Hydrothermal synthesis, structure and magnetic properties of barium diferrite

inGrcstBritain I. Phyr.Chem. Soli& 1975, Vol.36,pp.591-595. Pergamon Press.Printed

HYDROTHERMAL SYNTHESIS, STRUCTURE AND MAGNETIC PROPERTIES OF BARIUM DIFERRITE S. OKAMOTO,H. The Institute

of

SEKIZAWA and S. I. OKAMOTO Physical and Chemical Research, Wako-shi, Saitama-ken 351, Japan

(Received 24 September 1974) Abstract-A new barium ferrite, BaO.ZF%O,, was found to be crystallized by heating suspension of a-Fe*O, in aqueous barium hydroxide solution under hydrothermal conditions at around 260°C.It is hexagonal, space group P6,/m, with the lattice dimensions (I = 5160 and c = 13~811 A; Z = 2. The hexagonal unit cell consists of six layers of large oxygen and barium ions, and among the interstices ferric ions are distributed equally into two different sites; one surrounded tetrahedrally and the other octahedrally by oxygen ions. MSssbauer studies reveal a hyperfme structure which suggests an antiferromagnetic order with the easy-axis along the c-axis below TN= 945K. A small ferromagnetic component of magnetization observed is attributed to unavoidable impurities in the sample. From the structural and magnetic features a tentative magnetic structure is proposed.

‘INTRODUCTION

Ba(OH)* solution. The vessel was heated at a rate of about

Because of the technical importance of Ba0*6FezOj as a

2OO”C/hr,kept at 260°C for 16hr,* and finally cooled to room temperature. Prior to the heating the vessel was filled with 10kg/cm* of oxygen in order to protect the specimen from the reduction by hydrogen formed by the reaction between stainless-steel vessel and water. The product was washed with very diluted HCl solution and then with distilled water until na, Ba” was detected in the supematant, and finally it was dried at about 50°C in air. It contained red and shiny hexagonal platelets up to about 60 pm in diameter and several pm thick, together with finer crystals. Many of the thinner platelets were transparent and ruby-red colored in transmission. The crystalline product gave a single phase X-ray diffraction pattern with a hexagonal unit sell. Standard wet chemical analyses showed that the product is composed of 30.7 wt% of BaO and 69.3 wt% of Fe203, and all iron ions in the structure are trivalent. This composition is very close to the calculated value for BaO*2Fe203(32.4 wt% of BaO and 67.6 wt% of Fe203). It is to be noted that if the specimens were slightly reduced during preparation by hydrogen resulting from the corrosion of the stainless-steel vessel by water, a dark brown strongly magnetic and very fine grained powder was found to be formed in the solution. This magnetic product had a Curie temperature of about 500 K, and gave a single phase X-ray diffraction powder pattern characterized by a series of strong reflexions with d = 14*58/n A where n = 1,2,3,. . . . . It consisted of Ba2’, Fe2’, Fe3+and oxygen and water. Independent of our observation, Kiriyama and Kitaharna reported on a compound which seemed to be the same with ours. According to them, it is monoclinic with space group CZ/m, and the composition corresponds to Ba4Fe901,(0H)s[l2]. For convenience, this compound is called here the monoclinic phase. In order to obtain cation substituted barium diferrites, the mixtures of (Y-F~O~and vtious oxides, such as ZnO, CdO, CuO, NiO, A120, and Cr203,were heated in aqueous Ba(OH)2 solutions under similar conditions as in preparing Ba0*2Fe209. It was found that no cation substituted barium diferrite was obtained, but the formation of

material for permanent magnetsill, the system BaO-FezOs has long been investigated by many workers [2,3,41. Barium hexaferrite, BaO*6Fe203,has the magnetoplumbite structure (hexagonal, P6/mmc) with cell dimensions a = 5.888 and c = 23.22 8, and is ferrimagnetic with Tc = 723 K[5,6]. Barium monoferrite, BaO*FetOs, is of an orthorhombic structure (a = 19.074, b = 5.372, c = 8.450 A), the superstructure of hexagonal Ba0.A1201 and is an antiferromagnet with TN = 880 K[7]. According to Mori, BaFeO, (x = 2*5-3.0)exists in various forms depending on the oxygen content x; a hexagonal phase similar to BaTiO,, and phases having perovskitelike structure[l]. Most of all the magnetic oxides are synthesized mainly by either powder reactions or flux techniques at temperatures higher than 600°C. Some magnetic oxides, such as alkaline orthoferrites, however, can be crystallized even at temperatures as low as 150°C by reactions between aqueous alkaline hydroxide solutions and aFe203 powder[9]. In making use of aqueous barium hydroxide solution under hydrothermal conditions, we succeeded in synthesizing barium containing magnetic compounds[lO]. Among them, tiny hexagonal plates of a new compound BaO*2Fe20, were found to be crystallized, of which the crystal structure was analyzed[ll]. This paper reports the chemical, structural and magnetic features of the hydrothermally synthesized new barium diferrite. EXPERIMENTAL DETAILS AND RESJLs’IS (a) Hydrothermal synthesis The synthesis involved the solid-liquid type reaction

between cr-Fe20, crystal and aqueous Ba(OH)* solution under hydrothermal conditions. Teflon beaker containing 3 g of fine grained (Y-Fe203 powder dispersed in 60 cc of 0.5 M Ba(OH)z solution was placed in a stainless-steel pressure vessel which contained another 50 cc of 0.5 M *Below 240°C no reaction was observed.

591

592

S. OKAMOTO, H.

SEKZAWA and

S. I. OKAMO~~

strongly magnetic compounds was observed invariably, some of which were very similar to the monoclinic phase. On the other hand, when AgzOwas mixed, a new hexagonal phase of silver ferrite was obtained, of which crystal structure was analyzed [ 131. (b) Crystal structure Detailed structure analysis has been reported elsewhere[ll], and here it will be described briefly. The crystallographic data obtained are listed below: Barium diferrite, Ba0.2Fez09. Fw = 472.7. Hexagonal, space group P&/m. a = 5.160 +0*003 A, c = 13+311~0*009A, 2 = 2. v = 318.4320.46 A’. 4 = 4.93 g/cm’, D, = 4.93 g/cm’ (by picnometer). The crystal structure is shown schematically in Fig. 1. Interatomic distances and bond angles around ferric ions are shown in Fig. 2*. C’ (z-3/4) A (z-7/12)

B (z-5/12)

B (2=1/12)

A (z--1/12)

C’ (z--1/4)

0 FE3+(1)

. k3+(2)

0

02-

0

BA2+

Fig. 1. The hexagonel structure of Ba0.2FeZ0,.

The hexagonal unit cell is composed of six layers stacked along the c-axis. If we denote the three successive layers at z = cu. - l/12, ca. + l/12 and +1/4, as A, B, and C respectively, the packing scheme ABC is similar to that found in the f.c.c. lattice. Both A and B contain three close-packed oxygen ions whereas C contains only one oxygen and one barium ions. Thus, on the barium-oxygen mixed layer, an open space is formed. The remaining three layers are related to the block ABC by the mirror plane through C. Therefore, the overall stacking of the layers can schematically be represented as ABCBAC’. The interlayer spacings are: A - B = 2.18 and B-C=2*36A. *Although two structures (structure (I) and structure (II)) are possible in regards to the position of twelve O(2) oxygen atoms, structure (I) is preferable, as has been reported previously11Il. Here, we will discuss the magnetic structure on the basis of the structure (I), which is shown in Fii. 1 in this report. Me-O-Me or O-Me-O bond angles and distances are practically the same in the two structures, but O-O distances of FeO, octahedra differ very slightly.

0

Q+(l)

l

Fs3+(2)

0

02-

Fig. 2. Interatomic distances and bond angles around ferric ions.

Because of the open spaces, each barium ion is surrounded by nine oxygen ions instead of twelve. Similar conliguration is also found in the structure of Ba0*A120j and BaO*Fe203. A remarkable feature of the structure is found in the distribution of ferric ions on two cation sites with equal concentrations. Four of the eight ferric ions, Fe”(2), are surrounded octahedrally by oxygen ions with FeO distances of 1~98(O+l) and 2W(OW)A. The others, Fe”(l), are surrounded tetrahedrally with Fe-0 distances of 1+33(0*03) and 1*87(0+01) A. It should be noted that the pairs of the tetrahedra along the c-axis share a comer with a linear Fe-O-Fe bond, whereas each octahedron shares edges with the adjacent three octahedra forming pseudo-hexagonal layers of octahedra with a nearly rectangular Fe3’(2@--Fe3’(2) bond (95.1’). The bond angles between octahedrally coordinated and tetrahedrally coordinated ferric ions, Fe”(2)-0*--Fe”‘(l) are found to be approximately 120”. (c) Static magnetic properties

The magnetization was measured with a vibrating sample magnetometer within the temperature range from 77 to IOOOK. The results are shown in Figs. 3-6. Figure 3 shows the field dependence of the magnetization below 600 K. The magnetization per g., u appeared to obey the expression u = u. t ,yB.H, where cro is fer-

Fig. 3. (r vs H for the sample of Ba0.2Fe203 below 600K.

Hydrothermal

synthesis, structure asdmsgnetic properties ofbarium diierrite

Fig.4. Thermalhysteresisof the sampleof BaO.ZF&O,.

ANNEALING

TERPERATURE

(K)

Fig.5. u vs H for the sampleof BaO*F&O,above600K.

Fig.6. u0vs annealingtemperature. romagnetic component of the magnetization obtained by extrapolating u-H curves to H =O, and xS is the magnetic susceptibility per g. The thermal hysteresis of the ferromagnetic component of the magnetization is really complicated as can be seen in Fig. 4. In so far as the temperature did not exceed about *loOmg of the sample was. annealed at 395K.-for 64 hr, quenched and then subjected to the nqnetic measurementat room temperature.nK same sample was treated repeatedly in similarfashion at several tem@zaturesup,to 1025K.

593

6OOK, u. is reversible with respect to the temperature changes (curve I), and the dominant component of r. vanishes at around 500 K, which coincides with the Curie temperature of the monoclinic phase. Although it was not detected by means of X-ray, it seems that the sample was contaminated by a slight amount of the monoclinic phase. When the sample was heated once above around 600 K, u. increased and did not return to the original value even if the temperature was lowered. This can be seen in Fig. 5, where u. at room temperature is plotted against annealing temperatures.* The increase in uo above 600K is interpreted by the formation of BaO*6Fe203 due to the decomposition of the monoclinic phase because the Curie temperature of the ferromagnetic component appears at 725 K as can be seen in the thermal hysteresis of the curve II in Fig. 4. On increasing the temperature further above 725 K, u. vanished at about 850 K (Fig. 6) which is far below 945 K, the NCel temperature of the main phase Ba0.2FezOj, established by Miissbauer measurement described later. Only the magnetization of above about 850 K are thought to be that of the main antiferromagnetic phase of Ba0.2FezOs (xs = 14 x 10e6e.m.u./g at 873 K), while the magnetization in the lower temperature range seems to be contaminated by the contribution from the ferrimagnetic impurity phases and cannot bear detailed quantitative consideration. It was observed that when the sample was cooled after being heated as high as 950K, the ferromagnetic component of the magnetization increased further, as is seen from the curve III in Fig. 4 and Fig. 5. This is due to the decomposition of the main phase Ba0.2Fe203 into BaO*6Fe203 and BaO*FezOS,which was confirmed by means of X-ray. From Fig. 5, the decomposition temperature was estimated to be 925 K. By comparing the observed saturation magnetiaation with that of BaO*6Fe203,the ferromagnetic components in the curve II and III in Fig. 4 correspond to the mixing of approximately 0.5 and l.Owt% of BaO*6FezOs in the sample. (d) Miissbauer spectra Miissbauer spectra were obtained using a source of nCo in Cu and a combination of a constant acceleration electromagaetic drive and a 400 channel pulse-height analyser operated in time mode. The spectrum for the polycrystalline absorber is shown in Fig. 7(A). The Mossbauer parameters obtained are summarized in Table 1. The spectrum is easily interpreted as the one due to two six-line hyperline patterns corresponding to “Fe in two lattice sites. It is well established [14] that in various magnetic oxides the ferric ion in the tetrahedral site is more covalent than that in octahedral site. Consequently, the magnitude of the isomer shift and the internal magnetic field is smaller for the ion in tetrahedral site. On the basis of the structure determined, it can easily be deduced that the spectrum of the line group (a) originates from the ferric ions in the octahedral sites, whereas that of the line group (b) from the ferric ions in the tetrahedral sites. Furthermore, the intensities of the line group (a) and (b) are comparable to each other, which reflect the structural features of

594

S. OKAMOTO, H. SEKUAWA and S. I. OKAMOTO

400 -

(A)

= 3009 CF 200-

I

RANDOMLY ORLENTED

I

100-

0

(B)PARTIAL”

OR,ENTED

lb la

fi

1

-10

-5

0 h3CiTY

+5

(MM/SEC)

Table 1.Mossbauer parameters of BaO.ZFe,O,(R.T.)

0.51 0.31

1000

assuming S = 5/2 (solid line in the figure). From the plot of the hyperfine field as a function of v/(7’- TN), the Ntel temperature for BaO*2Fe203 was determined to be 945 + 3 K, which is slightly lower than that of cu-FelOl, TN = 965 K.* DISCUSSION

(mmlsec) 0.08 0.23

800

Fig. 8. Temperature dependence of internal field of BaO*ZFe,O,.

observed static magnetic behaviors of the barium dlferrite were rather complicated in contrast to the Miissbauer characteristics which were found to be simple. The Miissbauer characteristics reflect the properties of the majority of iron ions in the lattice whereas the static magnetic behaviors are liable to be influenced by ferromagnetic impurities when the majority phase is only weakly magnetic. It is concluded uniquely from the Mijssbauer spectra that the ferrite is in magnetically ordered state with an ordering temperature of 945 K. As the ordinary ferri- or ferromagnetism is denied by static magnetic measurements, the ordered state is established to be antiferromagnetic. Thus, the ferromagnetic part is attributable either (1) to weak parasitic ferromagnetism due to Zyaloshinsky-Moriya mechanism, or (2) to ferromagnetic impurities. It is well established that the indirect exchange coupling between ferric ions acting through the oxygen ions is stronger when the bonding angle Fe”-O*--Fe”’ is closer to 180”,the bonding distance of Fe”-O*- is shorter, and the number of interacting neighbors are larger. With regard to BaO*2Fe203,we can expect on the basis of the crystal structure shown in Figs. 1 and 2; (1) strong antiferromagnetic coupling between tetrahedral ferric ions forming linear array of Fe”‘-02--Fe3’, (2) strong antiferromagnetic coupling between octahedral and tetrahedral ferric ions and (3) weak interaction between nearest octahedral ferric ions in the same hexagonal layer of octahedra. These suggest that magnetically the lattice can be divided into four sublattices, in which antiferromagnetically coupled spins on tetrahedral ferric ions couple antiferromagnetically with spins on octahedral ferric ions, which are parallel with the next c-layer. Thus, on the assumption that the spins are parallel to the crystallographic c-axis, as is suggested from the Miissbauer experiments and the following discussions, a The

a b

600

+10

Fig. 7. Mijssbauer spectra of Ba0.2Fe203 at room temperature.

&* (mm/set)

400

TEMPERATURE (K)

(7 II C-AXIS)

I

200

516 486

*Referred to stainless steel.

BaO*2Fe203 that the concentrations of ferric ions in octahedral and tetrahedral sites are the same. To obtain information about the spin axis, the MBssbauer spectrum was collected for a partially oriented absorber, in which the hexagonal platelets of crystals are placed so as to cause preferred orientation of each crystal with c-axis parallel to y-ray direction. The spectrum is shown in Fig. 7(B). The intensities of the lines with unchanged nuclear magnetic quantum number (Am = 0) relative to those of innermost two lines vary as 3 sin’ 0, whereas 0 is the angle between the direction of the internal magnetic field (the spin axis) and the direction of the y-ray. The lines corresponding to the Am = 0 transitions (the lines 2a, 2b, Sa and 5b) have clearly decreased by the orientation, indicating that the spin axis of BaO*2Fe203does not lie in the c-plane. On considering the difficulties in aligning very small crystals completely, it is likely to be that the spin axis lies along the crystallographic c-axis (hence 0 = 0). In order to determine the NCel temperature of the antiferromagnet, temperature dependence of the hyperfine field was measured. The mean values of the hyperfhre fields acting on tetrahedral and octahedral “Fe are plotted in Fig. 8. As can be seen, the change follows the curves deduced from the Brillouin function for *For calibration, we measured the temperature dependence of the hypertine field in cr-Fe203 around its NCel temperature employing the same experimental setup. The T, for a-Fe203was determined to be 965k 3 K which is in good agreement with the values in literatures [ IS].

Hydrothermal synthesis, structure andmagneticpropertiesof bariumdiierrite

595

CONCLUSION

TET ,

TET.

A new compound BaO~ZFaQ was synthesized hydrothermally and its crystal structure was determined. Experimental results of magnetostatic measurements combined with those from Miissbauer study established that the material is an antiferromagnet with TN = 945 K. A small ferromagnetic component of magnetization observed is attributed to unavoidable impurities in the specimen.

OCT.

TET ,

Acknowledgemeals-The authors exgress their sincere thanks to Dr. T. Ito for the structure analysis and to Dr. T. Okada for Mijssbauer spectra measurement. HelRfulldiscussions with Dr. K. Kohn, Waseda University, contributed greatly to our group theoretical consideration.

TET .

ocr I

Fig. 9. Tentative antiferromagnetic spin arrangement of BaO-2Fe20,.

tentative magnetic structure is proposed as shown in Fig. 9. Group theoretical consideration leads to a conclusion that in our ferrite with the space group P6Jm, Dyaloshinsky-Moriya type weak ferromagnetism is allowed only when the easy axis of the antiferromagnet is not parallel to the c-axis[l6]. As mentioned in the previous section, Miissbauer experiment suggests that the c-axis is the easy axis. A possibility of finding the easy axis somewhat deviated from the c-axis cannot be excluded absolutely, but as it looked like unrealistic on various reasons, such a possibility was dropped from consideration. The final possibility left, i.e. the case (2) above, seems to be acceptable, with a few reservations. It is concluded that the pure hexagonal phase of Ba0*2Fe203 is an antiferromagnet with TN = 945 K, without parasitic weak ferromagnetism.

PCS

VOL. 36, NO. 6-H

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