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Several materials have or are currently being investigated for nuclear waste sequestration applications, including crystalline ceramic oxides, glasses, and glass-ceramic composites. Rare-earth phosphates have been investigated extensively for this application owing to the range of structures that the hydrous or anhydrous versions can adopt as well as the fact that naturally occurring rare-earth phosphates have been found to contain U or Th. The purpose of this mini-review is to discuss (generally) the properties that must be considered when identifying nuclear wasteform materials and (more specifically) the structure and properties of rare-earth phosphates with special attention being given to the resistance of these materials to radiation-induced structure damage. The last section of the mini-review contains an introduction to the development of glass-ceramic composite materials that contain rare-earth phosphate crystallites dispersed in a glass matrix. These composite materials have been suggested to be superior to using just glass or ceramic materials for nuclear waste sequestration applications owing to improved waste loading capabilities., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

The series BaIn 1- x Fe x O 2.5+δ , x = 0.25, 0.50, and 0.75, has been prepared under air-fired and argon-fired conditions and studied using X-ray diffraction, d.c. and a.c. susceptibility, Mössbauer spectroscopy, neutron diffraction, X-ray near edge absorption spectroscopy (XANES), and X-ray pair distribution (PDF) methods. While Ba 2 In 2 O 5 (BaInO 2.5 ) crystallizes in an ordered brownmillerite structure, Ibm 2, and Ba 2 Fe 2 O 5 (BaFeO 2.5 ) crystallizes in a complex monoclinic structure, P 2 1 / c , showing seven Fe 3+ sites with tetrahedral, square planar, and octahedral environments, all phases studied here crystallize in the cubic perovskite structure, Pm 3̅ m , with long-range disorder on the small cation and oxygen sites. 57 Fe Mössbauer studies indicate a mixed valency, Fe 4+ /Fe 3+ , for both the air-fired and argon-fired samples. The increased Fe 3+ content for the argon-fired samples is reflected in increased cubic cell constants and in the increased Mössbauer fraction. It appears that the Pm 3̅ m phases are only metastable when fired in argon. From a slightly modified percolation theory for a primitive cubic lattice (taking into account the presence of random O atom vacancies), long-range spin order is permitted for the x = 0.50 and 0.75 phases. Instead, the d.c. susceptibility shows only zero-field-cooled (ZFC) and field-cooled (FC) divergences at ∼6 K [5 K] for x = 0.50 and at ∼22 K [21 K] for x = 0.75, with values for the argon-fired samples in [ ]. Neutron diffraction data for the air-fired samples confirm the absence of long-range magnetic order at any studied temperature. For the air-fired x = 0.50, a.c. susceptibility data show a frequency-dependent χ'(max) and spin glass behavior, while for x = 0.75, χ'(max) is invariant with frequency, ruling out either a spin glass or a superparamagnetic ground state. These behaviors are discussed in terms of competing Fe 3+ -Fe 3+ antiferromagnetic exchange and ferromagnetic Fe 3+ -Fe 4+ exchange. The PDF and 57 Fe Mössbauer data indicate a local structure at short interatomic distances, which deviates strongly from the average Pm 3̅ m model. Fe Mössbauer, PDF, and XANES data show a systematic dependence on x and indicate that the Fe 3+ sites are largely fourfold-coordinated and Fe 4+ sites are fivefold- or sixfold-coordinated., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

We report the synthesis, structure, and redox behavior of the cation-ordered tetragonal Sc 2 VO 5+δ defect fluorite superstructure previously thought to be the oxygen precise A 3+ 2 B 4+ O 5 phase. Four synthesis routes in oxidative, reductive, and inert atmospheres are demonstrated. Ex situ and in situ powder X-ray and neutron diffraction analyses reveal vanadium disproportionation reactions. The structure-reaction map illustrates the oxygen-dependent competition between the tetragonal cation and anion ordered Sc 2 VO 5+δ and the disordered cubic Sc 2 VO 5+δ' (δ < δ' ≤ 0.5) phases as a function of temperature. Oxidation states and oxide stoichiometries were determined with DC magnetometry and XANES experiments. The tetragonal cation ordered Sc 2 VO 5+δ phase with δ = -0.15(2) for as-synthesized samples reveals vanadium charge ordering. V 3+ and V 4+ cations occupy octahedral sites, whereas V 5+ predominantly occupies a tetrahedral site. The paramagnetic 8 g {V 3+/4+ } 4 clusters are isolated by diamagnetic 2 c V 5+ cations. At temperatures below 500 °C the 8 g {V 3+/4+ } 4 clusters can be topotactically fine-tuned with varying V 3+ /V 4+ ratios. Above 600 °C the tetragonal structure oxidizes to the cubic Sc 2 VO 5+δ' fluorite phase-its disordered competitor. The investigation of the cation- and anion-ordered Sc-V-O phases, their formation, and thermal stability is important for the design of low-temperature solid state oxide ion conductors and vacancy structures.

SrLaMnO 4 is a layered antiferromagnetic (AF) oxide with the tetragonal ( I4/ mmm) n = 1 Ruddlesden-Popper phase structure (also known as the K 2 NiF 4 structure) with T N = 128 K. Remarkably, substitution of Sr 2+ by Ba 2+ , forming the solid solution Ba 1- x Sr x LaMnO 4 , results in the destruction of long-range magnetic order and of the ordered moment on Mn 3+ for x > 0.35, although the effective paramagnetic moment remains unchanged, an unprecedented behavior for this class of magnetic materials. Four members, x = 0.0, 0.25, 0.35, and 1.0, have been studied using XYZ neutron polarization analysis which permits isolation of the magnetic, nuclear and nuclear spin components of the scattering and the measurement of the absolute value of the magnetic cross section. Data analysis is done using model independent reverse Monte Carlo methods (SPINVERT). The results for x = 0.0 (SrLaMnO 4 ), T > T N (128 K) , show an asymmetric diffuse peak which evolves into resolution limited Bragg peaks below T N and a fully ordered AF ground state with a Mn 3+ moment of 3.06 μ B . For x = 0.25 the magnetic scattering below T N displays a remarkable phase separation-Bragg reflections coexisting with diffuse scattering. The ordered Mn 3+ moment is 1.1 μ B , much reduced from that obtained via unpolarized neutrons. There are no Bragg peaks for x = 0.35 at any measured temperature ( T > 3 K) but there is highly structured diffuse scattering indicating strong short-range order reminiscent of x = 0 and 0.25 above their respective transition temperatures. For x = 1.00 (BaLaMnO 4 ) the diffuse scattering roughly follows a paramagnetic form-factor indicating no short or long-range magnetic correlations. It is argued that the observed phenomena are due to a competition between AF and ferromagnetic (F) superexchange interactions for the 180° Mn 3+ -O-Mn 3+ geometry within the ab plane and that the changes in the local geometry of the Mn-O octahedron leads to reduction of the AF interaction with a likely enhancement of the F interaction with increasing Ba content, ultimately giving rise to a glassy ground state. Analysis of the diffuse magnetic components show clear 2D AF spin correlations above T N for x = 0.00, 0.25, and 0.35 with correlation lengths, ξ ∼ 14-7 Å and no spin correlations for x = 1.00.

Oxide materials that adopt the garnet-type structure (X 3 A 2 B 3 O 12 ) have received attention for a wide variety of applications, one of which is as potential wasteforms for the sequestration of radioactive actinide elements. The actinides are able to be accommodated in the eight-coordinate X site of the garnet structure. This study focuses on the investigation of Ce substitution into the X site as a surrogate for Pu because of their similar chemical properties. This is accomplished through analysis of the Y 3- z Ce z AlFe 4 O 12 (0.05 ⩽ z ⩽ 0.20) materials. The effects of the Ce concentration, oxidation state of the Ce in the starting materials, annealing environment, and cooling rate on the local structure and Ce and Fe oxidation states were investigated through analysis of the powder X-ray diffraction patterns and Ce L 3 -edge and Fe K-edge X-ray absorption spectroscopy (XANES) spectra. Analysis of Ce L 3 -edge XANES spectra indicated that Ce was present as both 3+ and 4+ oxidation states, the ratios of which depended on the synthetic conditions. The largest concentration of Ce 4+ was observed when the materials were postannealed at 800 °C following synthesis of the materials at 1400 °C. Variations in the Ce oxidation state are the result of the temperature-dependent Ce 3+ /Ce 4+ redox couple, with Ce 4+ being favored at lower temperatures. Analysis of the Fe K-edge spectra indicated that Fe was only present in the 3+ oxidation state and the Fe coordination number increased with increasing concentration of Ce 4+ , which is necessary to charge balance the system. The materials in this study can be described as an oxygen-intercalated garnet-type structure with the formula Y 3- z Ce z AlFe 4 O 12+δ .

A new 1-step method for synthesizing glass-ceramic composites consisting of rare earth phosphates (REPO 4 ) dispersed in borosilicate glass (BG) is reported herein as an alternative to the 2-step approach that is traditionally used. The effect of annealing time and annealing temperature on the formation of the 1-step glass-ceramic composites was investigated. Backscattered electron images and energy dispersive X-ray maps were collected to observe the morphology and chemical distribution in the glass-ceramic composites. X-ray diffraction was used to study the long-range order and X-ray absorption near edge spectroscopy was used to study the local environment of La, Y, P and Si. All analyses showed glass-ceramic composites made by the 1 and 2-step methods were similar to each other except for the Si L 2,3 -edge XANES spectra, which showed a slight change between the glass-ceramic composite materials made by the different synthesis methods. Xenotime-type phosphates (YPO 4 ) were observed to be more soluble in the borosilicate glass than the monazite-type phosphates (LaPO 4 ). This was attributed to the difference in the field strength of the rare-earth ions as a result of the difference in the ionic radii. Glass-ceramic composites made by the 1-step method were shown to form in 1 day at 1100 °C and in 3 days at 1000 °C without a significant change in glass or ceramic composition compared to the 1-step composite synthesized at 1100 °C for 3 days., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Hollandite is a class of metal oxide material with the general formula A 2 B 8 O 16 . Several methods have been used in the synthesis of this type of metal oxide, and the synthetic methods reported have typically employed high annealing temperatures between 1200 and 1300 °C. Appropriate synthetic methods must be employed to successfully synthesize these hollandite-type oxides at lower annealing temperatures. Hollandite compounds have been synthesized using ceramic (high annealing temperature only) and coprecipitation (high and low annealing temperatures) methods. Annealing temperatures ranging from 1200 to 700 °C have been employed in the thermal treatment process. Powder X-ray diffraction and X-ray absorption near-edge spectroscopy (XANES) were conducted on hollandite-type oxides (Ba x Al 2 x Ti 8-2 x O 16-δ ; x = 1.2; and Ba x Al x Fe x Ti 8-2 x O 16-δ, Ba x Fe 2 x Ti 8-2 x O 16-δ ; x = 1.16). Structural comparisons between materials annealed in the temperature range from 1200 to 800 °C were made, and an examination of the XANES spectra and powder X-ray diffraction patterns has provided confirmation of the absence of significant structural changes in these hollandite materials. This study has shown that hollandite-type materials can be formed using annealing temperatures as low as 700-800 °C when a coprecipitation method is used, with little change to the local and long-range structures being detected.

Rhabdophane- (REPO 4 ·nH 2 O; RE = La to Dy), monazite- (REPO 4 ; RE = La to Gd), and xenotime-type (REPO 4 and RE'PO 4 ·nH 2 O; RE = Tb to Lu and Y; RE' = Ho to Lu and Y) rare-earth phosphate materials are being considered for a number of applications including as photonic materials, for biolabeling studies, and as potential nuclear wasteforms. Structural studies of hydrous rare-earth phosphates are rather limited when compared to anhydrous rare-earth phosphates. In this study, rhabdophane- (REPO 4 ·nH 2 O; RE = La, Nd, Sm, Gd, and Dy) and xenotime-type (REPO 4 ·nH 2 O; RE = Y and Yb) materials were synthesized by a precipitation-based method and investigated using X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES). Examination of the powder XRD data from rhabdophane-type materials has confirmed that the rhabdophane structure crystallizes in the monoclinic crystal system rather than the hexagonal structure that has most often been reported. Materials adopting the rhabdophane- or xenotime-type structure were studied as a function of temperature to understand how the structure varies with increasing annealing temperature. Local structural information was obtained by collecting P K- and RE L 1 -edge XANES spectra. Examination of P K-edge XANES spectra from rhabdophane- and xenotime-type materials revealed changes in the local environment around P as a function of temperature. These changes were attributed to the removal of water from these structures as a result of high temperature annealing.

Inert matrix fuels (IMF) consist of transuranic elements (i.e., Pu, Am, Np, Cm) embedded in a neutron transparent (inert) matrix and can be used to "burn up" (transmute) these elements in current or Generation IV nuclear reactors. Yttria-stabilized zirconia has been extensively studied for IMF applications, but the low thermal conductivity of this material limits its usefulness. Other elements can be used to stabilize the cubic zirconia structure, and the thermal conductivity of the fuel can be increased through the use of a lighter stabilizing element. To this end, a series of Nd(x)Sc(y)Zr(1-x-y)O(2-δ) materials has been synthesized via a co-precipitation reaction and characterized by multiple techniques (Nd was used as a surrogate for Am). The long-range and local structures of these materials were studied using powder X-ray diffraction, scanning electron microscopy, and X-ray absorption spectroscopy. Additionally, the stability of these materials over a range of temperatures has been studied by annealing the materials at 1100 and 1400 °C. It was shown that the Nd(x)Sc(y)Zr(1-x-y)O(2-δ) materials maintained a single cubic phase upon annealing at high temperatures only when both Nd and Sc were present with y ≥ 0.10 and x + y > 0.15.

Four new double perovskites, SrLaMReO(6) (M = Mg, Mn, Co, Ni) in which Re(5+) (5d(2)) is present, were prepared via conventional solid state reactions and characterized by X-ray and neutron powder diffraction, XANES, SQUID magnetometry, and muon spin relaxation (μSR). Synchrotron X-ray and neutron diffraction experiments confirmed that all compounds crystallize in the monoclinic P2(1)/n structure type, which consists of alternately corner-shared octahedra of MO(6) and ReO(6). Rietveld refinement results indicated anti-site mixing of less than 7% on the M/Re sites. Bond valence sum calculations (BVS) suggest all M and Re ions are 2+ and 5+, respectively, and for the Mn-containing phase this is also supported by XANES measurements. All of the materials are paramagnetic at room-temperature and their Curie-Weiss temperatures are positive (except for Mg) indicating net ferromagnetic interactions. No evidence for long-range magnetic order is evident in the dc magnetic susceptibility and μSR measurements for SrLaMgReO(6) to 2 K. The Mn-phase shows long-range order at T(C) = 190 K and neutron diffraction revealed a ferromagnetic structure with a refined net moment of ∼3.7μ(B). Both Co- and Ni-containing phases exhibit spin glass behavior at T(G) = 23 and 30 K, respectively, which is supported by neutron diffraction and a.c. susceptibility data. The structure and physical properties of these four new rhenium based ordered double perovskites are compared to the closely related "pillared perovskites", La(5)Re(3)MO(16), the isoelectronic Os(6+) (5d(2)) double perovskite Sr(2)CoOsO(6), and the Re(6+) (5d(1)) double perovskites, Sr(2)MReO(6), (M = Mg, Ca, Mn, Co, Ni).

The geochemical model for Mo mineralization in the JEB Tailings Management Facility (JEB TMF), operated by AREVA Resources Canada at McClean Lake, Saskatchewan, was investigated using X-ray Absorption Near-Edge Spectroscopy (XANES), an elemental-specific technique that is sensitive to low elemental concentrations. Twenty five samples collected during the 2013 sampling campaign from various locations and depths in the TMF were analyzed by XANES. Mo K-edge XANES analysis indicated that the tailings consisted primarily of Mo(6+) species: powellite (CaMoO4), ferrimolybdite (Fe2(MoO4)3·8H2O), and molybdate adsorbed on ferrihydrite (Fe(OH)3 - MoO4). A minor concentration of a Mo(4+) species in the form of molybdenite (MoS2) was also present. Changes in the Mo mineralization over time were inferred by comparing the relative amounts of the Mo species in the tailings to the independently measured aqueous Mo pore water concentration. It was found that ferrimolybdite and molybdate adsorbed on ferrihydrite initially dissolves in the TMF and precipitates as powellite.

The JEB Tailings Management Facility (TMF) is central to reducing the environmental impact of the uranium ore processing operation located at the McClean Lake facility and operated by AREVA Resources Canada (AREVA). The geochemical controls of this facility are largely designed around the idea that elements of concern, such as Mo, will be controlled in the very long term through equilibrium with supporting minerals. However, these systems are far from equilibrium when the tailings are first placed in the TMF, and it can take years, decades, or centuries to reach equilibrium. Therefore, it is necessary to understand how these reactions evolve toward an equilibrium state to understand the very long-term behavior of the TMF and to ensure that the elements of concern will be adequately contained. To this end, the Mo speciation in a series of samples taken from the JEB TMF during the 2008 sampling campaign has been analyzed. This analysis was performed using powder X-ray diffraction (XRD), X-ray fluorescence mapping (μ-XRF), and X-ray absorption near-edge spectroscopy (XANES). These results show that only XANES was effective in speciating Mo in the tailings samples, because it was both element-specific and sensitive enough to detect the low concentrations of Mo present. These results show that the predominant Mo-bearing phases present in the TMF are powellite, ferrimolybdite, and molybdate adsorbed on ferrihydrite.

Magnetoelectric materials couple both magnetic and electronic properties, making them attractive for use in multifunctional devices. The magnetoelectric AFeO3 compounds (Pna2(1); A = Al, Ga) have received attention as the properties of the system depend on composition as well as the synthetic method used. Al(1-x)Ga(x)FeO3. (0 ≤ x ≤ 1) was synthesized by the sol-gel and coprecipitation methods and studied by X-ray absorption near-edge spectroscopy (XANES). Al L(2,3-), Ga K-, and Fe K-edge XANES spectra were collected to examine how the average metal coordination number (CN) changes with the synthetic method. Al and Fe were found to prefer octahedral sites, while Ga prefers the tetrahedral site. It was found that composition played a larger role in determining site occupancies than synthetic method. Samples made by the sol-gel or ceramic methods (reported previously; Walker, J. D. S.; Grosvenor, A. P. J. Solid State Chem. 2013, 197, 147-153) showed smaller spectral changes than samples made via the coprecipitation method. This is attributed to greater ion mobility in samples synthesized via coprecipitation as the reactants do not have a long-range polymeric or oxide network during synthesis like samples synthesized via the sol-gel or ceramic method. Increasing annealing temperature increases the average coordination number of Al, and to a lesser extent Ga, while the average coordination number of Fe decreases. This study indicates that greater disorder is observed when the Al(1-x)Ga(x)FeO3. compounds have high Al content, and when annealed at higher temperatures.

The titanate and stannate pyrochlore-type oxides have been investigated because of their potential applications in different fields. Pyrochlore-type oxides exhibit a wide variety of properties such as fast ionic conduction, resistance to radiation induced structural damage, and ferro- and antiferro-magnetism. These properties mainly depend on the metal-oxygen bonding interactions and electronic structure of the materials, both of which can change with composition. The Gd2Ti(2-x)Sn(x)O7 (0 ≤ x ≤ 2) system was synthesized by the ceramic method and investigated by X-ray absorption near edge spectroscopy (XANES), which allows for the examination of the effect of substitution on bonding and the electronic structure of materials. Examination of metal K- and L3-edge XANES spectra from the Gd2Ti(2-x)Sn(x)O7 system allowed for the elucidation of how the metal-oxygen bond covalency effects the electronic structure of these materials with increasing Sn content. The ionic character of the Ti-O and Gd-O bonds increases while the Sn-O bond becomes more covalent as x increases in the formula, and resulted in changes in energy and/or line shape of the spectra. This study shows that the bonding interactions between metal and oxygen vary strongly with composition, which may affect the fast ionic conduction and resistance to radiation induced structural damage that has been previously reported for these materials. It was also observed in this study that the intensity of the intersite-hybrid peaks found in the pre-edge region of the Ti K-edge XANES spectra resulting from excitations of 1s electrons into next-nearest neighbour Ti 3d states decreased significantly with increasing Sn incorporation.

Amorphous quaternary [(ZrO(2))(x)(TiO(2))(y)(SiO(2))(1-x-y)] and ternary [(ZrO(2))(x)(SiO(2))(1-x)] silicates were synthesized using a sol-gel method and examined via XPS and XANES. Metal silicates are important industrial materials, though structural characterization is complicated because of their amorphous nature. Hard (Ti K- and Zr K-edge) and soft (Ti L(2,3)-edge) X-ray XANES spectra suggest the Ti and Zr coordination numbers in the quaternary silicates remain constant as the metal identity or total metal content (x, y, or x + y in the chemical formula) is varied. XPS core-line spectra from the quaternary silicates show large decreases in Ti 2p(3/2), Zr 3d(5/2), Si 2p(3/2), and O 1s binding energies due to increasing final-state relaxation with greater next-nearest neighbour substitution of Si for less-electronegative Ti/Zr, which was confirmed by analysis of the O Auger parameter. These decreases in binding energy occur without any changes in the ground-state energies (e.g., oxidation state) of these atoms, as examined by Ti L(2,3)-edge, Si L(2,3)-edge, and O K-edge XANES. Because most spectroscopic investigations are concerned with ground-state properties, knowledge of the contributions from final-state effects is important to understand the spectra from materials of interest.

Rare-earth orthoferrites, REFeO₃ (RE D rare earth; Y), are tremendously adaptable compounds that are being investigated for use in a wide variety of applications including gas sensors, vehicle catalytic converters, and solid-oxide fuel cells. They also exhibit interesting magnetic properties such as high-temperature antiferromagnetism, making them useful for data storage applications. The compounds adopt a distorted perovskite-type structure where the tilt angle of the octahedra increases (Fe-O-Fe bond angle decreases) as the size of the rare-earth atom decreases. Despite intensive study of the physical properties of these compounds, very few studies have investigated how the bonding and electronic structure of these systems change with substitution of the RE. X-ray absorption near-edge spectroscopy (XANES) is a technique well-suited for such a study, and, in view of this, Fe L-, Fe K- and O K-edge spectra from a series of REFeO₃ compounds (RE D La, Pr, Nd, Sm, Eu, Gd, Ho, Yb, Y) have been collected, and are presented here. Fe L-edge spectra show that Fe is octahedrally coordinated and that the Fe-centered octahedra do not appear to distort with changes in the identity of the RE. The Fe K-edge spectra contain an intersite hybrid peak, which is an ill-studied feature that is attributed to non-local transitions of 1s electrons to 3d states on the next-nearest-neighbor atom that are hybridized with 4p states on the absorbing atom through O 2p states. In this study, it is shown that the intensity of this feature is strongly dependent on the Fe-O-Fe bond angle; the lower the Fe-O-Fe bond angle, the less intense the intersite hybrid peak is.

Sr(2)FeMnO(5+y) was synthesized under two different conditions, in air and in argon, both of which resulted in a cubic, Pm ̅3m, structure with no long-range ordering of oxygen vacancies. The unit cell constants were found to be a(0) = 3.89328(1) Å for argon (y = 0.0) and a(0) = 3.83075(3) Å for air (y = 0.5). In contrast, Ca(2)FeMnO(5) retains long-range brownmillerite oxygen vacancy ordering for either air or argon synthesis. Remarkably, Sr(2)FeMnO(5.0) oxidizes spontaneously in air at room temperature. A neutron pair distribution function (NPDF) study of Sr(2)FeMnO(5.0)(Ar) showed evidence for local, brownmillerite-like ordering of oxygen vacancies for short distances up to 5 Å. Mössbauer spectroscopy results indicate more than one Fe site for Sr(2)FeMnO(5+y)(Ar and air), consistent with the noncubic local structure found by NPDF analysis. The isomer shifts and quadrupole splittings in both air- and argon-synthesized materials are consistent with the 3+ oxidation state for Fe in sites with coordination number four or five. This is confirmed by an L-edge XANES study. Mn is almost entirely in the 3+ state for Sr(2)FeMnO(5.0)(Ar), whereas Mn(4+) is predominantly present for Sr(2)FeMnO(5.5)(air). Magnetic susceptibility data show zero-field-cooled/field-cooled (ZFC/FC) divergences near 50 K for the Ar sample and 25 K for the air sample, whereas Ca(2)FeMnO(5) is long-range G-type antiferromagnetically ordered at 407(2) K. Hyperfine magnetic splitting, observed in temperature-dependent Mössbauer measurements, indicates short-range magnetic correlations that persist up to 150 K for Sr(2)FeMnO(5.0)(Ar) and 100 K for Sr(2)FeMnO(5.5)(air), well above the ZFC/FC divergence temperatures. Neutron diffraction data confirm the absence of long-range magnetic ordering at room temperature and 4 K for Sr(2)FeMnO(5.0)(Ar) but indicate the presence of domains with short-range G-type order at 4 K with an average dimension of ∼50 Å (y = 0); thus, this material is actually a superparamagnet rather than a true spin glass. In sharp contrast, corresponding data for Sr(2)FeMnO(5.5)(air) show mainly a very weak magnetic Bragg peak, indicating that ∼4% of the sample has G-type antiferromagnetic ordering at 4 K., (© 2011 American Chemical Society)

Rare-earth-filled transition-metal pnictides having the skutterudite-type structure have been proposed for use as high-temperature thermoelectric materials to recover waste heat from vehicle exhaust, among other applications. A previous investigation by this research group of one of the most studied skutterudites, CeFe(4)Sb(12), found that, when exposed to air, this material oxidized at temperatures that are considerably below the proposed maximum operating temperature. Here, by the combined use of TGA, powder XRD, and XANES, it has been found that the substitution of Ce(3+) and Fe(2+) for larger rare-earth and transition-metal elements (Eu(2+) and Ru(2+)) results in a significantly higher oxidation temperature compared to that of CeFe(4)Sb(12). This increase can be related to the increased orbital overlap provided by these larger atoms (Eu(2+) and Ru(2+) vs Ce(3+) and Fe(2+)), enabling the development of stronger bonds. These results show how selective substitution of the constituent elements can significantly improve the thermal stability of materials., (© 2011 American Chemical Society)

The Fe K-edge X-ray absorption near-edge (XANES) spectra from Fe(1-x)Ga(x)SbO(4), having a rutile-like structure, have been investigated. Similar to the Ti K-edge XANES spectrum from TiO(2) (rutile), the low-energy pre-edge region observed in the Fe K-edge spectra is too broad to be representative of only a local, quadrupolar 1s → 3d excitation. The broadness of this peak results from the presence of a nonlocal transition, referred to as an intersite hybrid, which involves the excitation of 1s electrons to unoccupied 3d states of a next-nearest-neighbor Fe atom. (These 3d states overlap Fe 4p states of the absorbing atom through O 2p states.) With increasing Ga concentration, the intensity of the intersite hybrid peak decreases because of a deficiency of unoccupied next-nearest-neighbor 3d states. This observation provides important information on how the peak intensities of these nonlocal excitations are affected by substitution of the constituent elements.

The binary transition-metal monophosphides CrP, MnP, FeP, and CoP have been studied with X-ray photoelectron spectroscopy. The shifts in phosphorus 2p(3/2) core line binding energies relative to that of elemental phosphorus indicated that the degree of ionicity of the metal-phosphorus bond decreases on progressing from CrP to CoP. The metal 2p(3/2) core line binding energies differ only slightly and show similar line shapes to those of the elemental metals, reaffirming the notion that these transition-metal phosphides have considerable metallic character. The satellite structure observed in the Co 2p(3/2) X-ray photoelectron spectra of Co metal and CoP was examined by reflection electron energy loss spectroscopy and has been attributed to plasmon loss, not final state effects as has been previously suggested. Valence-band spectra of the transition-metal phosphides agree well with the density of states profiles determined from band structure calculations. The electron populations of the different electronic states were extracted from the fitted valence-band spectra, and these confirm the presence of strong M-P and weak P-P bonding interactions. Atomic charges determined from the P 2p core line spectra and the fitted valence-band spectra support the approximate formulation M(1+)P(1-) for these phosphides.