As recently reported for Sr4 MMn2O9 (M = Cu, Zn) [32], the rational value of γ = 2/3 indicates that our compound can accurately be expressed in the form A4 MB2 O9 , A = Ba, M = (Na, Mn), B = (Mn, Na). Two methods can be used to derive the relation γ = c1 /c2 = (1 + x)/2: The first one is based on the chemical formula A1+x Ax B1−x O3 which both expresses the charge balance and the composite character of the crystal. Then the a, b, c1 hexagonal unit cell contains one anionic layers O3 , i.e., 3 [A1+x Ax B1−x O3 ] units, while the a, b, c2 hexagonal unit cell contains two cationic A layers, i.e., 6/(1 + x) [A1+x Ax B1−x O3 ] units, leading to γ = c1 /c2 = (1 + x)/2. The second method uses a different approach, calculating apart c1 and c2 as a function of x and the heights of the prisms (height = Dp ) and octahedra (height = Do ) stacked along z. Then within the commensurate case, γ = c1 /c2 = n2 /n1 is a sufficient approximation if n1 and n2 (both integers) are well chosen, the c supercell parameter is then given by c = n1 c1 = n2 c2 . Over a period c of one metallic column are counted n1 x prisms and n1 (1 − x) octahedra. Assuming that Do and Dp are constant throughout the crystal, we find: c1 = xDp +(1− x)Do . In the same way, c2 can be calculated by averaging the three kinds of apical Ba–Ba distances occurring along c; Dp , 2Do and Do + 1/2Dp . Note that because of the AB stacking of these materials two shifted Ba columns are to be considered including then 2n2 Ba–Ba distances. Among these, n1 xDp and 2n1 x(Do + 1/2Dp ) are obviously found. Consequently, there are (2n2 − 3n1 x)(2Do ) Ba–Ba distances. Then the average gives: n1 xDp + 2n1 x(Do + Dp /2) + (2n2 − 3n1 x)2Do 2n2
 n1 n1 = xDp + 2 1 − x Do . n2 n2

c2 =

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Fig. 1. The hk0 layer showing strong and weak spots arising from the [Ba1+x ] sublattice and weaker spots of the [Nax Mn1−x O3 ] sublattice which respect the R-lattice condition.

Fig. 2. The γ = 9/14 case. (a) The [A B1−x O3 ] sublattice: evidence of the (1 − x) octahedra of height Do and x prisms of height Dp . (b) The [A1+x ] sublattice: numbering of the three kinds of apical Ba–Ba distances.

E. Quarez et al. / Solid State Sciences 6 (2004) 931–938

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Table 2 Atomic coordinates and thermal parameters for Ba4 Mn2 NaO9 Atom

Fig. 3. The occupational AMF scheme for γ = 2/3.

It results then from the c1 /c2 calculation that γ = (1 + x)/2, independently of the Do and Dp values. This method is illustrated in the Fig. 2a and 2b, for the γ = 9/14 case. In our case, where γ = 2/3, the deduced formula is then A4/3 A1/3 B2/3 O3 with A = Ba, A = Na, B = Mn. This corresponds to the Ba3n+3m Nan Mn3m+n O9m+6n formula with m = 1 [BaO3 ] layers and n = 3 [Ba3 NaO6 ] layers.

Site

x

Ba1 3f Ba2 3e Ba3 6g Mn1 2d Mn2 2d Mn3 2c Na1 2d Na2 1a O1 6g O2 6g O3 6g O4 6g O5 3f Atom U 11

0.3265(7) 0.3510(7) 0.3146(4) 1/3 1/3 0 1/3 0 0.185(3) 0.503(2) 0.825(2) 0.161(6) 0.850(4) U 22 U 33

Ba1 Ba2 Ba3 Mn1 Mn2 Mn3

0.042(3) 0.013(2) 0.008(1) 0.008(2) 0.009(2) 0.009(4)

0.024(2) 0.014(2) 0.009(1) 0.008(2) 0.009(2) 0.009(4)

y

z

0 0 0.3372(4) 2/3 2/3 0 2/3 0 0.524(3) 0.320(2) 0.492(2) 0.010(6) 0 U 12

0 1/2 0.2542(6) 0.399(2) 0.076(2) 0.846(2) 0.725(3) 1/2 0.225(3) 0.479(3) 0.043(3) 0.277(8) 0 U 13

0.014(2) 0.014(2) 0.014(2) 0.032(8) 0.003(3) 0.019(7)

Ueq 0.025(2) 0.014(2) 0.0105(9) 0.016(3) 0.007(2) 0.013(3) 0.007(6) 0.01(1) 0.017(6) 0.001(4) 0.011(4) 0.09(2) 0.011(8) U 23

0.021(2) 0.0025(9) 0.005(2) 0.007(1) −0.0052(7) −0.010(2) 0.005(1) 0.0057(2) 0.0024(8) 0.004(1) 0 0 0.005(1) 0 0 0.005(2) 0 0

3.3. Polyhedral sequence Another interesting signature of this structural flexible system is the possibility to predict the octahedra–prisms sequence of the [Ax B1−x O3 ] sublattice which is imposed by the x value (for a review on other predictable systems, see Ref. [18]). In the present case, it is trivial that the only possible sequence built up from m = 1 and n = 3 (i.e., γ = 2/3) is –(oct–oct–prism)–. Therefore, a general method to determinate the polyhedra sequence is to specify in which unit cells (c1 ) are found the antagonist A and B atoms. Within the 4D approach, this result can be obtained from the representation of their atomic occupancy functions against the internal parameter: x4 = γ z0  + n3 · γ , where z0  is the average z coordinate of A and B ((0, 0, 0, 0) positions in the superspace group R3m(00γ ) 0s) and n3 is the integer defining the unit cell position along z. As it has been shown by Perez-Mato et al. [6] for symmetrical reasons related to the superspace group, these functions are crenel functions symmetric with respect to x4 = 1/4 and 3/4 showing a width compatible with the formula. So they are centered at x4 = 1/4 and 3/4 for A (= Na+ ); width = γ − 1/2, and for B (= Mn) centered at x4 = 0 and 1/2; width = 1 − γ . In the commensurate case, only some particular x4 values are involved in the [0, 1] interval; x4 = 0, 2/3 and 1/3 are found for γ = 2/3. It should be noted that the two later values stand at the border of two possible successive domains but since two successive prisms is not possible only the two equivalent sequences –oct–prism–oct– and –oct–oct–prism– are to be considered along c = 3c1 . Fig. 3 shows the scheme of the octahedra/prism sequence along x4 for x = 1/3, γ = 2/3. The way to use this scheme is rather easy: starting from the n = 0 value (situated in an

octahedral domain), the next points are given by successive incrementation of γ (= 2/3). The concerned polyhedron is so-given by assignment to the corresponding domain (point 1: octahedral; point 2: prismatic, etc.). It leads to the expected (oct–oct–prism) and could be successfully adapted to any x values. 3.4. Structural refinement The three predicted possible space groups compatible for γ = 2/3 are P 3, P 321 and P -3 [6]. Only the two latter lead to satisfactorily models, but P -3 results show a disordered [Mn2 NaO9 ] lattice with half occupancy on every sites. P 321 was so-retained. Barium atoms have been located with help to the Patterson function calculation. Manganese, sodium and oxygen have been found on subsequent Fourier difference syntheses maps. The full-matrix leastsquares refinement was performed on F values using a unit weight scheme in order to enhance the weak spots influence. As commonly observed for this family an obverse/reverse twin was introduced (hkl/khl). In the last cycles of the refinement, anisotropic thermal parameters have been considered for barium and manganese atoms yielding the final R = 3.21%, wR = 3.52%. Attempts to refine oxygen occupancies lead to fully occupied positions within esd’s. Atomic positions, thermal parameters and pertinent interatomic distances are reported in Tables 2 and 3.

4. Results and discussion The different compounds adopting the m = 1 and n = 3 crystal structure are listed in Table 4. Owing to ionic ra-

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Table 3  Inter-atomic distances (Å) and bond valences ( Sij ) for Ba4 Mn2 NaO9 (for bond sum calculations, data are taken from Ref. [40] assuming Ba (II), Mn (IV) and Na (I)) Ba1 environment Ba1–O1 Ba1–O3 Ba1–O3 Ba1–O4

×2 ×2 ×2 ×2

Ba1–O5  Sij

×2

Mn1 environment 2.66(2) 2.91(2) 2.97(3) 2.85(7) 2.83(2)

Mn1–Mn2 Mn1–Na1 Mn1–O1 Mn1–O2  Sij

2.3

Mn2 environment

Ba2 environment

×3 ×3

2.66(2) 2.68(3) 2.04(3) 1.87(2) 3.6

Mn2–Na1

2.89(3)

Ba2–O1 Ba2–O2

×2 ×2

2.89(2) 2.78(2)

Ba2–O2

×2

2.86(3)

Mn2–O1 Mn2–O3  Sij

Ba2–O4  Sij

×2

2.68(7)

Mn3 environment

2.0

Mn3–Mn3

2.53(2)

Mn3–Na2

2.85(2)

Ba3 environment

×3 ×3

4.3

Ba3–O1 Ba3–O1

×1 ×1

2.77(4) 2.82(4)

Ba3–O2

×1

2.71(2)

Mn3–O5 Mn3–O4  Sij

Ba3–O2

×1

2.74(2)

Na1 environment

Ba3–O3 Ba3–O3

×1 ×1

2.62(3) 2.94(2)

Ba3–O4

×1

2.84(5)

Na1–O2 Na1–O3  Sij

Ba3–O4

×1

2.83(5)

Na2 environment

Ba3–O5  Sij

×1

2.74(2)

Na2–O4  Sij

2.4

1.90(2) 1.86(2)

×3 ×3

1.96(3) 1.86(7) 4.0

×3 ×3

2.30(3) 2.48(3) 1.3

×6

2.41(7) 1.2

dius and easy reducibility, Mn4+ and mixed Mn4+/3+ stand at the limit between cubic and hexagonal stacking depending on the nature of their assorted A cation. Thus, BaMnO3 adopts the 2H-crystal while SrMnO3 and high temperature CaMnO3 show a 4H-type structure, e.g., a number of cubic Mn3+ /Mn4+ perovskite, parent of the colossal magnetoresistant materials have been prepared. In the same way, manganese may also appear to be an ideal candidate for

A1+x Ax B1−x O3 2H-related perovskites even if for x = 1/3, only Sr4 MMn2 O9 (M = Ni, Cu, Zn) have been reported to date [31,32]. In the latter oxides, the presence of anti-site and/or stacking faults yield a deviation from the ideal stoichiometry towards the final Sr4 Mn2.09Cu0.91 O9 from a single crystal study [31] and Sr4 Cu1.01 Mn1.99O9 and Sr4 Zn0.95Mn2.05O9 from neutron diffraction powder results [32]. During our crystal structure refinement, a special attention has been paid to possible mixed cationic sites occupancies but finally leads to the ideal Ba4 Mn2 NaO9 formula. The relative rarity of manganese containing compounds with other m and n terms [8] opens a wide field of future investigations and possible new materials as pictured by the recent synthesis and characterization of Sr1+x (Cox Mn1−x )O3 (x ∼ 0.266, the m = 5, n = 9 term and x ∼ 0.280 the m = 1, n = 2 term) [33] and Ba6 A Mn4 O15 (A = Mg, Ni) (the m = 1, n = 1 term) [34]. The crystal structure of Ba4 Mn2 NaO9 with its label scheme is shown on Fig. 4a. Na(1) is slightly off centered in its prismatic O(2)3O(3)3 polyhedron towards Mn(1) with two types of distances Na(1)–O(2) and Na(1)–O(3) of 2.30(3) and 2.48(3) Å, respectively. In the shifted column, Na(2) is located at the center of its O(4)6 prism with Na(2)–O(4) = 2.41(6) Å. As discussed below, this different environment denotes a difference observed in the manganese environment for the two columns, but at least, Na–O bonds are included in the typical range for prismatic sodium in this kind of structure. Several A3 A BO6 (m = 0, n = 1) are reported with [(Ba, Sr, Ca)3 NaO6 ] layers, e.g., Ca3 Na(Ir/Ru)O6 [35,36], (Sr/Ba)3 Na(Sb/Bi)O6 [37], Ba3 Na(Ru/Ir)O6 [38], Ba3 Na(Nb/Ta)O6 [39]. The shortest and longest Na–O bond length reported in this coordination are 2.302(3)–2.485(2) Å for Ca3 NaRuO6 and NaBa3 BiO6 , respectively. As a matter of fact, the bond valence sum calculations using Brown and Altermatt data [40] for Na(1) and Na(2) yield acceptable values 1.26 and 1.16, respectively. The two [NaMn2 O9 ]∞ columns slightly contrast by their Mn environments and displacements. In this compound, the

Table 4 The reported A4 A B2 O9 compounds (m = 1, n = 3) Compounds

A

a (Å)

Sr4 Cr3 O9 Sr4 Ru2 O9 Sr4 Na0.5 Ni2.5 O7.67 Sr4 Ni2.5 O9 Sr4 Ni3 O9 Sr4 CuIr2 O9 Sr4 NiMn2 O9 Ba12 (Bax Pt3−x )Pt6 O27 Ba1.317 (Cu, Pt)O3 (Sr0.5 Ca0.5 )4 Co3 O9 Sr4 Mn2.09 Cu0.91 O9 Sr4 Cu1.01 Mn1.99 O9 Sr4 Zn0.95 Mn2.05 O9 Ba4 NaMn2 O9

Cr

9.618 9.642 9.429 9.474 9.477 9.685 9.601 10.098 → 10.156 10.075 9.28 9.5817 9.5918 9.5894 10.006

Na, Ni Ni Ni Cu Ni Pt → Ba 0
x
3 Cu/Pt Co Cu, Mn Cu, Mn Zn, Mn Na

* Calculated using the modulated composite formalism.

c (Å) 7.874 8.104 7.896 7.802 7.826 8.047 7.765 8.618 → 8.830 8.361* 8.06 7.8290 7.8114 7.5039 8.229

Space group

References

P3 ¯ P 62c P 321 P 321 P 321 P 321 P 321 P 321 (x = 3) – P 321 P 321 P 321 P 321 P 321

[19] [20] [21] [22] [23,24] [25] [26] [27] [28,29] [30] [31] [32] [32] This work

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Fig. 4. (a) The Ba3 NaMn2 O9 crystal structure and label scheme. (b) View of the cationic displacement inside their coordination polyhedra and resulting CDW-like shape.

rare Mn +4.5 valence is discussed below. The Mn–O distances are include in the 1.86(2)–2.04(2) range compatible with the Sr4 MMn2 O9 results (M = Cu, Zn) [32]. In the first column Mn(1) and Mn(2) are displaced towards their closest sodium neighbors picturing a Mn–Mn electrostatic repulsion (Mn(1)–Mn(2) = 2.66(2) Å). On the contrary, in the second column, Mn(3) are displaced towards the shared face of dimers which may suggest a metal–metal bond scheme (Mn(3)–Mn(3) = 2.53(2) Å). As observed on the viewing of adjacent columns along [110], the displacement has a particular role. It appears that Mn atoms are shifted towards anionic planes in order to ensure a kind of charge density wave (CDW)-like along c with respect to the crystallographic periodicity, Fig. 4b. So, within the –[Ba3 O9 ]–[Ba3 NaO6 ]a – [Ba3 NaO6 ]b –[Ba3 NaO6 ]c – stacking two manganese atoms approach the central [Ba3 NaO6 ]b layers while four manganese approach the peripheral [Ba3 O9 ] layers as shown on Fig. 4. Most of the isomorphous compounds show a minimal split within dimeric metal–metal distances of independent columns, e.g., Ru–Ru = 2.78/2.77 Å in Sr4 Ru2 O9 [20], Mn/Mn = 2.55/2.57 Å in Sr4 Mn2 NiO9 [26]. For the former, the interlayers contrast is reduced by vacancies; e.g., [Sr3 O9 ]/[Sr3 O6 ]. Therefore, strong variations are also reported as Ni–Ni = 2.43/2.67 Å in Sr4 Ni2.5 O9 [22] and Pt/Cu–Pt/Cu = 2.57/2.84 Å for Ba1.317(Cu,Pt)O3 [28,29], but for these compounds the partial occupancy and mixed nature of the cationic sites may suggest a deviation from the ideal n = 3, m = 1 crystal structure. The mean manganese valence deduced from the formula is Mn+4.5 . As discussed above, no evidence for distinct Mn(IV) and Mn(V) sites is observed in the present crystal structure. It suggests mixed valent +4.5 state within Mn2 O9 pairs. This observation is reinforced by the Mn(V) preference

for tetrahedral sites, polyhedron unobserved in our phase. On such mixed valent systems, valence bond calculations is poorly informative since data have been tabulated from unambiguous monovalent oxides. The results reported in Table 3 show valences calculated for Mn(1)–Mn(3) with Mn(IV) data lead to valence bond sums between +3.6 and +4.3. Even if few hexagonal perovskites that show an average valence close to +4.5 have been recently reported, their charge is segregated over Mn3+ /Mn4+ in octahedra and Mn5+ in independent layers formed by tetrahedra, e.g., Ba7 Ca2 Mn5 O20 [41], Ba4 (Ca,Cr,Mn)4−y O12−x [42], Ba4 Ca1−x Mn3+x O12−δ [43]. In fact, to our knowledge, only the Ba3 ErMn2 O9 compound shows a Mn4+ /Mn5+ mixed valence. It is a 6H-perovskite with a (chc)2 sequence [44]. The Mn–Mn distance within Mn2 O9 dimers is longer than in the title compound (Mn–Mn = 2.84 Å) but the involved Mn– O distances are comparable to those observed here (1.71– 2.09 Å) comforting our results. References [1] J. Darriet, M.A. Subramanian, J. Mater. Chem. 5 (1995) 543. [2] T. Negas, S. Roth, J. Solid State Chem. 3 (1971) 323. [3] M. Parras, J. Alonso, J.M. Gonzàlez-Calbet, M. Vallet-Regi, Solid State Ionics 106 (1993) 614. [4] J.M. Gonzàlez-Calbet, M. Parras, J. Alonso, M. Vallet-Regi, J. Solid State Chem. 106 (1993) 99. [5] M. Parras, J.M. Gonzàlez-Calbet, J. Alonso, M. Vallet-Regi, J. Solid State Chem. 113 (1994) 78. [6] J.M. Perez-Mato, M. Zakhour-Nakhl, F. Weill, J. Darriet, J. Mater. Chem. 9 (1999) 2795. [7] M. Evain, F. Boucher, O. Gourdon, V. Petricek, M. Dusek, P. Bezdicka, Chem. Mater. 10 (1998) 3068. [8] K.E. Stitzer, J. Darriet, H.C. zur Loye, Curr. Opin. Solid State Mater. Sci. 5 (2001) 535.

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