Characterization of Tin Substituted Fe2TiO5 Based Materials

Iron titanate is a pseudobrookite which exhibits many interesting properties such as spin glass behaviour, thermal microcracking, high resistivity, etc. A cluster approach may be used for a description of spin glass behaviour of the pseudobrookites [1]. D. A. Kharmov, et.al [1] have studied the spin glass transition in the Fe2 Ti1-x Snx O5 pseudobrookite and found that Sn4+ ions appear to occupy only M1/ 4c octahedral sites and spin glass transition temperature Tg depends on tin concentration monotonically. According to the XRD analysis and Sn-Mossbauer Spectral (MS) data the maximum solubility of Sn4+ ions in the pseudobrookite structure at 1250oC is x = 0.22 [1], where the unit cell volume increases linearly with the increasing tin concentration. The XRD analysis of the limit of solid solution Fe2 Ti1-x Snx O5 (0

2. RESULTS AND DISCUSSION:

The structural properties of the samples namely Fe2TiO5 using rutile TiO2 [FTR], Fe2TiO5 using anatase TiO2 [FTA], Fe2 Ti0.75 Sn0.25 O5 using rutile TiO2 [FTSR] and Fe2 Ti0.75 Sn0.25 O5 using anatase TiO2 [FTSA] all synthesized at 1250oC are reported here. The corresponding XRD data is reported in Figures 1-4 and Tables1-4 respectively. The data is indexed in a single phase orthorhombic structure having the space group Bbmm of the pseudobrookite.

Space group: Bbmm, Lattice parameters;a=9.7780 Å, b=9.9608 Å, c=3.7262 Å.

Space group: Bbmm, Lattice parameters; a =9.795 Å, b=9.992 Å, c=3.732 Å.

Space group: Bbmm, Lattice parameters; a =9.856 Å, b=10.016 Å, c=3.747 Å. [FTSR] and [FTSA] are pseudobrokites. The values of unit cell volume V, XRD density, practical density, Debye particle size, porosity and inhomogeneity are included in the Table - 5.

It is interesting to note that substitution of Ti 4+ by Sn4+ is responsible to increase the unit cell volume and X-Ray or theoretical density. This may be attributed to the larger ionic radius and larger atomic mass of Sn4+

[14].

On the other hand, the comparison of [FTR] with [FTA] shows that the use of anatase in the reaction causes the decrease in practical density and the increase in the porosity, Debye particle size and the inhomogeneity. This may be attributed to the vertex –sharing network of anatase which is opposite to the edge –sharing network of the pseudobrookite [18]. However, the comparison of [FTR] with [FTSR] and [FTA] with [FTSA] shows that tin has exactly opposite effect on the anatase as the practical density, porosity, inhomogeneity and the Debye particle size are concern. The tendency of tin to increase the practical density or to decrease the porosity and the Debye particle size comes from its abilities to (i) increase the speed of rutilation and to inhibit the grain growth [6], (ii) to occupy both interstitial and substitutional sites[13] and (iii) to be present both on the surface and within the structure [9]. The effect of tin on the porosity and the Debye particle size is however less dominant.

It is thought worthwhile to compare these observations with the cation distributions of these four samples reported in Table - 6 obtain from new empirical model [19] . It is interesting to note that Sn4+

occupies only M1 sites. This agrees well with the earlier work [1, 2]. Moreover, the relative percentage intensities of (230) planes which passes through the M1 sites (Figure - 5) increase from 87 in [FTR] to 105 in [FTSR] and 44 in [FTA] to 123 in [FTSA] (Tables 1- Secondly Sn4+ displaces Fe3+ to M2 sites. Knowing the preference of Fe3+ for lower tetrahedral symmetry [18], the symmetry of the M2 site is lowered by Sn4+. Also, the octahedral symmetry that Sn4+ prefers raises the symmetry of the M1 site. Overall Sn4+ causes net increase in the asymmetry of the pseudobrookite and therefore the increase in volume (Table - 6).

Also, more Fe3+ shifts to the M2 site when anatase is used in place of rutile. Thus, [FTA] is more asymmetric than [FTR]. This is likely to decrease the practical density, increase the porosity, the Debye particle size and the inhomogeneity. This trend reverses when one compares the cation distribution of [FTSR] with that of [FTSA] and [FTSR] is now more asymmetric. This is because Sn4+ prefers the M1 site [6, 9,13] and attracts rutile Ti4+ to the M1 site as both prefer the edge sharing. These effects are summarized by defining the order parameter λ’ [20] which shows that the order parameter λ’ is decreased by use of anatase and increased by substitution of Sn4+.

The IR spectra of the samples recorded on the „PERKIN-ELMER‟ 683 Spectro Photometer at room temperature are as shown in the Figure -6 and the possible octahedral assignment [4] of two major bands of frequencies 1 and 2 corresponding to the octahedral sites M1 and M2 [4] respectively is reproduced in the Table-7. lower frequency side and 2 to shift to higher frequency side. Moreover, the sharpness and the intensity difference increase due to the anatase. It is observed that both the use of anatase and the substitution of Sn4+ decreases the frequency difference (1- 2) between these bands. The shift of 1 to the high frequency side appears to be due to the larger charge of Sn4+on M1 site. On the other hand, the high

frequency shift of 2 causes from the increase in Fe3+

It is interesting to note that substitution of Ti 4+ by Sn4+ is responsible to increase the unit cell volume and X-Ray or theoretical density. On the other hand, the use of anatase in the reaction causes the decrease in practical density and the increase in the porosity, Debye particle size and the inhomogeneity. This may be attributed to the vertex –sharing network of anatase which is opposite to the edge –sharing network of the pseudobrookite. However, tin has exactly opposite effect on the anatase as the practical density, porosity, inhomogeneity and the Debye particle size are concern.

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