Results

Figure 8.1: Room temperature Mössbauer spectra for all toner samples.

Figure 8.2: 77K Mössbauer spectra for all toner samples.

All of the room temperature spectra showed that the iron oxide content was not pure Fe$_{3}$O$_{4}$. In pure Fe$_{3}$O$_{4}$ there is a distinctive ratio between the intensity of the two spectral components. This ratio, $R$, should equal 1:1.88 (A:B).^8.1 In all cases for the toner samples $R$ was greater than this. This indicated another iron oxide being present and comparison with the example spectra in Reference dacosta_95 shows this to be $\gamma$-Fe$_{2}$O$_{3}$ (maghemite). In this case we can use the areas of the two components to calculate the Fe$^{2+}$/Fe$^{3+}$ ratio and the Fe$_3$O$_4$/Fe$_2$O$_3$ ratio.

Taking $y$ as the the proportion of Fe$_{2}$O$_{3}$ to the whole (between 0, pure Fe$_3$O$_4$, and 1, pure Fe$_2$O$_3$)

$$y = \frac{N_2}{N_1 +N_2}$$ (8.1)

where $N_1$ is the number of Fe atoms in the Fe$_{3}$O$_{4}$ compound and $N_2$ is the number of Fe atoms in the Fe$_{2}$O$_{3}$ compound. The ratio of the areas of the two components, $R$, can be used to calculate $y$ from the following relation[50]:

$$R=\frac{1+0.94\left( \frac{5y}{3}\right) }{0.94(2-2y)}$$ (8.2)

where the factor of 0.94 arises from the Fe ions on the B sites having a slightly smaller recoil-free fraction.[51] Once $y$ is obtained the ratio of Fe$^{2+}$/Fe$^{3+}$ is given by:

$$\frac{Fe^{2+}}{Fe^{3+}}=\frac{1-y}{2+0.67y}$$ (8.3)

Laporte supplied five samples for study: two of their own (L1 and L2) and three competiting brands (C1, C2 and C3). No information was given concerning the composition of any of the samples. The room temperature Mössbauer spectra obtained from these samples are shown in Figure 8.1 and the fitting parameters in Table 8.1.

Table 8.1: Final room temperature fit parameters for all toner powder samples.

A-site B-site

Sample IS QS Field Area IS QS Field Area

$$\% \%$$ (mms) (mms) (kG) (mms) (mms) (kG)

$$+0.35 -0.02 494.2 60 +0.72 +0.02 459.4 40$$ L1

$$+0.28 -0.02 489.7 46 +0.66 +0.02 457.5 54$$ L2

$$+0.29 -0.01 489.0 50 +0.65 -0.01 456.6 50$$ C1

$$+0.31 -0.02 491.3 66 +0.64 +0.01 454.1 34$$ C2

$$+0.30 -0.02 492.7 63 +0.67 +0.01 456.6 37$$ C3

Using Equations 8.2 and 8.3 the ratio of Fe$^{2+}$ to Fe$^{3+}$ can be calculated. These values are shown in Table 8.2. Rearranging Equation 8.1 to obtain the ratio of atoms, $N_1$:$N_2$, gives the relation

$$\frac{N_1}{N_2} = \frac{1-y}{y}$$ (8.4)

the results of which are also shown in Table 8.2. The molecular ratio of Fe$_3$O$_4$ to $\gamma$-Fe$_2$O$_3$ can be obtained by multiplying the $N_1$:$N_2$ ratio by a factor of $0.67$, to take account of the fact that a molecule of $\gamma$-Fe$_2$O$_3$ contains 2 Fe atoms while a molecule of Fe$_3$O$_4$ contains 3. The ratio of magnetite to maghemite was thus calculated to an accuracy between $1\%$ and $17\%$ from these results, where higher concentrations of magnetite have a larger error.

Table 8.2: Calculated Fe$^{2+}$:Fe$^{3+}$ and $N_1$:$N_2$ ratios for the room temperature spectra.

$$R y ^{2+} ^{3+} N_1 N_2$$ Sample

$$1.56\pm.05 0.43\pm.01 0.25\pm.01 1.33\pm.03$$ L1

$$0.84\pm.05 0.18\pm.03 0.39\pm.03 4.56\pm.75$$ L2

$$1.01\pm.05 0.26\pm.02 0.34\pm.01 2.85\pm.20$$ C1

$$1.92\pm.05 0.50\pm.01 0.21\pm.01 1.00\pm.02$$ C2

$$1.71\pm.05 0.46\pm.01 0.23\pm.01 1.17\pm.02$$ C3

$$_3 _4$$ 0.53 0 0.50 NA

Mössbauer spectra were recorded from the same samples at $77\ensuremath{\unskip\,\mathrm{K}}$. These spectra are shown in Figure 8.2.

At $77\ensuremath{\unskip\,\mathrm{K}}$ the samples are below the Verwey transition temperature, $T_{v}$, of $119\ensuremath{\unskip\,\mathrm{K}}$. The single B site component has now become two separate components for $2+$ and $3+$ ions. The hyperfine fields for Fe$^{3+}$ A sites and Fe$^{3+}$ B sites in magnetite and the Fe$^{3+}$ A and B sites all lie close to each other, meaning they overlap in the spectrum. This, combined with the broad linewidth of the Fe$^{2+}$ B site makes these data much less reliable for accurate assessment of area ratios than the room temperature spectra. Thus the final results were based on the room temperature fits only.