Perovskite-Related Oxide Fluorides : The Use of Mössbauer Spectroscopy in the Investigation of Magnetic Properties

We review here some of our recent work on the synthesis and characterisation of new perovskite-related oxide fluorides. We demonstrate the use of low temperature fluorination methods for the preparation of new phases with high fluorine contents. We also show how fluorine can be accommodated in different sites according to the structural details of the initial oxide and the fluorine content. Importantly, we describe how Mössbauer spectroscopy is a powerful technique for monitoring changes in cation oxidation state as a result of fluorination and for examining the complex magnetic interactions which result from the accommodation of fluorine within the structures and how these can be related to structural properties and changes to the superexchange pathways.


INTRODUCTION
−3] We have, in recent years, synthesised new perovskite-related oxide fluorides and have found Mössbauer spectroscopy to be an important technique for investigating magnetic interactions in these materials.In this Feature Article, written to commemorate the 70 th birthday of Dr. Svetozar Musić and his contribution to inorganic materials chemistry and Mössbauer spectroscopy in Croatia, we draw together our work to illustrate the power of Mössbauer spectroscopy in the investigation of new inorganic oxide fluorides.We refer the reader to standard texts [4,5] for information concerning the principles of Mössbauer spectroscopy and focus attention in this article on the syntheses of these phases and their magnetic properties.

Synthesis of Inorganic Oxide Fluorides
The fluorination of inorganic oxides is best achieved by low temperature fluorination routes because the number of oxide fluorides that can be prepared by the standard high temperature solid state routes is limited by the high stability of the simple fluoride starting materials.In terms of low temperature fluorination, a range of fluorinating agents have been used including gaseous fluorine, NH4F, MF2, (M = Cu, Ni, Zn) and XeF2.More recently a new method has been identified which entails heating the precursor oxide with the polymer poly(vinylidene fluoride). [6]This method was shown to produce high quality samples without significant metal fluoride impurities which are often formed by other low temperature fluorination methods.

T SrFeO2F
Our initial work [7,8] in synthesising and characterising fluorinated cubic perovskite-related oxides used poly(vinylidene fluoride) for the fluorination of the oxygen deficient phase SrFeO3-d which, because of oxygen deficiency, distorts to an orthorhombic unit cell.The AMO3 unit cell, depicted in Figure 1 (n = infinity), contains strontium (green spheres) coordinated by twelve oxygen atoms whilst iron is located in an octahedral array of oxygen (red spheres).
Fluorination of SrFeO3-d was achieved by mixing with poly(vinylidene fluoride) and heating the mixture at 400 °C for 24 hours in flowing nitrogen.The initial characterisation 7 by X-ray powder diffraction showed that fluorination resulted in a material with larger cell parameters consistent with the partial replacement of oxygen by fluorine (while X-ray diffraction suggested cubic symmetry, subsequent neutron diffraction studies showed that the cell symmetry was actually orthorhombic). 57Fe Mössbauer spectroscopy demonstrated [7] that fluorination induced a reduction of the oxidation state of iron from Fe 4+ /Fe 3+ in SrFeO3-d to Fe 3+ in the compound SrFeO2F.
Subsequent investigations [8] of SrFeO2F by magnetometry in the temperature range 10−400 K showed small aligned moments indicating the absence of ferro-or ferrimagnetism and the separation of field cooled and zero field cooled magnetisations at temperatures around 300 K.The 57 Fe Mössbauer spectra recorded between 80 and 700 K are shown in Figure 2. The spectra recorded between 285 and 650 K showed clear splitting of all the lines contributing to the sextet pattern and showed increasing line-widths and decreasing magnitudes of magnetic hyperfine field until, at 700 K, the magnetic field collapsed to a quadrupole split absorption indicative of the paramagnetic state.The variation of the average magnetic hyperfine field with increasing temperature enabled a magnetic ordering temperature of ca.685 K to be deduced for SrFeO2F.The spectra recorded in the temperature range 285−700 K were interpreted in terms of a model in which each Fe 3+ ion has four O 2− and two F − ions in the nearest neighbour anion shell corresponding to charge neutrality within the perovskite-related cell.In this case there are two types of arrangements for the fluoride ions, that with the two F − ions at opposite corners of the octahedron (trans arrangement) and that where F − is located at adjacent corners (cis arrangement).The spectral areas of the two components in the spectra recorded between 285 and 650 K were interpreted in terms of a model involving cis and trans arrangements of two F − ions per perovskite-related unit cell and a single antiferromagnetic axis.
The 57 Fe Mössbauer spectra recorded from SrFeO2F between 10 and 317 K are collected in Figure 3 and are different in that they show no splitting of the spectral lines but significant broadening of each line contributing to the sextet pattern.The difference between these spectra recorded below ca.300 K and those above ca.300 K is significant.Several different models were initially used to fit the spectra recorded between 10 and 310 K and at least three sextet components found to be required to achieve a reasonable fit and were interpreted in terms of a model in which the Fe 3+ moments were randomly orientated with respect to any axis of maximum electric field gradient (EFG).For spectra between 200 and 317 K an additional non-magnetic doublet component arising from an impurity phase was also required.
The Mössbauer results, taken together with the magnetisation data, were interpreted in terms of SrFeO2F ordering into an antiferromagnetic state at TN = ca.685 K which persists as the temperature is decreased to ca. 300 K. Below this temperature a state characterised by random spin orientation occurs, which shows different ground states in field cooled and zero field cooled conditions.The randomness was associated with the substitution by fluorine on the oxygen anion site causing different superexchange Fe 3+ -O 2− -Fe 3+ and Fe 3+ -F − -Fe 3+ pathways.The significance of the work 7,8 is vested in it showing that the magnetic structure of SrFeO2F is significantly more complex than that of the antiferromagnetic structurally related PbFeO2F [9] in which the trans configuration of fluorine in the distribution of anions around Fe 3+ is favoured over the cis arrangement.The differences may be related to the lower temperature synthesis route to produce SrFeO2F in contrast to the high pressure, high temperature synthesis route needed to produce PbFeO2F.

SrFe1-xSnxO3-yFy
The fluorination of orthorhombic SrFeO3−d was followed by attempts to fluorinate the SrFeO3-d phase in which iron had been partially replaced by other metals.The first of these studies [10] involved tin-doped materials of composition SrFe0.69Sn0.31O2.94 and SrFe0.46Sn0.54O2.88. 57Fe Mössbauer spectroscopy indicated that iron in these phases is present as Fe 5+ and Fe 3+ .The disproportionation of Fe 4+ in SrFeO3-d to Fe 5+ and Fe 3+ upon substitution of Sn 4+ (as demonstrated by EXAFS) on the iron site was associated with the reduction of lattice strain.
The two materials were fluorinated by reaction with poly(vinylidene fluoride) at 400 °C in flowing nitrogen to give products of composition SrFe0.69Sn0.31O2.31F0.69 and SrFe0.46Sn0.54O2.54F0.46 with larger unit cells than the parent oxides.The 57 Fe Mössbauer spectra recorded between 10 and 300 K are shown in Figure 4.The results showed that all the Fe 5+ in the pure oxide is reduced to Fe 3+ in the fluorinated samples (and was consistent with the increased iron to oxygen or fluorine distances as revealed by EXAFS).The results were interpreted in terms of fluorination involving both replacement of some oxide ions by fluoride ions and the filling of vacant anion sites by fluoride ions.Furthermore, all the spectra except that recorded from SrFe0.46Sn0.54O2.54F0.46 at 300 K showed the coexistence of magnetic and non-magnetic components and the proportion of the sextet component was found to decrease with increasing tin content and with increasing temperature.
The coexistence of the sextet and doublet components in the 57 Fe Mössbauer spectra was interpreted in terms of the presence of clusters or networks of coupled Fe 3+ ions with the superexchange coupling being broken by Sn 4+ ions which consequently limit the size of the networks.Hence networks larger than a critical size at a given temperature give rise to the magnetic sextet components whilst those less than critical size give rise to the non-magnetic doublet component.The increase in tin content resulted in increased disruption of the magnetically coupled networks

La1-xSrxFe1-yCoyO3-d/F
Fluorinated derivatives of phases in which cation substitution had been performed on both the A and M sites of the perovskite-related AMO3 structure have also been examined by Mössbauer spectroscopy. [11]Fluorination was achieved by reaction of the pure oxides with poly(vinylidene fluoride) at 400 ○ C in flowing nitrogen.X-ray absorption and Mössbauer spectroscopy showed a reduction in the oxidation state of iron and/or cobalt upon fluorination.The fluorinated materials gave complex magnetically split Mössbauer spectra reflecting the interactions between Fe 3+ ions which are not possible in the pure oxides which contain Fe 4+ and Fe 3+ .

BaFeO2F
A cubic phase of composition BaFeO2F was made by low temperature fluorination of BaFeO3-x at 400 ○ C in flowing nitrogen using poly(vinylidene) fluoride. [12,13]The magnetic properties were found to be substantially different from the related compound SrFeO2F (see above).Mössbauer spectra collected between 400 and 650 K showed decreasing magnitudes of magnetic hyperfine field with increasing temperature and from the variation of the average magnetic hyperfine field with increasing temperature (Figure 5) a magnetic ordering temperature of ca.645 K was deduced, some 40 K lower than the ordering temperature for SrFeO2F.The Mössbauer spectra recorded between 10 and 300 K were analysed in terms of the random occupation by fluorine of the anion sites assuming a charge neutral perovskite-related unit cell.In this material neutron diffraction indicated a large displacement of the iron off-site and established the Fe 3+ moments to be ordered in a G-type antiferromagnetic structure.Magnetisation measurements indicated the presence of a weak ferromagnetic moment that was assigned to the canting of the antiferromagnetic structure.

Ba2SnO2.5F3 • xH2O
Ruddlesden-Popper phases of general formula An+1MnO3n+1 may be viewed as intergrowths between perovskite and rock salt units.The situation where n = 1 is also known as the K2NiF4-type structure and is depicted in Figure 1.These n = 1 phases have also attracted attention for fluorination and an example, involving 119 Sn Mössbauer spectroscopy, reported [14] on the fluorination of Ba2SnO4 through a low temperature reaction at 240 °C with ZnF2 in flowing nitrogen.X-ray and neutron powder diffraction showed fluorination to result in a significant enlargement by ca. 3 Å along the c-axis of the unit cell precursor oxide.A structural model based on the perovskite-related K2NiF4-type structure of the oxide was proposed in which there was direct replacement of oxygen in octahedral SnO6 units by fluorine as well as the presence of F − at interstitial sites between BaO rock salt layers.Atomistic computer modelling indicated that apical fluorine substitution is favoured.The structural model was supported by the results of 19 F and 119 Sn MAS NMR spectroscopy as well as tin K-and barium K-edge EXAFS.Thermal analysis revealed the presence of water in the synthesised material and this was assigned to interstitial sites.The 119 Sn Mössbauer spectrum recorded from Ba2SnO4 at 77 K (Figure 6) was composed of a single absorption with chemical isomer shift of 0.22(1) mms −1 and the spectrum recorded from the fluorinated derivative was similar (Figure 6) but with a smaller chemical isomer shift of 0.12(1) mms −1 indicative of the electron withdrawing effect of electronegative fluorine on the electron density around Sn 4+ .The result was endorsed by tin K-edge XANES.

Sr3Fe2O5+xF2-x
Sr3Fe2O7-x is a n = 2 Ruddlesden-Popper phase (see Figure 1) in which complete filling of the ideal oxygen anion sites within the perovskite and rock salt layers would lead to a total oxygen content of 7.0.An interesting additional aspect of these phases is that they also have the possibility of incorporating anions into interstitial fluorite-type positions within the rock salt layers.Sr3Fe2O7-x might nominally be expected to contain mixed Fe 3+ /Fe 4+ species.However, 57 Fe Mössbauer spectroscopy has suggested [15−20] the disproportionation of Fe 4+ into Fe 3+ and Fe 5+ .Fluorination of this system has also been examined [21] by reducing Sr3Fe2O7-x to Sr3Fe2O6 and treating with gaseous fluorine to give Sr3Fe2O6F0.87 which also contained Fe 3+ and Fe 5+ .Subsequently [22] a higher degree of fluorine content was achieved in a material of composition Sr3Fe2O5.44F1.56 in which all the normal anion sites were filled by using polytetrafluoroethylene (PTFE) as the fluorination agent.
We have recently [23] fluorinated Sr3Fe2O7-x by mixing with poly(vinylidene fluoride) (PVDF) in 1:1, 1:2 and 1:2.8 molar ratios and heating at 375 °C.X-ray powder diffraction showed successful fluorination with the unit cell volumes increasing with increasing levels of PVDF.Rietveld refinement of the powder X-ray diffraction data from the 1:1 sample indicated complete filling of the normal anion sites and a composition Sr3Fe2O5+xF2-x. 57Fe Mössbauer spectroscopy (Figure 7) showed this phase to contain both Fe 3+ and Fe 4+ and a composition Sr3Fe2O5.28F1.72 was deduced.The phase was considered to be similar to that reported previously [22] by fluorination using PTFE.Our work [23] showed that by using PVDF as the fluorinating agent higher fluorine contents could be achieved.For the 1:2 composition Rietveld refinement of the X-ray powder diffraction data showed complete filling of the normal anion sites and partial filling of the interstitial sites. 57Fe Mössbauer spectroscopy (Figure 8) indicated that all the iron was present as Fe 3+ and a composition Sr3Fe2O4F4 with half the interstitial sites filled was determined.For the sample made with the highest amount of PVDF 57 Fe Mössbauer spectroscopy (Figure 9) showed only the presence of Fe 3+ and an approximate composition  of Sr3Fe2O3F6 corresponding to complete filling of both the normal anion sites and interstitial sites was proposed.The Mössbauer data in the temperature ranges 10−300 K (Figures 7, 8 and 9) showed complex magne c interactions in each of the three phases.For Sr3Fe2O5.28F1.72 (Figure 7) the paramagnetic component accounting for ca.30 % of the spectral area at 300 K decreased in intensity with decreasing temperature until 56 K when all the Fe 3+ ions were magnetically ordered.The quadrupole interactions of the two Fe 3+ sextets were quite large and indicative of a distortion of the anion or electronic array around Fe 3+ ions and reflective of the presence of both oxygen and more electronegative fluorine.Assuming that fluorine is located on the apical sites (as indicated by refinement of the X-ray diffraction data and bond valence calculations) the magnetic Fe 3+ -O 2− -Fe 3+ superexchange interactions within planes were associated with the sextet components in the Mössbauer spectra.For Sr3Fe2O4F4 (Figure 8) the main magnetic component was characterised by relatively large quadrupole interactions indicative of a distorted electronic or anionic environment around Fe 3+ and reflective of the presence of more electronegative fluorine in some of the anion sites and local distortions around the Fe 3+ ions.For Sr3Fe2O3F6 (Figure 9) the Mössbauer spectra showed most of the Fe 3+ to be paramagnetic at 300 K. Some of the sextet components were associated with the presence of impurity phases.The behaviour of the Mössbauer spectra with decreasing temperature was characterised by the coexistence of the paramagnetic doublet and different sextet

La2BaFe2O5F4
We have also recently reported [24] the fluorination of the n = 2 Ruddlesden-Popper phase (see Figure 1) La2BaFe2O7 in a flowing mixture of gaseous 10 % fluorine/90 % nitrogen at ca. 300 °C to form La2BaFe2O5F4. Structural characterisation from neutron powder diffraction data showed the oxide fluoride to contain two fluoride ions in interstitial sites within the rock salt regions and two fluoride ions that had substituted for oxide ions in apical sites within the rock salt layers.The material antiferromagnetically ordered at temperatures below ca.500 K.
The 57 Fe Mössbauer spectra recorded from La2BaFe2O5F4 are shown in Figure 10.The major part (ca.97 %) of the spectrum recorded at 298 K differed from that recorded [25] from La2BaFe2O7 at 298 K in the appearance of a significant (ca. 25 %) paramagnetic Fe 3+ component.The lower intensity of the paramagnetic doublet at 16 K (ca.6 %) suggested that the fluorination process induces a decrease in particle size which is represented by the paramagnetic doublet observed at 298 K.At lower temperatures the small particle La2BaFe2O5F4 would be expected to magnetically order and give a smaller paramagnetic contribution to the spectrum.This was confirmed in the spectrum recorded at 16 K where the doublet decreased in magnitude to ca. 6 % of the spectral area.The two Fe 3+ sextet patterns observed in the spectrum recorded at 16 K reflect the distribution of lanthanum and barium around the FeO6 octahedra and these components accounted for ca.94 % of the spectral area.The minor (ca. 3 %) singlet component in the spectrum recorded at 298 K was attributed to a barium-doped LaFeO3 impurity phase present in the La2BaFe2O7 starting material which magnetically orders at low temperature and becomes incorporated within the dominant sextet patterns observed at 16 K.
An important contribution of the Mössbauer spectra recorded from La2BaFe2O5F4 was the demonstration that all the iron in La2BaFe2O5F4 is present as Fe 3+ and shows that fluorination of La2BaFe2O7 is achieved without oxidation of iron.This is in contrast with the situation when similar phases containing manganese are fluorinated and manganese is oxidised.This was attributed to the higher reluctance of the d 5 Fe 3+ ion to oxidation as compared to Mn 3+ .

CONCLUSION
Our investigations of the fluorination of perovskite-related structures have produced new phases in a range of materials.Of particular importance has been the development of low temperature routes for achieving higher fluorine contents.The structural characterisation of these materials has given new insight as to how fluorine might accommodate itself within these structures.Mössbauer spectroscopy has shown itself to be an important technique for monitoring the changes in oxidation state upon fluorination.It has also revealed the complexity of magnetic interactions within the new oxide fluorides and has sought to relate these to structural properties and the effect of fluorine on superexchange pathways.We are confident that this area of chemistry has many possibilities for the synthesis of new materials in the future and that Mössbauer spectroscopy has a strong place alongside other techniques in the characterisation of these phases.

Figure 1 .
Figure 1.Representation of the perovskite AMO3 unit cell (n = infinity) and the Ruddlesden-Popper structures with n = 1 and n = 2.

Figure 6 .
Figure 6.119  Sn Mössbauer spectra recorded from Ba2SnO4 and its fluorinated derivative at 77 K. Reproduced with permission from Ref.[14].

Figure 5 .
Figure 5. Variation of the magnetic hyperfine field in BaFeO2F with temperature.Reproduced with permission Ref. [13].