Spectroscopic Study of Al 3 +-Substituted Strontium Hexaferrite

The substituted Y type hexaferrites Sr 2 Zn 2 Al x Fe 12−x O 22 (where x= 2, 4, 6, 10) are prepared by standard ceramic technique. The infrared absorption spectra of the prepared compounds are studied in the range 400 cm to 4600 cm. These spectra are used to locate the vibrational ranges due to substituted cations with the nearest oxygen layers and also to understand the band positions attributed to the lattice sites. The absorption regions found around 460 cm and 600 cm are the common features of all spinel ferrites. BothX-ray and IR spectroscopic studies in these ferrites do not detect the presence of Fe. Alsowith increase in aluminium substitution, the higher frequency bands start disappearing.This may be due to reduced vibrations of trivalent cations Fe–O bonds at both octahedral and tetrahedral sites which in turn affect Fe–O–Fe superexchange interactions present in the structure.


Introduction
Infrared spectroscopy is the most powerful technique for the chemical identification of crystal structure.It provides useful information about the structure of molecules without tiresome evaluation methods, which are applied in other usual methods of structural analysis such as X-ray diffraction and neutron diffraction.With the absorption of infrared radiations, the molecules of the chemical substance vibrate at many rates of vibration, giving rise to close packed absorption bands.Thus such infrared spectra give the vibrational behavior of the crystal structure.It is interesting to note here that till date much less effort has been taken in studying the vibrational spectrum of the ferrites.In that, most of the work is concentrated on spinel ferrites [1][2][3].Infrared spectral analysis has been carried out for several ferrites by Waldron in 1955, who reported two absorption bands within the wave numbers 200-800 cm −1 , which are attributed to the tetrahedral and octahedral complexes of the spinel structure.Cr substituted Ni spinel ferrite is also studied [4] and authors attributed the existence of fine structure to the Jahn-Teller effect.Gd 3+ substituted Cd-Ca spinel ferrite [5] and Nd 3+ substituted Zn-Mg spinel ferrite [6] are also studied by using infrared absorption spectroscopy.Thus it is clear that as far as the hexaferrite is concerned, no previous infrared studies of these materials were found in the literature with the exception of the reflection spectrum of Ba 2 -Y type [7], Sr 2 -Y with divalent substitution [8], and absorption spectra of Ba-M type ferrite [9].
In the present paper, the results regarding the infrared absorption spectral analysis of four hexagonal ferrites with general chemical formula Sr 2 Zn 2 Al  Fe 12− O 22 ( = 2 to 10) are discussed.The efforts are taken to find out the effect of less massive, nonmagnetic aluminum substitution in the Sr-Y type hexagonal ferrite using spectroscopic studies.

Experimental
The system of polycrystalline samples Sr 2 Zn 2 Al  Fe 12− O 22 ( = 2 to 10) was prepared by standard ceramic technique.The analar grade reactants SrCO 3 , ZnO, Al 2 O 3 , and Fe 2 O 3 are mixed in the proper molar ratio and ground in the agate mortar for about 7 hours using acetone to achieve uniform grain size and homogeneity.With polyvinyl acetate as a binder and by applying a pressure of 10 tones psi for 5 to 7 minutes, pellets were prepared.These pellets are slowly heated in the furnace at 600 ∘ C for about 6 hours to remove Later on, the furnace is cooled at room temperature in natural way.The pellets are then finely powered and sieved through fine sieve.To confirm the completion of the solid state reaction, the compositions were subjected to characterization by X-ray diffraction.
For infrared studies, the technique used by Mazen et al. [11] is applied to the sample preparation.Nearly 2 mg of ferrite powered is mixed with powdered KBr in the ratio 1 : 100 by weighing to ensure uniform dispersion.The mixed powder is then pressed in a die of 12 mm diameter and thickness about 1 mm by applying a pressure of 10 tones per square inch for about 5 minutes.The spectra were recorded on Infrared Spectrophotometer model FTIR-8001 with resolution 4.0 cm −1 in the region of 400 cm −1 to 4000 cm −1 .

Results and Discussion
X-ray diffractograms of powdered compounds under investigation reveal the formation of single phase hexagonal Y type phase, showing well-defined reflections of allowed planes.The results regarding X-ray analysis are reported elsewhere [12].
The infrared spectra of the prepared Al 3+ substituted Sr-Y ferrite compounds are recorded in the range 400 cm −1 to 4600 cm −1 .The spectra are shown in Figure 1.In the spectra, no absorption bands were present above 1250 cm −1 except the band at 3460 cm −1 from the OH group of the moisture present (always) in the sample, which is the major disadvantage of the pressed pellet technique used in the infrared spectroscopy [13].The spectra of all the aluminium substituted Sr-Y type ferrite are used to locate the band positions, which are shown in Table 2.
The Y type hexagonal ferrite, namely, Ba 2 Zn 2 Fe 12 O 22 , has alternate stacking of the spinel S and hexagonal T block along hexagonal  axis as (TS)  | (TS)(TS)  (TS)  | (TS) where the prime means that the corresponding block is rotated 120 ∘ around the  axis.The space group is R3m [14].Here the cations occupy six different tetrahedral and octahedral sites present in the structure in different blocks S and T, respectively, as shown in Table 1.We tried to throw some light on the I. R. spectra of such compounds and showed that the vibration frequencies are below 1250 cm −1 [8].In the substituted Y type ferrite, the divalent Zn 2+ cations and two Fe 3+ ions are statistically distributed among the tetrahedral 6c positions of the spinel and hexagonal blocks and the remaining 10 Fe 3+ ions occupy 3a and 6c positions on the T block, 3b site in the S blocks while 18h on the T-S block boundaries [15].Now with the replacement of Fe 3+ ions by Al 3+ ions, aluminium is distributed among the 3b, 18h, and 6c octahedral positions in the whole complex structure [16].
As mentioned in Introduction, very few efforts are taken for spectroscopic measurements of hexaferrites, so we tried to interpret the infrared spectra of our prepared hexaferrite samples in the shade of spinel ferrites.This is owing to the following.(1) Lot of work has been done on spectroscopic study of spinel ferrites.
(2) Spinel infrared study has been started with rhombohedral primitive cell by Waldron in 1955 where Y type hexaferrite is rhombohedral itself.
(3) Y type hexagonal ferrite already consists of spinel block alternately stacked with T block along  axis.
(4) In both the ferrites, only tetrahedral and octahedral sites are present.
As far as the interpretation of the low frequency bands and the first element in the medium frequency is concerned, from the literature reported data, Waldron [1] and Hafner [17] attributed the low frequency band around 400 cm −1 to the stretching vibrations of octahedral complexes while the band around 600 cm −1 to the intrinsic vibrations of tetrahedral complexes in the spinel structure as these absorption ranges are the common features of all spinel ferrites.For these bands, Tarte and coworkers [3,18] observed that this attribution is true only when tetrahedral cation is of higher valence than octahedral cation.In the present work, Tarte's assumption feels true, so here the absorption at (462 ± 3) cm −1 is assigned to the stretching vibrations of the tetrahedral complexes in spinel blocks.The intensity of this band decreases slowly with decrease in Fe 3+ concentration, which indicates the decrease in the occupancy of the iron cations at tetrahedral sites in S blocks.Also the absorption band at (600 ± 12) cm −1 is attributed to the stretching vibrations of the octahedral group in spinel blocks.The broadness of this band decreases with decrease in magnetic substitutions, so this may be the effect of magnetic substitutions in the vibrations.Splitting is observed at higher frequency side near to (692 ± 2) cm −1 .The increase in significance of this absorption band may be attributed to the presence of aluminium vibration at octahedral sites in T-S block boundaries.
In a crystal, the cation-oxygen vibration frequencies depend on the mass of the cations, the cation oxygen bonding force, distance related to cation electronic structure, ionic radius and chemical nature of the neighboring cations, and also on the unit cell parameters [19].Thus as the aluminium has less atomic mass (atomic weight 26.982) compared to iron (atomic weight 55.847) and also less ionic radius of the Al 3+ cation, the cell volume as well as bond strength Al-O decreases with Al 3+ substitution.This weakening of the bond strength increases the vibrations of the octahedral aluminium cations at 759.2 cm −1 in both S and T blocks.This band is sharp and more significant.The increase in the intensity may be due to occupancy of Al 3+ cation on the octahedral sites in both S and T blocks of the whole complex Y structure.
Towards higher frequency side in the absorption spectra of Sr-Y type hexaferrite, it is observed that with the dilution of Fe 3+ by Al 3+ , the absorption bands gradually start disappearing.These higher frequency absorption bands are assigned to the vibrations of octahedral and tetrahedral substitutions of the metallic cation Fe 3+ with oxygen layer.By replacing trivalent Fe cation by less massive trivalent Al cation, the contribution of Fe-O vibrations decreases.Thus the disappearance of the higher frequency bands is attributed to the decrease of Fe cations with decrease in Fe-O vibrations in the whole complex structure.

Table 1 :
Lattice sites present in the Y type hexaferrite structure.10] and fired at 1200 ∘ C, sintering temperature for 120 hours.The samples then cooled at a rate of 20 ∘ C per hour up to 1100 ∘ C and then at a rate of 60 ∘ C per hour up to 500 ∘ C.

Table 2 :
Infrared absorption bands in the Sr 2 Zn 2 Al  Fe 12− O 22 system.