Susan D. Forder

Soft Chemical Control of Superconductivity in Lithium Iron Selenide Hydroxides Li1–xFex(OH)Fe1–ySe

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li(1-x)Fe(x)(OH)Fe(1-y)Se (x ∼ 0.2; 0.02 < y < 0.15) with a wide range of iron site vacancy concentrations in the iron selenide layers. This iron vacancy concentration is revealed as the only significant compositional variable and as the key parameter controlling the crystal structure and the electronic properties. Single crystal X-ray diffraction, neutron powder diffraction, and X-ray absorption spectroscopy measurements are used to demonstrate that superconductivity at temperatures as high as 40 K is observed in the hydrothermally synthesized samples when the iron vacancy concentration is low (y < 0.05) and when the iron oxidation state is reduced slightly below +2, while samples with a higher vacancy concentration and a correspondingly higher iron oxidation state are not superconducting. The importance of combining a low iron oxidation state with a low vacancy concentration in the iron selenide layers is emphasized by the demonstration that reductive postsynthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li(1-x)Fe(x)(OH) reservoir layer to fill vacancies in the selenide layer.

Topochemical Fluorination of Sr3(M0.5Ru0.5)2O7 (M = Ti, Mn, Fe), n = 2, Ruddlesden–Popper Phases

Reaction of the appropriate Sr3(M(0.5)Ru(0.5))2O7 (M = Ti, Mn, Fe), n = 2, Ruddlesden-Popper oxide with CuF2 under flowing oxygen results in formation of the oxide-fluoride phases Sr3(Ti(0.5)Ru(0.5))2O7F2, Sr3(Mn(0.5)Ru(0.5))2O7F2, and Sr3(Fe(0.5)Ru(0.5))2O(5.5)F(3.5) via a topochemical anion insertion/substitution process. Analysis indicates the titanium and manganese phases have Ti(4+), Ru(6+) and Mn(4+), Ru(6+) oxidation state combinations, respectively, while Mössbauer spectra indicate an Fe(3+), Ru(5.5+) combination for the iron phase. Thus, it can be seen that the soft fluorination conditions employed lead to formation of highly oxidized Ru(6+) centers in all three oxide-fluoride phases, while oxidation states of the other transition metal M cations remain unchanged. Fluorination of Sr3(Ti(0.5)Ru(0.5))2O7 to Sr3(Ti(0.5)Ru(0.5))2O7F2 leads to suppression of magnetic order as the fluorinated material approaches metallic behavior. In contrast, fluorination of Sr3(Mn(0.5)Ru(0.5))2O7 and Sr3(Fe(0.5)Ru(0.5))2O7 lifts the magnetic frustration present in the oxide phases, resulting in observation of long-range antiferromagnetic order at low temperature in Sr3(Mn(0.5)Ru(0.5))2O7F2 and Sr3(Fe(0.5)Ru(0.5))2O(5.5)F(3.5). The influence of the topochemical fluorination on the magnetic behavior of the Sr3(M(0.5)Ru(0.5))2O(x)F(y) phases is discussed on the basis of changes to the ruthenium oxidation state and structural distortions.

Backscatter Fe-57 Mössbauer studies of iron(II) phthalocyanine

Abstract Phthalocyanines are established gas sensitive materials. We have successfully recorded backscatter conversion electron Mossbauer spectra (CEMS) of iron(II) phthalocyanine. Further, we have shown that the introduction of nitrogen dioxide, a typical analyte for these materials, into the detector system can be achieved without deleterious effects. The method allows surface sensitive spectra of these materials to be recorded under normal sensor operating conditions of temperature and pressure. The high vacuum conditions of other surface sensitive techniques used to study materials of this type are not required by this method.

A Mössbauer investigation of phases formed in Al-Fe alloys

During the semi-continuous casting of aluminium alloy ingots, the solidification rate varies substantially with position in the ingot. For commercial purity alloys, which contain small amounts of iron and silicon, intermetallic phases are formed inter-dendritically from the final liquid to solidify, and comprise typically 1% of the microstructure. The development in the work reported in this paper is that the phases of interest have been extracted from super-purity based, Bridgman Grown model aluminium-iron binary alloys by dissolving the aluminium matrix in butanol. This has the advantage of providing the pure phase and thus increases the proportion of iron (and Fe-57) which gives an improved signal to noise ratio for Moessbauer spectroscopy. Using phases extracted in this way, detailed variable temperature Moessbauer studies have been carried out on Al{sub 13}Fe{sub 4} and Al{sub 6}Fe. The variation with temperature of the isomer shift and absorption area were obtained. This enabled the recoil-free fractions of the two phases to be determined. With this information it was then possible to determine the proportion of Al{sub 13}Fe{sub 4} and Al{sub 6}Fe in an extracted sample containing both phases, and in a rod of the aluminium containing both phases in situ.

Topochemical Reduction of the Ruddlesden–Popper Phases Sr2Fe0.5Ru0.5O4 and Sr3(Fe0.5Ru0.5)2O7

Reaction of the Ruddlesden-Popper phases Sr2Fe(0.5)Ru(0.5)O4 and Sr3(Fe(0.5)Ru(0.5))2O7 with CaH2 results in the topochemical deintercalation of oxide ions from these materials and the formation of samples with average compositions of Sr2Fe(0.5)Ru(0.5)O(3.35) and Sr3(Fe(0.5)Ru(0.5))2O(5.68), respectively. Diffraction data reveal that both the n = 1 and n = 2 samples consist of two-phase mixtures of reduced phases with subtly different oxygen contents. The separation of samples into two phases upon reduction is discussed on the basis of a short-range inhomogeneous distribution of iron and ruthenium in the starting materials. X-ray absorption data and Mössbauer spectra reveal the reduced samples contain an Fe(3+) and Ru(2+/3+) oxidation state combination, which is unexpected considering the Fe(3+)/Fe(2+) and Ru(3+)/Ru(2+) redox potentials, suggesting that the local coordination geometry of the transition metal sites helps to stabilize the Ru(2+) centers. Fitted Mössbauer spectra of both the n = 1 and n = 2 samples are consistent with the presence of Fe(3+) cations in square planar coordination sites. Magnetization data of both materials are consistent with spin glass-like behavior.

Concerning the use of standards for identifying coordination environments in glasses

It is an established methodology to use crystallographically well-defined standard materials for understanding site geometries in glasses. Here we discuss the benefits and the limitations of this approach for the investigation by Mossbauer and EXAFS of Fe2+ Fe3+ in aluminosilicate glasses. As a case study we specifically consider [5]Fe2+ [5]Fe3+ sites and whether these exist in our glasses; and if so, whether they have defined site geometries or occur simply as a consequence of the site distortions and Fe-O bond length distributions. Results are consistent with the existence of [5]Fe2+ and [5]Fe3+ but do not prove this because site distortion and a mixture of 4- and 6-coordinated sites can produce comparable results. This exemplifies the need for caution when interpreting glass data based on standards.

Organometallic cation-exchanged phyllosilicates: variable-temperature 57Fe Mössbauer spectroscopic and related studies of the adsorption of dimethylaminomethylferrocene on clays and pillared clays

Variable-temperature 57Fe Mossbauer spectroscopy, thermogravimetry (TG), powder X-ray diffraction (PXRD) and temperature-programmed solid insertion probe mass spectrometry (TP-SIP-MS) have been used to study the interaction of dimethylaminomethylferrocene (DMAMF) with Westone-L (WL), a low iron montmorillonite. The hydrochloride salt of DMAMF, (ferrocenylmethyl)dimethylammonium chloride (FMDMACl), was prepared and studied both prior and subsequent toexchange on the interlamellar sites of WL. X-Ray diffraction confirmed that the FMDMA+ cations were incorporated between the clay lamellae and the observed spacing of 15.1 A was thermally stable up to 200 °C in air. TP-SIP-MS indicated that a small proportion of the incorporated metallocene was volatilised at temperatures below 400 °C, but that the majority decomposed via loss of cyclopentadienyl ligands leaving the metal centre between the sheets. A similar thermal degradation path was observed for DMAMF on aluminium pillared clay (Al-PILC). 57Fe Mossbauer spectroscopy revealed that the FMDMA+ cation occupied a similar environment in the chloride salt, FMDMA+–WL and DMAMF–Al-PILC insofar as the isomer shift, 6, and quadrupole splitting, Δ, of the incorporated metallocene were essentially the same in all complexes and virtually identical to that observed for FMDMACl(δ= 0.34 mm s–l, δ= 2.32 mm s–l at 300 K). The values for the Debye temperature θ, and recoil-free fraction f determined from variable-temperature 57Fe Mossbauer spectroscopy, were typically 140 K and 0.12, respectively, for FMDMACl and FMDMA+–WL, thus confirming the similar environment occupied by the cation in the chloride salt and in WL. In contrast, the corresponding values for DMAMF–A1-PILC were 118 K and 0.06, respectively, indicating that the the metallocene enjoyed much greater freedom in the galleries of the Al-PILC which exceed the dimensions of the metallocene compared to FMDMA+-WL where the organoiron cation itself determines the layer separation.

Synthesis and Characterization of Li11Nd18Fe4O39−δ

Li(11)Nd(18)Fe(4)O(39-δ) has been synthesized by the solid-state reaction of pellets, covered with powder of the same composition to avoid lithium loss, with a final reaction temperature of 950 °C. This phase has been reported previously to have various stoichiometries: Li(5)Nd(4)FeO(10), Li(8)Nd(18)Fe(5)O(39), and Li(1.746)Nd(4.494)FeO(9.493). The crystal structure of Li(11)Nd(18)Fe(4)O(39-δ) is closely related to that reported previously for two of the other three compositions but contains extra Li and differences in Li/Fe site occupancies. Fe is present in a mixture of 3+ and 4+ oxidation states, as confirmed by Mössbauer spectroscopy. The oxygen content of 39 - δ is variable, depending on the processing conditions. Samples slow-cooled in air from 800 °C are semiconducting, attributed to the presence of Fe(4+) ions, whereas samples quenched from 950 °C in N(2) are insulating.

A Mössbauer study of iron in vitrified wastes

57Fe Mossbauer spectroscopy has been used to study the environment and oxidation state of iron in a series of vitrified sewage sludge ash (SSA) wastes, which are broadly similar in composition and variability to some intermediate-level legacy wastes (ILLW). The SSA wastes studied here are incapable of forming an homogeneous glass when melted at 1450 °C although increasing additions of CaO reduce the crystalline content, which consists of Ca3(PO4)2 and Ca3Mg3(PO4)4. Bulk glass transition temperatures of 670–850 °C have been measured, the value decreasing with increasing CaO content. Tetrahedrally coordinated Fe3+ appears to exist in vitrified SSA only at high CaO contents (> ∼ 29 mol. %) and whilst broadly similar to our previous results for simulated vitrified SSA, behavioural differences have been noted between fitted Mossbauer parameters for real and simulated vitrified SSA. We suggest that these could be attributable to the buffering action of carbon and other reducing agents in the real SSA wastes.

57Fe Conversion Electron Mössbauer Spectroscopy of Factors Influencing Fe–Sn Intermetallic Phase Formation in Tinplated Steel

The formation of intermetallic phases influences the properties of commercial alloys. CEMS can be used to reveal the presence of Fe-Sn intermetallics at the interface on tinplated steels. The work presented investigates the sensitivity of CEMS to detect intermetallics. Varying amounts of Fe-Sn intermetallics have been identified by CEMS having been produced by heat treating samples of tinplated steel at selected temperatures and times. The effect of surface roughness of the substrate on the Mössbauer signal has also been investigated. The intermetallic formation was found to reach a limiting value with increased heat treatment. Using CEMS, intermetallic formation was observed at temperatures as low as 190°C. Sample roughness was found to affect intermetallic formation and their observation using CEMS and GA-XRD.