Barium hexaaluminoferrites with new structural features

Mat. R e s . B u l l . , Vol. 28, p p . 435-443, 1993. P r i n t e d in t h e USA. 0025-5406/93 $6.00 + .00 C o p y r i g h t (c) 1993 P e r g a m o n P r e s s L t d .

BARIUM HEXAALUMINOFERRITES WITH NEW STRUCTURAL FEATURES

V. Delacarte, A. Kahn-Harari, J. ThEry Laboratoire de Chimie Appliqude de l'Etat Solide (URA 1466 du CNRS) ENSCP 11 rue Pierre et Marie Curie 75231 Paris Cddex 05

(Received January

28, 1993; C o m m u n i c a t e d b y P. H a g e n m u l l e r )

ABSTRACT Barium hexafenite and hexaaluminate are of hexagonal symmetry (P63/mmc) with a structure respectively of magnetoplumbite (MP) and g-alumina (B) type. The BaAlxFet2_xO19 phases (BAF) have been synthesized by solid state reactions at high temperatures (1350°C - 1500°C). The X-ray powder diffraction patterns are close to those of a MP or B structure type but for 4 < x < 8 they also exhibit some extralines. These new lines can be interpreted as 001, 1 = 2n+l which are "forbidden" reflexions in the P63/mmc space group. Their intensities are particularly strong for BaA16Fe6019 (x = 6) and decrease apart from this composition. The a unit cell constant shows a linear decrease with increasing x, whereas the c vs. x curve presents a marked inflection point for intermediate x values. The c/a ratios show that for x < 4 the structure should be of MP-type while compounds with x > 9 would adopt the B structure. For the intermediate compositions, corresponding to the appearance of the extra lines, a new structural type appears. Models are proposed to describe these new phases. MATERIALS INDEX':

Hexaferrites,

m a g n e t o p l u m b i t e s , fl - a l u m i n a , b a r i u m

Introduction

M- type barium hexagonal ferfite BaFel2019 has been widely studied because of its relevance in technological applications such as permanent magnets, microwave devices, and recording media (1,2). Its c r y s t a l structure is s i m i l a r to that of the m i n e r a l m a g n e t o p l u m b i t e PbFe7.sMn3.sA10.sTi0.5019 . For this reason it is also called MP-type ferrite. The structure of 435

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BaFe12019 (SG P6Vmmc) can be described as a stacking of spinel-like blocks separated by mirror planes containing the large cation Ba2+, three 02- ions and one small trivalent cation Fe 3+. Substituted M-type ferrites BaFel2-xAxO19 have been widely investigated in order to improve the magnetic characteristics of barium fenite (3-5). Among these the study of Al-substituted hexagonal ferrites has been carried out by many authors (6,7). In particular Batti et al (8) report a miscibility gap in the system barium ferrite - barium aluminate. According to these authors barium hexaaluminate was supposed to have a magnetoplumbite structure ("BaAl12019"). However, Stevels et al (9,10) for the first time have considered barium hexaaluminate as having B-alumina structure (Nal+xAlllO17+x/2, 0 < x < 1). Afterwards several authors (11-14) showed that barium hexaaluminate (phase I) has an approximate composition of Ba0.7AlllO17.2. The B-alumina structure is closely related to that of MP structure. The main difference between the two structures consists in the composition and site occupancy of the mirror plane perpendicular to the c axis which is close-packed for MP and loose-packed for B : in the ideal B-structure, these planes contain only one 02- ion (instead of three in MP), the Ba 2+ ion and no small cation (A13+). This remark concerning the B-structure of barium aluminate has led us to review the results previously described concerning the miscibility between BaFe12O19 (MP) and Ba0.7AlllO17.2 (B). It was of interest to study the evolution of the structure of BaFel2-xAlxO19 from MP (BaFe12019) to B (Bao.TAlllO17.2). Up to now, no continuous evolution from the B-structure to the MP-structure has been observed whatever the system considered. According to the case one finds either a mixture of two phases (B+MP) or a phase whose structure belongs to a different type: the sodium-lanthanide mixed aluminate (SLnA) type (15). SLnA is of hexagonal symmen'y (SG P6m2) and the unit cell can be described as an alternate stacking of g-Na aluminate and MP-lanthanide aluminate half unit-cells (16,17). The SLnA phases are easily identified by their X-ray powder diffraction patterns which are close to those of B and MP type compounds (SG P63/mmc) but exhibits 001 and hhl (1 odd) reflections which are forbidden in the P63/mmc space group. Such phases have been observed for alkaline-lanthanide (18,19), barium-lanthanide (17), sodium-calcium (20,21) mixed hexaaluminates and potassium-barium mixed hexaferrites (22). Generally speaking it appears that SLnA-type phases are formed only when one of the parent compounds is of magnetoplumbite type and the other of the B one.Therefore we can expect that such structure appears for BaFel2-xAlxO19 compounds. However these SLnA phases have been observed when the parents compounds differ for the nature of the large cation. BaFe12019 and Ba0.TAlllO17.2 are respectively of MP and 13 type but have the same large cation Ba2+. The difference lies in small cations Fe 3+ and A13+. This paper reports the solid state synthesis and clTstal growth of BaFel2-xAlxO19 compounds and their charactelization by x-ray diffraction. Exverimental

The nature and number of phases is determined by x-my powder diffraction at each step of the heat treatments. Debye Scherrer diagrams have been recorded on a Theta 60 CGR diffractometer using CoKe~ wavelength. Solid state s v n t h e s i s * BaFe12_xAlx019

BaFel2-xAlxO19 compounds (x = 0, 2, 4, 5, 6, 7, 7.5, 8, 9, 10, 11) are obtained by heating at high temperature a mixture of the following starting materials in the appropriate amounts : A1203,

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Fe203, BaCO3. The barium carbonate can eventually be replaced by BaF2. About l g of this intimated mixture is pressed into pellets, heated at 1350°C for 12 hours in order to allow the decomposition of the carbonate and prereaction of the oxides. For compositions x = 0 and x = 2, a single phase of MP structure -see parameters in Table I- appears at 1350°C. For higher x values two phases are formed with closely related structure (of 13 or MP type). The samples are then crushed, pressed again and annealed at 1500°C for 12 hours. This leads to the melting and the decomposition of the samples x = 0 and x = 2. For compounds with x > 4 a single phase appears (Fig.l). 107

0.0.1

1.1.4

20:11 2.20

2.05

006

I

I

20

I

40

80

60

2e, FIG. 1 X-ray powder diagram of BaFe6A16019 heated at 1350°C and 1500°C For the whole range of x values, the X-ray powder patterns contain diffraction lines, characteristic of 13or MP-type structure, but for x = 5, 6, 7, 7.5 they exhibit some extra lines. These ones can be interpreted as 001, 1 = 2n+l which are "forbidden". reflections in the P63/mmc space group. Their intensities are particularly strong for the composition x = 6 and decrease apart from this composition (Fig.2).

001 O01 X=2

X=6

X=5

002 0O3 O04

5

6"

10

5

O"

10

003 ~r

X=7

X = 10

~1 6e

~--~

FIG.2 Comparison of the 001 and 003 lines intensities for different compositions. The lattice constants a and c and the values of c/a are given in Table I.

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TABLE I Unit cell parameters and c/a ratio for samples obtained by solid state reaction at 1500°C (except for x = 0 and 2 which are annealed at 1350°C).

a (A)

c

(A)

c/a

5.895

23.203

3.94

5.853

23.050

3.94

5.795

22.927

3.96

5.773

22.926

3.97

6

5.754

22.901

3.98

7

5.718

22.913

4.01

7.5

5.716

22.894

4.01

8

5.710

22.884

4.01

9

5.671

22.815

4.02

10

5.646

22.847

4.05

11

5.618

22.781

4.06

11.5

5.602

22.750

4.06

12

5.589

22.703

4.06

The variation of a with the degree of substitution x is represented in Fig.3 : a shows a linear decrease with increasing x. The variation of c appears to be more complicated (Fig.4). The c = f(x) curve reveals different parts : for small (x < 4) and high x values (x > 8) the decrease of c is almost linear whereas for intermediate x values it slows down. The curve exhibits a marked inflexion point in this area corresponding to the occurence of the new peaks 001 (1 odd) in the Xray powder diagrams. 5.90

23.3,

5.85"

23"2 k

5.80 5.75" o

5.70 -: 5.65

-'

5.60 ¸ 5.55 "

I

I

I

I

I

1

I

I

I

l

2

3

4

5

6

7

8

9

I

I

l0 II

12

X

FIG.3 a versus x for BaFel2.xAlxOl9

::I "1 :::1,,,,,,,,,,, 0

1

2

3

4

5

6

7

8

9

10 11 12

X

F I G .4 c versus x for BaFe12-xAlxO19

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According to Verstegen et al (24), c/a values range from 3.91 to 3.95 for MP-type compounds and from 4.02 to 4.08 for g-type ones. This allows a first explanation of the structure : from x = 0 to x = 4, the structure should be of MP-type while compounds with x > 9 would adopt the 13 structure. For the intermediate compositions (5 < x < 8), corresponding to the appearance of the extra lines, it is not possible to conclude between the 13 or the MP structure, which leads us to think of a new type of structure. * BaFe6AI6019

BaFe6A16019 was of particular interest because the extra lines in the X-ray powder patterns were the strongest for this composition. Therefore, this compound was studied in detail in order to precise the conditions of formation of this new phase. Several treatments are then carried out the results of which are given in Table II. Each one consists in maintaining the sample for 12 hours at the considered temperature (1350°C or 1500°C). The pellets are crushed between each treatment. TABLE II Number and nature of the phases obtained for several thermal treatments Successive heat treatments First treatment 12h Second treatment 12h Third treatment 12h

Annealing temperature number and nature of the phase(s) obtained (1) 1350"C (4) 1500"C 2 phases 2 phases MP or ~ structure MP or 13 structure (2) 13500C (2bis) 1500°C (5) 1500"C 1 phase 1 phase 1 phase MP new structure new structure (3) 1500"C 1 phase new s t r u c t u r e

The 001 lines with 1 odd never appear at 1350°C. As already mentioned two phases are obtained at this temperature after one heat treatment (exp. n°l). However two heat treatments at 1350°C (exp. n°2) lead to the formation of a MP-type single phase (a = 5.761 A, c = 22.750 A, c/a = 3.95). When the samples annealed at 1350°C (exp. n°l and 2) are heated at 1500°C (exp. n°3 and 2bis) a single phase is obtained whose X-ray pattern contains 001, 1 = 2n+l reflections. So this new phase is only formed at 1500°C. However a direct heating at 1500°C (exp. n°4) does not allow its formation : after one treatment at this temperature two phases are present (alike to one treatment at 1350°C). If annealed a second time at 1500°C (exp. n°5) the new phase appears. S i n g l e c r v s t a l s synthe~i~ BaFel2-xAlxO19 compounds melt incongruently. Therefore crystallization from the liquid state can not be considered. Therefore, crystals growth was tried by two flux methods, performed at two different temperatures: 1350°C and 1500°C. As already seen the nature of the phase obtained depends upon the temperature. In each experiment, the crystallized material is separated from solidified flux by leaching with dilute nitric acid. The chemical composition of the crystals is determined by electron probe microanalysis.

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* Slow cooling

We have carried out a slow cooling down from 1350°C of a supersatured solution. Two experiments are performed using a BaB204 flux and either the starting materials {BaCO3 + 3 A1203 + 3 Fe203} (exp.a Table III) or the BaA16Fe6019 (exp.b Table III) prereacted (by solid state synthesis exp. n°2bis of table II). BaB204 is a successful flux for crystallization of oxide compounds and especially for hexaferrites (23). Moreover this flux does not bring pollution by foreign cations. Flux + ferrite are mixed in a 1:1 weight ratio. The mixture is heated 24 h at 1350°C, cooled at a cooling rate of 5°C/h down to 1000°C and 7°C/rain down to room temperature. In both experiments, the small hexagonal platelets obtained are some mm in diameter. Their X-Ray patterns are typical of a MP-type single phase (without any extralines). The composition and unit cell parameters are given in Table III. TABLE HI Compositions and unit cell parameters for crystals obtained by slow cooling. Starting materials

Composition

a (]~)

c (A)

c/a

a) BaCO3 + 3 A1203 + 3 Fe203

BaAI2.3Fe9.5018.7

5.85

23.02

3.94

b) BaA16Fe6019

BaAI2.9Fes.9018.7

5.83

22.98

3.94

It appears that crystals synthesized by this method are Fe-deficient compared to the starting composition corresponding to x = 6. For the MP compounds (x = 0, 2, 4 of Table I, x = 6 (2) of Table II and x = 2.3, 2.9 of Table III) the c unit cell shows a linear decrease with increasing x (Fig.5). We find here the results previously described in the literature (4). Departure from this straight line caracteristic of MP structure occurs only for sample annealed at 1500°C.

23.3

23,2

-

23.1

23.0

22.9 -

22.8 -

22.7

t

X

FIG 5 c versus x for MP-structure compounds * Flux evaporation

The flux evaporation is performed at a constant temperature of 1500°C : BaC12 is chosen for this experiment with a BaC12/{BaCO3 + 3 A1203 + 3 Fe203} weight ratio equal to 1. The mixture is slowly heated to 1500°C, maintained for four days and then cooled down (100°C/h) to the room temperature. This technique yields small hexagonal platelets (about 0,2-0,3 mm in diameter) embedded in a polycrystalline bulk. The mixture is crushed and analyzed by X-ray powder diffraction. The X-ray pattern reveals two phases of 13or MP structure but exhibits also 001, 1 odd lines. Therefore the crystals have been studied by LaiJe, oscillation crystal and Weissenberg methods. No extra spots have been observed so we can suppose that the new phase is in the polycrystalline bulk.

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Discussion As already seen we have not been able to obtain the new phase BaA16Fe6OI9 with non P63/mmc structure in a single crystal form. Nevertheless, to get a first insight into the structure of the phase obtained with this composition, the "lazy routine" (25) was employed which allows a simulation of the X ray diffraction intensities when entering the description of the structure. The occurrence of the new lines 001 with I odd, forbidden in the P63/mmc space group reveals a loss of symmetry. Therefore the selected model should correspond to a space group which allows these reflections. We first thought of a SLnA-type structure generally described as an alternate stacking of [3 and MP half unit-cells (16). So different models in the P6m2 group were tested. This group corresponds to P63/mmc with no inversion center. This permits a variable occupation of the successive mirror planes (which were related by 1 in P63/mmc). The model giving the best results can be described as follows (see on Fig.6a): one mirror plane is similar to a MP-type mirror plane with 3 0 2- ions, 1 Fe 3+ ion and 1 Ba 2+ ion. A part from it, in the neighboring part of spinel blocks, 2 Fe 3+ are in octahedral sites. The other mirror plane of the cell is a ~-type plane: 1 Ba 2÷, no A13+ ion in the mirror plane and 2 A13+ in neighboring tetrahedral sites apart from the bridging oxygen of the mirror plane. Within the spinel blocks, the tetrahedral sites are A13+ occupied while the other sites are occupied both by A13+ and Fe 3+ in a statistic distribution, to keep the unit cell constants close to the observed ones. The pattern corresponding to this suggested model I is given on Fig.6b. a)

b) I - -

X Ray powder pattern P-6m2 I

i

;~ ,

O O iO ()

I O

'+l old....... ;;.;....... ~.;....... ~.0....... ;~.;....... ~.0....... d.~....... 7;.0....... ~J.;.... 2-theta In degrees

O

O 2-

•

A13+

Ba 2+

Q Fe 3+ (11 A13+ / Fe3+ (50/50) A13+ / Fe 3+ (33/67)

FIG.6 : Model I in P6m2 a) unit cell b) theoretical X-ray powder pattern

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We may also consider a lower symmetry (compare to P63/mmc) with the P3m 1 space group which corresponds to P63/mmc with no (001) mirror plane. In the P3ml like in the P~]m2 space group the 001, 1 = 2n+l reflections are permitted. Here the ex-mirror planes are symmetric (1) and cannot be different. In these limits, several models were tested. The best one, the unit cell of which is given on Fig.7a, can be described as follows: the ex-mirror planes are of BaFel2019 MP type, with 3 O 2-, 1 Fe 3+ and 1 Ba 2+. The two octahedral sites a part from it are differentiated: one is an Fe 3+ ion and belongs to a mostly Fe3+-occupied spinel block, whereas on the other octahedral site lies a A13+ ion belonging to a mostly A13+-occupied spinel block. The corresponding pattern is given on Fig. 7b.

a)

b) -

-

X Ray powder pattern P-3rnl

2-theta in degrees

O

02-

• A13+

•

Ba2+

~) Fe3+ •

A13+/ Fe 3+ (50/50)

X inversion center FIG.7 : Model II in P3ml a) unit cell b) theoretical X-ray powder pattern These two proposed models are surely not the only ones fitting the experimental powder pattern still they illustrate two ways of building a new structure in a space group of lower symmetry, i.e. P63/mmc --->P6m2 (model I) and P63/mmc ---> P3m 1 (model II). In absence of single crystals, further investigations, giving local arrangements for instance, could help us to decide which type of arrangement is the true one in our material. Conclusion

As a conclusion, under particular conditions, we have been able to prepare a new BaFe12_xAlxO19 phase for which the X-Ray diffraction pattern exhibits the reflections of a f~-alumina or

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magnetoplumbite type structure, with some forbidden extra lines 001 with 1 odd. This occurs when: - the starting composition satisfies the formula : BaCO3, x/2 A1203, (12-x)/2 Fe203 5 < x < 7.5 - the annealing temperature is at least 1500°C - the time reaction is long enough: the new phase is formed when annealing the sample during 24 hours. Mechanical treatment (crushing) between successive annealings influences the kinetics of the solid state reaction. Tentative crystal growth was not successful. Therefore two theoretical models are proposed to describe the new phase of lower symmetry. Their calculated X-Ray pattern correctly fit the experimental one. Both models are based on partial ordering of A13+ and Fe 3+ on their common sites. Then the new phase can be described : - either in P6m2 space group with successive mirror planes (perpendicular to the c-->axis) having different atomic disu-ibution, - or, in P.3ml space group, with spinel blocks differentiated by the distribution of Fe3+/A13+ ions. Further investigations using the Rietveld method or Mossbauer spectroscopy of 57Fe are undertaken in order to get further in the sn'uctural and local description of the new phase. Reference

(l) J. Smit and H.P.J. Wijn, Ferrites, Phil. Techn. Lib., Eindhoven, p. 177(1959). (2) H. Kojima, Ferromagnetic Materials, Vol. 3 p. 305 E.P. Wohlfarth, North-Holland, Amsterdam(1953). (3) F. Bertaut, C.R. Acad. Sc., 2594(1958) (4) F. Bertaut, A. Deschamps, R. Pauthenet and S. Pickart, J. Physique 20, 404(1959). (5) G. Albanese, A. Deriu, Cer. Int., 5 n. 1 , 3(1979). (6) G. Albanese, G. Asti and P. Patti, Nuovo Cimento 58B, 467(1968). (7) G. Albanese, G. Asti and P. Patti, Nuovo Cimento 58B, 480(1968). (8) P. Batti and G. Sloccari, Ann. di Chim. 58, 213(1968). (9) A. L. N. Stevels and A. D. M. Schrama de Pauw, J. Electrochem. Soc. 123, 691(1976). (10) A. L. N. Stevels, J. Lumin. 17, 121(1978). (11) F. Haberey, G. Oehlschlegel, and K. SaM, Ber. Dtsch. Keram. Ges. 54, 373(1977) (12) S. Kimura, E. Bannai, and I. Shindo, Mater. Res. Bull. 17, 209(1982). (13) N. Iyi, Z. Inoue, S. Takekawa, and S. Kimura, J. Solid State Chem. 52, 66(1984) (14) A. Kahn, T. Gbehi, J. Th6ry, and J.J. Legendre, J. Solid State Chem. 74, 295(1988). (15) J.P. Barret, D. Vivien and J. Th6ry, Mater. Res. Bull., 18, 59(1983). (16) A. Kahn and J. Th6ry, J. Solid State Chem. 64, 102(1986) (17) T. Gbehi, J. Th6ry, D. Vivien, Mater. Res. Bull. 22, 121(1987). (18) A. Rafaoui, A. Kahn, J. Th6ry, and D. Vivien, Solid State Ionics 9-10, 331(1983). (19) G. Aka, J. Th6ry, and D. Vivien, J. Amer. Ceram. Soc. 7__Q,C-179(1987). (20) P.E.D. Morgan, Mater. Res. Bull. 18, 231(1983). (21) P.E.D. Morgan and J.A. Miles, J. Amer. Ceram. Soc. 69, C-157 (1986). (22) Shinya Nariki, Shigeru Ito, and Noboru Yoneda, J. Solid State Chem. 87, 159(1990). (23) F. Licci, T. Besagni, Mater. Res. Bull. 22,467(1987). (24) J. M. P. J. Verstegen and A. L. N. Stevels, J. Lumin. 9, 406(1974). (25) Lazy Programme, Cristallographie Rayons-X, Universit6 de Gen~ve, 24 Quai EmestAnsermet, CH 12l 1 Gen6ve 4, Switzerland.