57Fe Mössbauer spectra from fluorinated phases of Fe0.50M0.50(M = Co,Mg)Sb2O4

Fluorinated phases formed by reaction of Fe0.5Co0.5Sb2O4 and Fe0.5Mg0.5Sb2O4 with gaseous fluorine have been examined by 57 Fe Mössbauer spectroscopy between 298 and 5 K. The degree of oxidation of Fe2+to Fe3 +has been used to quantify the amount of fluorine incorporated within the channels of the schafarzikite-related structure and enable the evaluation of the compositions as Fe0.5Co0.5Sb2O4F0.41 and Fe0.5Mg0.5Sb2O4F0.31. The multiplicity of components observed in the spectra recorded in the paramagnetic regime can be related to the number of near neighbour fluoride ions which lie in the channels at the same value of the crystal z- coordinate as the iron ions. Comparison of the magnetically ordered spectra recorded at lower temperatures from Co0.5Fe0.5Sb2O4F0.41 with those recorded previously from FeSb2O4 indicates that the insertion of fluoride ions into the channels of the structure does not affect the angle between the EFG and magnetic hyperfine field.


Introduction
The insertion of fluorine into two-dimensional structures induces structural changes and electronic properties resulting from the oxidation of metal ions in the host material. The conversion of semiconducting La 2 CuO 4 to superconducting La 2 CuO 4 F x [1] illustrates this type of reaction and other examples are documented in a recent review [2]. As a part of our ongoing interest in the fluorination of inorganic structures we have recently reported on the accommodation of fluorine within the narrow one-dimensional channels of phases related to the mineral schafarzikite of composition FeSb 2 O 4 [3].
The compound FeSb 2 O 4 is isostructural with the tetragonal form of Pb 3 O 4 [4,5] (Fig. 1) and consists of rutile-related chains of FeO 6 octahedra along the c-axis linked by trigonal pyramidal Sb 3+ cations which, being bound to three oxygen anions, possess a lone pair of electrons which can be regarded as a fourth ligand. The Fe-Fe distance within the chains (2.96 Å) is shorter than the nearest Fe-Fe distance within the layers (6.07 Å) and is consistent with the one-dimensional character of FeSb 2 O 4 . The material undergoes an antiferromagnetic transition around 45 K [6,7]. The 57 Fe Mössbauer spectrum of FeSb 2 O 4 at ca. 4.2 K is unusual, being the result of combined magnetic hyperfine and electric quadrupole interactions of comparable strength [8][9][10] and, together with data recorded above T N [11], has been used to determine the order of the t 2g orbital energy levels. The literature also documents some structurally related compounds of composition MSb 2 O 4 (M = Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Mg 2+) [12][13][14][15][16][17].
We have also prepared new phases involving substitution on the M site. The magnetic properties deduced from neutron diffraction, magnetic susceptibility and Mössbauer spectroscopy measurements in the series of composition Fe 1 − x Co x Sb 2 O 4 suggested the existence of some short range correlations within the chains [18,19] whilst in materials of formulation Fe 1 Fig. 1 The structure of FeSb 2 O 4 : FeO 6 octahedra are shaded with Fe 2+ ions located within the octahedra; O 2− ions are shown as black spheres and Sb 3+ ions are shown as white spheres − x Mg x Sb 2 O 4 , the decrease in magnetic ordering temperature with increasing concentrations of magnesium was associated with non-magnetic Mg 2+ ions weakening the magnetic interactions between Fe 2+ ions [19,20]. We have also demonstrated the capacity of these materials to accommodate additional oxygen within their structures [21].
Our previous work [3] reported on the reaction of Mg 0.50 Fe 0.50 Sb 2 O 4 and Co 0.50 Fe 0.50 Sb 2 O 4 with fluorine gas at low temperatures to give topotactic insertion of fluorine into the channels which are an inherent feature of the structure. Neutron powder diffraction and solid state NMR studies showed that the interstitial fluoride ions were bound to antimony within the channel walls to form Sb − F − Sb bridges. 57 Fe Mössbauer spectra recorded at 300 K showed that oxidation of Fe 2+ to Fe 3+ was primarily responsible for balancing the increased negative charge associated with the presence of the fluoride ions within the channels.
In this work we report on the 57 Fe Mössbauer spectra recorded at low temperatures from new samples of the fluorinated phases of Mg 0.50 Fe 0.50 Sb 2 O 4 and Co 0.50 Fe 0.50 Sb 2 O 4 with the schafarzikite structure and their interpretation in terms of the number of near F − ions which lie in the channels at the same value of the crystal z-coordinate as the iron ions and the magnetic properties of these materials.

Experimental
Materials of composition Mg 0.50 Fe 0.50 Sb 2 O 4 and Co 0.50 Fe 0.50 Sb 2 O 4 were prepared by heating appropriate quantities of the dried metal oxides in evacuated sealed quartz tubes as previously described [18,20]. Fluorinated samples were obtained by heating the parent oxides in flowing fluorine gas (10% in nitrogen) at 230C and purging with gaseous nitrogen or argon [3]. The structural characterisation and analysis of the samples of composition Fe 0.5 Co 0.5 Sb 2 O 4 F 0.49 and Fe 0.5 Mg 0.5 Sb 2 O 4 F 0.31 has been reported previously [3]. In this work new samples were examined. 57 Fe Mössbauer spectra were recorded in a helium gas-flow cryostat in constant acceleration mode using a ca. 25 mCi 57 Co source. All spectra were computer fitted and all chemical isomer shift data are quoted relative to metallic iron at room temperature.

Results and discussion
The 57 Fe Mössbauer spectra recorded at 298 K, 80 K, 45 K, 20 K and 5 K are shown in Fig. 2 and the fitting parameters are listed in Table 1. The spectra are different from those reported previously [3] which is not unexpected given that we have already noted [3] that differences in fluorine content probably relates to differences in particle size of starting materials and the fluorination conditions not being identical.

Identification of components in spectra recorded at 298 K and 80 K
The spectrum recorded at 298 K showed a component with δ = 1.06 mms −1 which can readily be assigned to Fe 2+ and components with δ = 0.41 mms −1 and δ = 0.37 mms −1 characteristic of Fe 3+ (the partial superposition of the two components precludes their observation in Fig. 2  The fitting parameters listed in Table 1 show that in the spectrum recorded at 298 K the two Fe 3+ components have slightly different values of quadrupole splitting. In the spectrum recorded at 80 K there are two Fe 2+ components with slightly different values of quadrupole splitting. We have investigated whether this behaviour could be linked to different near neighbour fluoride configurations around the iron ions. In the ab -planes containing the iron-and fluoride-ions the structure [3] shows that there are four possible sites for an inserted fluoride ion around each iron ion. A simulation incorporating random occupation by iron and cobalt of the cation sites and random occupation of surrounding fluoride sites (subject to their relative abundance, x) indicated that the relative percentages of an iron ion having 0,1,2,3 near neighbour fluoride ions was 9%, 42%, 42% and 7% respectively. This provides some indication that the observed difference in splitting in the doublets observed in spectra recorded in the paramagnetic regime corresponds to the main probabilities of having 1 or 2 fluoride neighbours.

Spectra recorded at 45 K, 20 K and 5 K
The spectra recorded below 80 K all show the superposition of magnetic sextet components. This is consistent with a magnetic ordering temperature T N = 75 K recorded from magnetisation results recorded previously from the compound Fe 0.5 Co 0.5 Sb 2 O 4 F 0.49 [3]. Despite their complexity each spectrum is clearly dominated by a sextet of large spectral area with isomer shift characteristic of Fe 3+ . We associate these sextet components as the low temperature versions of the large-area Fe 3+ doublet components observed in the spectra recorded at 298 K and 80 K. In these sextet spectra the quadrupole shift Δ is evaluated from 2Δ = the energy difference between the splitting of lines 5 and 6 and lines 1 and 2 of the sextet. The angle Θ between the principal axis of the electric field gradient (EFG) and the direction of the magnetic hyperfine field B hf can be evaluated from the values of Δ and the quadrupole splitting e 2 qQ/2 observed in the 80 K spectrum using the relation Δ ¼ e 2 qQ=2: 1 =2 : 3cos 2 Θ-1 À Á : The values of Θ from the spectra at 45 K, 20 K and 5 K are evaluated as 69°, 67°and 64°r espectively. These values are effectively identical to those measured in the parent material FeSb 2 O 4 [10] and although there are differencesin FeSb 2 O 4 the iron is Fe 2+ and the magnetic structure is A-type instead of the present C-typeit seems that the insertion of fluoride ions into the channels of the structure does not affect the angle between EFG and magnetic hyperfine field. Given the complexity of the Fe 2+ spectral components we have not analysed these to a degree which justifies inclusion of parameters in Table 1.

Mg 0.5 Fe 0.5 Sb 2 O 4 F x
The 57 Fe Mössbauer spectra recorded at 298 K, 80 K, 45 K, 20 K and 5 K are shown in Fig. 3 and the fitting parameters are listed in Table 2.

Identification of components
The spectra recorded at 80 K and 45 K are well fitted with a single Fe 3+ quadrupole split component and a single Fe 2+ quadrupole split component.
The spectrum recorded at 298 K spectrum shows-in addition to the Fe 3+ and Fe 2+ quadrupole split components-a component with isomer shift δ = 0.71 mms −1 which, as in Co 0.5 Fe 0.5 Sb 2 O 4 F 0.41 , can be associated with electron mobility between Fe 2+ and Fe 3+ ions. At lower temperatures this component is not observed as electrons become localised on the Fe 2+ and Fe 3+ ions.
A simulation similar to that described in 3.1.2 above was performedwith x = 0.31. This indicated that the percentages of iron ions having 0,1 and 2 near neighbour fluoride ions is 25%, 50%, 25%. This distribution appears not to cause observable differences in quadrupole splitting for the Fe 3+ components in the 298 K, 80 K and 45 K spectra or for the Fe 2+ components in the 80 K and 45 K spectra.

Spectra at 20 K and 5 K
The spectra at 20 K and 5 K both show components representing magnetically ordered and nonordered states of Fe 2+ and Fe 3+ ions. However, these components are not sufficiently well defined to enable the evaluation of the angle theta or further analysis. In particular the complexity of the Fe 2+ spectral components precluded confident evaluation of parameters for inclusion in Table 2.

Conclusion
The 57 Fe Mössbauer spectra recorded from fluorinated Mg 0.50 Fe 0.50 Sb 2 O 4 and Co 0.50 Fe 0.50 Sb 2 O 4 show that a major effect of fluoride ion insertion involves the oxidation of Fe 2+ ions to Fe 3+ ions in the structure. The 57 Fe Mössbauer spectra enable the degree of fluorination to be evaluated and the compositions to be derived. We suggest that that the multiplicity of components in the spectra recorded in the paramagnetic regime may be related to the number of near fluoride ions which lie in the channels at the same value of the crystal zcoordinate as the iron ions. Comparison of the magnetically ordered spectra recorded at lower temperatures from Co 0.5 Fe 0.5 Sb 2 O 4 F 0.41 with those recorded previously from FeSb 2 O 4 indicates that the insertion of fluoride into the channels of the structure does not affect the angle between EFG and magnetic hyperfine field.