Single-crystal X-ray structure analysis of Mn-substituted barium hexaaluminates as-grown and after reduction? Hiroshi Inoue,aMasato Machida,b Koichi Eguchi' and Hiromichi Arai"" 'Department of Materials Science and Technology, Graduate School of Engineering Sciences, K yushu University, 6-1 Kasugakoen, Kasuga, Fukuoka 81 6, Japan bDepartment of Materials Science, Faculty of Engineering, Miyazaki University, 1-1 Gakuen-Kibanadai-Nishi, Miyazaki 889-21, Japan An X-ray single-crystal structure analysis was performed on manganese-substituted barium hexaaluminate (Bao~780Mno~254Al,o~706017~153)with the p-alumina structure, which was grown by the floating zone method and reduced by subsequent evacuation at 1150 "C. Both as-grown and reduced crystals have a hexagonal structure (P63/mm~, 2=2) and their unit-cell dimensions are a =0.559 1 (l), c =2.2659( 2) nm and a =0.5587( 2), c =2.2656( 3) nm respectively. The structure refinement from the X-ray reflection data collected by using the w28 scan technique was carried out by the least-squares method to give a final R value of 0.03.The resultant structure was identical with that of p-alumina (Ba0~75A111~0017~25) reported by Iyi et a1.l Di- and tri-valent manganese partially and preferentially replaced Al( 2) sites in a tetrahedral environment in the spinel block. UV-VIS spectra showed that the evacuation treatment brought about the reduction of some of the Mn3+ to Mn2+. The charge compensation for the reduction was found to be effected by 02-defect formation in the mirror plane whereas the occupancies of oxygen sites inside the spinel block remained unchanged.Hexaaluminate compounds are important functional inorganic materials which have been studied extensively in a number of fields of application, such as superionic conductors, host crystals for fluorescence, lasers and nuclear waste disposal. We previously reported the application of powdered hexaaluminate compounds in high-temperature catalytic processes. The most prominent feature of this material is its thermal stability against sintering, which is quite useful in retaining the large surface area necessary for catalytic The Mn-substituted hexaaluminate also exhibits catalytic activity for oxidation reactions, because of the reduction-oxidation cycle of Mn partially substituting the A1 site.The charge compensation in the redox process seems to be accomplished by the non-stoichiometry of lattice oxygen. Thus, the reactivity of lattice oxygen plays a key role in the catalytic activity. However, none of the structural changes during the redox process have been elucidated to demonstrate the structure-dependent cataly- sis of this material. With regard to the oxidation state of Mn in the hexaaluminate lattice, Laville et al. reported the result of spectroscopic investigations of LaMg, -xMn,All,Ol, single crystal^.^ The absorption spectra of this material, as grown under a reducing atmosphere, were assigned to Mn2+, while later oxidation treatment produced Mn3+ species. Although this result implies that redox cycles between Mn2+ and Mn3+ exist in the hexaaluminate lattice, the accompanying oxygen non-stoichiometry was not described.We have performed single-crystal structural analysis of Mn- substituted hexaaluminates to evaluate the structural changes that occur during the redox process. Comparing the structural parameters of the sample as grown in air and after subsequent reduction treatment is useful as it provided a new insight into the structure-dependent catalytic properties of this material. This enabled us to study the reactivity of lattice oxygen of different crystallographic sites. Experimenta1 Manganese-substituted hexaaluminate single crystals were used for the X-ray structural analysis. For single-crystal growth, a t Single-crystal data are available from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany.sintered rod was prepared from a cool, isostatically pressed, powder sample of Bao~80Mno~40Allo~,0017~3-cI by heating at 1450 "C. A single crystal of Mn-substituted barium hexaalumin- ate was grown by using a floating zone (FZ) apparatus (Nichiden Kikai Co.) equipped with a xenon arc lamp. Crystal growth was maintained at the constant rate of 3mm h-' in air. The as-grown transparent red crystals were subsequently evacuated at 1150°C in order to reduce the Mn species. The crystal turned transparent green upon this reduction treatment. Clear sections with no cracks were cut out from both as-grown and reduced crystals, and shaped into spheres (ca.0.3-0.2 mm in diameter), for use in X-ray diffraction measurements. X-Ray intensity data were collected on a Rigaku AFC5R diffractometer with graphite-monochromated Mo-Ka (A= 0.71069 A) radiation and a 12 kW rotating anode generator. Cell constants and an orientation matrix for data collection, obtained from a least-squares refinement using the setting angles of carefully centred reflections in the range 52.09 <28 <55.03" corresponded to a primitive hexagonal cell (Laue class 6/mmm) with cell parameters: a =0.5591(1)nm, c=2.2659(2) nm, V=0.6134(1) nm3, 2=2 for an as-grown red crystal and a =0.5587( 2) nm, c =2.2656( 3) nm, V= 0.6125(1)nm3, 2=2 for a reduced crystal. Based on the systematic absence of hhl, I # 2n, packing considerations, stat- istical analysis of intensity distribution, and the successful solution and refinement of the structure, the space group was determined to be P63/mm~ (no.194). The data were collected at temperature of 24+_ 1 "C using the w28 scan technique to a maximum 28 value of 120.4". Omega scans of several intense reflections, made prior to data collection, had an average width at half-height of 0.28" with a take-off angle of 6.0". Scans of (1.68 +0.30 tan 8)O were made at a speed of 16.Oomin-' (in omega). The weak reflections [I <lO.Oo(I)] were rescanned (maximum of 3 scans) and the counts were accumulated to ensure good counting statistics. Stationary background counts were recorded on each side of the reflection.The ratio of peak counting time to background counting time was 2: 1. The diameter of the incident beam collimator was l.Omm, the crystal to detector distance was 258mm, and the detector aperture was 9.0~13.0mm (horizontal x vertical). J. Muter. Chem., 1996, 6(3), 455-458 455 Intensities were collected by applying Lorentz polarization and absorption corrections (transmission factors 0 5345-1 2028 for an as-grown crystal and 09425-1 0444 for a reduced crystal) Of the 3700 reflections which were collected, 1997 were unique The intensities of three representative reflections were measured after every 150 reflections and showed no significant deviations from the mean The structure was solved by and expanded using Fourier techniques with secondary extinction correction Refinements were based on the values for barium hexaaluminate (Ba, 75Alll ,O,, 25) reported by Iyi et a1 ,6 including general temperature factors, positional par- ameters, and the occupation factors of all sites Full-matrix least-squares refinement minimized Cw(lF,1 -(Fc1)2, w = l/a2(F,) Neutral atom scattering factors were taken from Cromer and Waber8 The values for Af' and A7 used were J LtL,mEa +b --c +ak Fig.1 Refined crystal structure of Mn-substituted barium hexaalumin- ate with 95% probability ellipsoids those of Creagh and Mcauley' The final least-squares cycle converged with unweighted and weighted agreement factors of R=O029 and R,=O039 for an as-grown crystal, and R= 0026 and R,=O033 for a reduced crystal All calculations were performed using the TEXSAN crystallographic software package of the Molecular Structure Corporation Results and Discussion The crystal structure of Mn-substituted hexaaluminates refined in this study is shown in Fig 1 The final structural parameters of the as-grown Mn-substituted hexaaluminate crystal are listed in Table 1 The occupancy was expressed as the number per unit cell divided by 24 The refined crystal structure agrees essentially with that of Ba, 75Alll ,O,, 25, which was refined by Iyi et a1 to be a defective p-alumina type' The ideal p-alumina structure is constructed by alternate stacking along the c axis of a spinel block and a monoatomic interlayer plane (mirror plane) containing Ba2+ and oxide ions (Fig 1) According to the report by Iyi et al, however, barium p-alumina contains interstitial oxygen with barium ion vacancies in the mirror plane due to Frenkel defects of A1 ions Their structural model is constructed from a random distribution of two types of half unit cells, one containing a barium and an oxygen ion in the mirror plane, and the other containing interstitial aluminium and oxygen ions with no barium ions Such a complicated defect structure arises from a charge compensation mechanism for non-stoichiometry to accommo- date divalent cations into the p-alumina structure As is evident from the refined structural parameters in Table 1, barium ion occupies the 2(d) (Beevers-Ross) site in the mirror plane (z=O 25) Aluminum ions in the spinel block are distributed on Al(1) and Al(4) sites in an octahedral environment and on Al(2) and Al(3) sites in a tetrahedral environment An interstitial Al( 5) site is created by the Frenkel defect of Al(1) First refinements indicated the partial occu- pancy of Al(2) of about lo%, whereas no deviation for Al(3) and Al(4) was found To determine the site occupancy of Mn, therefore, the refinement was conducted by assuming that Al(1) and/or Al(5) contain Mn with the same coordinates, but the occupancies of Mn on these sites converged to zero In the subsequent refinement, therefore, Mn was placed on Al(2) with the same coordinates and the total occupancy of Al(2) and Mn was constrained to 1/6 Under these conditions, the occupancy of Mn on the Al(2) site converged to 00212 Such preferential replacement is consistent with the result on man- ganese-substituted lanthanum hexaaluminate reported by Gasperin et al loand also supported by the UV-VIS absorption results described below Among the seven types of oxygen Table 1 Final structural parameters of an as-grown crystal ~~~~~ atom position ~ ~ ~ occupancy' X Y Z Be: 0 0650( 1) 213 113 114 1135(2) 0 0212( 7) 113 213 0 02374( 4) 0 43(4) 0 4702( 12) 0 1455( 7) 0 1667 0 0833 0 02654 12) 0 83309(6) 113 113 0 0 840( 1) 213 213 0 66618( 12) 0 0 680(2) 0 10528(2) 0 02374(4) 0 17512(3) 0 0 1774(3) 0 465(5) 0 357(6) 0 537(4) 0 426(6) 07(1) 0 5000 0 5000 0 1667 0 1667 0 15683( 10) 0 5039( 1) 213 0 031366(20) 0 0078(2) 113 0 0 05013(4) 0 14734(4) 0 05676( 7) 0 14207(7) 0 674( 10) 0 568(9) 0 584( 9) 0 513(9) 0 0196( 15) 0 0667( 17) 0 0097( 16) 113 0 2949(9) 0 880(5) 213 0 7051(9) 0 760( 10) 114 114 114 1 04(4) 0 55( 7) 0 9(5) 'The weighted sum of the occupancies corresponds to the formula Bao,,,Mno,,,All,,o,Ol, 153 The thermal parameters are of the form 4 n2[U,l(ua*)2+ U22(bb*)2+ U3,(cc*),+2Ul,aa*bb*cos y +2~,,aa*cc*cos/3+ 2U,,bb*cc*cos a] 'Interstitials 456 J Muter Chem , 1996,6(3), 455-458 sites, O(l), 0(2), O(3) and O(4) are close packed in the spinel block, whereas 0(5),O(6) and O(7) are in the mirror plane.The interstitial O(7) ion is placed near the mid-oxygen site, coordinating with the interstitial Al(5) ion.At the first stage of refinement, the occupancies of O(1)-0(4) remained unchanged. In the subsequent refinements, therefore, occupan- cies of these sites were fixed and only those of O(5)-0(7) were refined. We also examined refinement of all parameters simul- taneously. However, the result showed a good agreement with the stepwise refinement described above. Note that O(5)-0(6) bondlengths and 0(5)-A1(3)-0(6) angles are very small, but these two types of 0 sites are not occupied at the same time. The average oxidation number of Mn was estimated to be +2.472 from the refined structural formula, Bao~780Mno~25,All,~706~17~153,by assuming that the valences of the other elements is Ba2+, A13+ and 0,-.The refined structural formula agreed with that determined by inductively coupled plasma optical emission spectrometry (ICP-AES).Gasperin et al. reported that the strong diffuse scattering observed on manganese-substituted lanthanum hexaaluminates leads to a systematic error in the compositional calculation." The diffuse scattering was also observed on Nd3 +-exchanged sodium P-aluminogallate as reported by Kahn et ul." Since these diffuse scatterings arise from coherent shifts of La3+ or Nd3+ in combination with a local ordering of the cation vacancy, hexaaluminates with trivalent large cations tend to suffer from the same problem in determination of the site occupancies. In case of barium hexaaluminates, however, no result has yet been reported on the diffuse scattering.Taking electrical neutrality into consideration, the decrease in positive charge brought about by manganese substitution for A13+ should be compensated by an increase in cation concentration and/or by a decrease in anion concentration. As about no significant change in the final positional parameters as shown in Table 2. However, thermal parameters of O(5) and O(7) for the as-grown crystal are higher than those for the reduced crystal. The reason for the increased thermal parameters is not clear at the present stage, but one possible explanation could be as follows. The occupancy of the Al(5) site in the as-grown crystal was lower than that in the reduced crystal. The electrostatic bond between Al(5) and 0 in the mirror plane [0(5)and/or 0(7)] in the as-grown crystal is, therefore, supposed to be weakened by the reduction to produce the increased thermal parameters. A slight change can be also seen in the oxygen stoichiometry as a result of the elimination of lattice oxygen during evacuation.This is reflected by decreased occupancies of the 0(5),O(6) and O(7) sites in the mirror plane, whereas those of O(1)-0(4) sites in the spinel block remained unchanged. The cumulative change of their occupancies corresponds to ca. 4% of the interlayer oxygen being eliminated during the evacuation. The resultant structural formula of the reduced crystal was calculated as Bao~786Mn,~238Allo~705017~120,which implies an average oxi- dation state for the manganese ion of +2.324.The results of the present structural analysis suggest that the apparent oxi- dation state of manganese ion in the hexaaluminate was reduced from +2.472 to +2.324 during evacuation at 1150"C. The change in the oxidation state of Mn can be confirmed from the colour of the single crystal, which was initially red and turned transparent green upon evacuation at 11 50 "C. The UV-VIS absorption spectra of both crystals (Fig. 2) showed four independent bands at 362, 378, 425 and 447nm, which are well explained by the Tanabe-Sugano interaction matrix of a d5 ion (Mn2+) in tetrahedral symmetry.' However, the red as-grown crystal showed a significant red-shift of the absorption edge (-400 nm), compared with the reduced crystal, due to the intense oxygen-to-Mn3+ charge transfer bands.Therefore, decoloration of the crystal accompanying the evacu- compared with the structural parameters of Bao~75Alll~oO17~25 refined by Iyi et ~l.,~increases in the occupancies of Ba and A1 and decreases in those of O(5)-0(7), were observed. This suggests that the charge compensation required by the manga- nese substitution was achieved by both mechanisms in the present system. In addition, it should be noted that the occupancy of the interstitial Al( 5)ion decreased, whereas that of the Al(1) ion in octahedral environment increased with a simultaneous decrease in the occupancy of the interstitial O(7) ion, which coordinates the interstitial Al( 5).Consequently, the charge compensation is mainly achieved by O(7) defect forma- tion, which leads to the displacement of the interstitial Al(5) to the original position, the Al( 1) site.Structural analysis was also performed on a single crystal after the reduction treatment. The reduction treatment brought ation treatment must be associated with the progress of the reduction of Mn3+ to Mn2+. Besides the oxygen non-stoichiometry, a small variation of occupancies can be seen on Ba and Mn(2) sites. One possible reason for these variation must be due to an error in the compositional determination of these minority species from crystallographic data. In addition, the diffraction data may have been influenced by the compositional fluctuation occur- ring during single-crystal growth.For the lowered Mn content of the reduced crystal, another possible reduction mechanism may be also pointed out. This mechanism consists of two stages: disproportionation of Mn3+ to Mn2+ and Mn4+, and subsequent combination of Mn4+ and two lattice oxygens to yield MnO, which is given off. Such a disproportionation Table 2 Final structural parameters of a reduced crystal atom position occupancy' X Y z Beqb 0.0655( 1) 213 113 114 1.153( 2) 0.0198( 6) 113 213 0.02373( 2) 0.34(3) 0.4680( 10) 0.83 308(5) 0.666 16( 10) 0.10520( 2) 0.460( 4) 0.1469 (6) 113 213 0.02373( 2) 0.370( 6) 0.1667 113 213 0.175 17( 3) 0.544( 4) 0.0833 0 0 0 0.42 1 (6) 0.0272( 9) 0.8408(8) 0.6816( 16) 0.1770( 3) 0.40(8) 0.5000 0.15672(9) 0.3 1344( 18) 0.05012(4) 0.656( 9) 0.5000 0.50401 (9) 0.00802( 18) 0.14733(4) 0.556(8) 0.1667 213 113 0.05660( 6) 0.578(9) 0.1667 0 0 0.14218(6) 0.484(8) 0.0191( 11) 113 213 114 0.18 (4) 0.0653(15) 0.2943 (7) 0.5886( 14) 114 0.81(6) O.O089( 13) 0.881(3) 0.762( 6) 114 0.5(3) 'The weighted sum of the occupancies corresponds to the formula Bao.,86Mno.238A1,0,705017.120.The thermal parameters are of the form: $ + U,,(CC*)~n2[U,, + U,,(~ZI*)~ +2U,,aa*bb*cos y+ 2U,,aa*cc*cos +2U,,bb*cc*cos a].Interstitials. J. Mater. Chem., 1996, 6(3), 455-458 457 I 1 I I 300 400 500 600 700 wavelengthlnm Fig. 2 Absorption spectra of Mn-substituted Ba-hexaalummate single crystal (a)As-grown, (b)after reduction is a well known feature of Sn2+ chemistry, eg 2Sn0+ Sno+ SnO, l2 However, the exact route of this compositional variation is not clear at the present stage The results of this study suggest that the Mn ions in the hexaaluminate can be reduced without structural deterioration The calculated oxidation number of Mn in the crystal as prepared in air, 2472, agrees with that in the powder sample of BaMnAll1019-d,, which was determined by thermogravime- try in an H2 flow to be 243 Thus, manganese tends to be in a mixed oxidation state between + 2 and + 3 in the hexaalum- inate matrix This is in contrast to the other 3d elements, z e , Co and Ni prefer divalent, whereas Fe prefers trivalent states The intermediate property of Mn must be the reason for easy reduction/oxidation, which cannot be attained by hexaalumin- ates partially substituted with other 3d elements The reduction/oxidation of Mn plays a key role in the catalytic activity of manganese-substituted hexaaluminate With regard to the site of Mn in the hexaaluminate structure, Laville et a2 implied from the spectroscopic characterization, that the oxidation of Mn2+ to Mn3+ in a lanthanum hexaalum- inate single crystal is accompanied by the migration of Mn ions from a tetrahedral to an octahedral environment However, the present study clearly indicates that Mn in barium hexaaluminate remains in the tetrahedral Mn( 2) site during the reduction/oxidation process From a kinetic point of view, this oxidation/reduction process without cation diffusion should facilitate reversibility Note also the charge compensation mechanism for the reduction of manganese-substituted hexaaluminate As shown in Table 2, reducing the as-grown crystal produced the oxygen defects, in particular, only on mirror plane sites, 0(5), O(6) and O(7) This may suggest that the oxygen atoms, which are loosely packed on the mirror plane, tend to be easily removed as compared to the close-packed oxygen atoms inside spinel blocks However, the differences of occupancies of these oxygen sites are too small to be regarded as significant and no other evidence was found to support the preferential O2-defect formation on the mirror plane Mirror plane oxygen in hexa- aluminate shows unique behaviour because of its interlayer character We previously reported the anisotropic oxygen self- diffusion in barium he~aaluminate,'~ l4 which is brought about by a preferential diffusion route in the mirror plane Large interatomic distances between Ba and interlayer oxygen [Ba-0(5) 0 32 nm, Ba-0(6) 0 31 nm] suggest that these oxygen ions in the mirror phane in a weak electrostatic potential are readily mobile with a small activation energy References 1 N Iyi, S Takekawa and S Kimura, J Solid State Chem, 1985, 59,250 2 M Machida, K Eguchi and H Arai, J Catal, 1987,103,385 3 M Machida, K Eguchi and H Arai, J Catal, 1989,120,377 4 M Machida, K Eguchi and H Arai, J Catal, 1990,123,477 5 F Laville, D Gouner, A M Lejus and D Vivien, J Solid State Chem, 1983,49,180 6 N Iyi, S Takekawa, Y Bando and S Kimura, J Solid State Chem , 1983,47,34 7 N Iyi, Z Inoue, S Takekawa and S Kimura, J Solid State Chem , 1984,52,66 8 D T Cromer and J T Waber, in International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, 1974, vol IV, Table 2 2 A 9 D C Creagh and W J Mcauley, in Internatmnal Tables for X-Ray Crystallography, Kluwer, Boston, 1992, vol C, Table 4 2 6 8 10 M Gasperin, M C Same, A Kahn, F Laville and M Lejus, J Solid State Chem ,1984,54,61 11 A Kahn, G Aka and J Thery, J Solid State Chem, 1991,91,71 12 C Decroly and M Ghodsi, Comp Rend, 1965,261,2659 13 M Machida, T Shiomitsu, Y Shimizu, K Eguchi and H Arai, J Solid State Chem, 1991,95,220 14 M Machida, T Shiomitsu, K Eguchi, H Haneda and H Arai, J Mater Chem, 1992,2,455 Paper 51072455, Received 2nd November, 1995 458 J Mater Chem, 1996, 6(3), 455-458