Sr2MnO2Na1.6Se2: A Metamagnetic Layered Oxychalcogenide Synthesized by Reductive Na Intercalation to Break [Se2]2- Perselenide Dimer Units.

Recent advances in anion-redox topochemistry have enabled the synthesis of metastable mixed-anion solids. Synthesis of the new transition metal oxychalcogenide Sr2MnO2Na1.6Se2 by topochemical Na intercalation into Sr2MnO2Se2 is reported here. Na intercalation is enabled by the redox activity of [Se2]2- perselenide dimers, where the Se-Se bonds are cleaved and a [Na2-x Se2](2+x)- antifluorite layer is formed. Freshly prepared samples have 16(1) % Na-site vacancies corresponding to a formal oxidation state of Mn of +2.32, a mixed-valence between Mn2+ (d5) and Mn3+ (d4). Samples are highly prone to deintercalation of Na, and over two years, even in an argon glovebox environment, the Na content decreased by 4(1) %, leading to slight oxidation of Mn and a significantly increased long-range ordered moment on the Mn site as measured using neutron powder diffraction. The magnetic structure derived from neutron powder diffraction at 5 K reveals that the compound orders magnetically with ferromagnetic MnO2 sheets coupled antiferromagnetically. The aged sample shows a metamagnetic transition from bulk antiferromagnetic to ferromagnetic behavior in an applied magnetic field of 2 T, in contrast to the Cu analogue, Sr2MnO2Cu1.55Se2, where there is only a hint that such a transition may occur at fields exceeding 7 T. This is presumably due to the higher ionic character of [Na2-x Se2](2+x)- layers compared to [Cu2-x Se2](2+x)- layers, reducing the strength of the antiferromagnetic interactions between MnO2 sheets. Electrochemical Na intercalation into Sr2MnO2Se2 leads to the formation of multiphase sodiated products. The work shows the potential of anion redox to yield novel compounds with intriguing physical properties.

Anionic Redox Topochemistry for Materials Design: Chalcogenides and Beyond.

Topochemistry refers to a generic category of solid-state reactions in which precursors and products display strong filiation in their crystal structures. Various low-dimensional materials are subject to this stepwise structure transformation by accommodating guest atoms or molecules in between their 2D slabs or 1D chains loosely bound by van der Waals (vdW) interactions. Those processes are driven by redox reactions between guests and the host framework, where transition metal cations have been widely exploited as the redox center. Topochemistry coupled with this cationic redox not only enables technological applications such as Li-ion secondary batteries but also serves as a powerful tool for structural or electronic fine-tuning of layered transition metal compounds. Over recent years, we have been pursuing materials design beyond this cationic redox topochemistry that was mostly limited to 2D or 1D vdW systems. For this, we proposed new topochemical reactions of non-vdW compounds built of 2D arrays of anionic chalcogen dimers alternating with redox-inert host cationic layers. These chalcogen dimers were found to undergo redox reaction with external metal elements, triggering either (1) insertion of these metals to construct 2D metal chalcogenides or (2) deintercalation of the constituent chalcogen anions. As a whole, this topochemistry works like a "zipper", where reductive cleavage of anionic chalcogen-chalcogen bonds opens up spaces in non-vdW materials, allowing the formation of novel layered structures. This Perspective briefly summarizes seminal examples of unique structure transformations achieved by anionic redox topochemistry as well as challenges on their syntheses and characterizations.

Controlling the Superconductivity of Nb2PdxS5 via Reversible Li Intercalation.

The Nb2PdxS5 (x ≈ 0.74) superconductor with a Tc of 6.5 K is reduced by the intercalation of lithium in ammonia solution or electrochemically to produce an intercalated phase with expanded lattice parameters. The structure expands by 2% in volume and maintains the C2/m symmetry and rigidity due to the PdS4 units linking the layers. Experimental and computational analysis of the chemically synthesized bulk sample shows that Li occupies triangular prismatic sites between the layers with an occupancy of 0.33(4). This level of intercalation suppresses the superconductivity, with the injection of electrons into the metallic system observed to also reduce the Pauli paramagnetism by ∼40% as the bands are filled to a Fermi level with a lower density of states than in the host material. Deintercalation using iodine partially restores the superconductivity, albeit at a lower Tc of ∼5.5 K and with a smaller volume fraction than in fresh Nb2PdxS5. Electrochemical intercalation reproduces the chemical intercalation product at low Li content (<0.4) and also enables greater reduction, but at higher Li contents (≥0.4) accessed by this route, phase separation occurs with the indication that Li occupies another site.

Anion-polarisation-directed short-range-order in antiperovskite Li2FeSO.

Short-range ordering in cation-disordered cathodes can have a significant effect on their electrochemical properties. Here, we characterise the cation short-range order in the antiperovskite cathode material Li2FeSO, using density functional theory, Monte Carlo simulations, and synchrotron X-ray pair-distribution-function data. We predict partial short-range cation-ordering, characterised by favourable OLi4Fe2 oxygen coordination with a preference for polar cis-OLi4Fe2 over non-polar trans-OLi4Fe2 configurations. This preference for polar cation configurations produces long-range disorder, in agreement with experimental data. The predicted short-range-order preference contrasts with that for a simple point-charge model, which instead predicts preferential trans-OLi4Fe2 oxygen coordination and corresponding long-range crystallographic order. The absence of long-range order in Li2FeSO can therefore be attributed to the relative stability of cis-OLi4Fe2 and other non-OLi4Fe2 oxygen-coordination motifs. We show that this effect is associated with the polarisation of oxide and sulfide anions in polar coordination environments, which stabilises these polar short-range cation orderings. We propose that similar anion-polarisation-directed short-range-ordering may be present in other heterocationic materials that contain cations with different formal charges. Our analysis illustrates the limitations of using simple point-charge models to predict the structure of cation-disordered materials, where other factors, such as anion polarisation, may play a critical role in directing both short- and long-range structural correlations.

Anion redox as a means to derive layered manganese oxychalcogenides with exotic intergrowth structures.

Topochemistry enables step-by-step conversions of solid-state materials often leading to metastable structures that retain initial structural motifs. Recent advances in this field revealed many examples where relatively bulky anionic constituents were actively involved in redox reactions during (de)intercalation processes. Such reactions are often accompanied by anion-anion bond formation, which heralds possibilities to design novel structure types disparate from known precursors, in a controlled manner. Here we present the multistep conversion of layered oxychalcogenides Sr2MnO2Cu1.5Ch2 (Ch = S, Se) into Cu-deintercalated phases where antifluorite type [Cu1.5Ch2]2.5- slabs collapsed into two-dimensional arrays of chalcogen dimers. The collapse of the chalcogenide layers on deintercalation led to various stacking types of Sr2MnO2Ch2 slabs, which formed polychalcogenide structures unattainable by conventional high-temperature syntheses. Anion-redox topochemistry is demonstrated to be of interest not only for electrochemical applications but also as a means to design complex layered architectures.

Molecular exploration of fossil eggshell uncovers hidden lineage of giant extinct bird.

The systematics of Madagascar's extinct elephant birds remains controversial due to large gaps in the fossil record and poor biomolecular preservation of skeletal specimens. Here, a molecular analysis of 1000-year-old fossil eggshells provides the first description of elephant bird phylogeography and offers insight into the ecology and evolution of these flightless giants. Mitochondrial genomes from across Madagascar reveal genetic variation that is correlated with eggshell morphology, stable isotope composition, and geographic distribution. The elephant bird crown is dated to ca. 30 Mya, when Madagascar is estimated to have become less arid as it moved northward. High levels of between-clade genetic variation support reclassifying Mullerornis into a separate family. Low levels of within-clade genetic variation suggest there were only two elephant bird genera existing in southern Madagascar during the Holocene. However, we find an eggshell collection from Madagascar's far north that represents a unique lineage of Aepyornis. Furthermore, divergence within Aepyornis coincides with the aridification of Madagascar during the early Pleistocene ca. 1.5 Ma, and is consistent with the fragmentation of populations in the highlands driving diversification and the evolution of extreme gigantism over shorts timescales. We advocate for a revision of their taxonomy that integrates palaeogenomic and palaeoecological perspectives.

High- versus Low-Spin Ni2+ in Elongated Octahedral Environments: Sr2NiO2Cu2Se2, Sr2NiO2Cu2S2, and Sr2NiO2Cu2(Se1-x S x )2.

Sr2NiO2Cu2Se2, comprising alternating [Sr2NiO2]2+ and [Cu2Se2]2- layers, is reported. Powder neutron diffraction shows that the Ni2+ ions, which are in a highly elongated NiO4Se2 environment with D4h symmetry, adopt a high-spin configuration and carry localized magnetic moments which order antiferromagnetically below ∼160 K in a √2a × âˆš2a × 2c expansion of the nuclear cell with an ordered moment of 1.31(2) µB per Ni2+ ion. The adoption of the high-spin configuration for this d 8 cation in a pseudo-square-planar ligand field is supported by consideration of the experimental bond lengths and the results of density functional theory (DFT) calculations. This is in contrast to the sulfide analogue Sr2NiO2Cu2S2, which, according to both experiment and DFT calculations, has a much more elongated ligand field, more consistent with the low-spin configuration commonly found for square-planar Ni2+, and accordingly, there is no evidence for magnetic moment on the Ni2+ ions. Examination of the solid solution Sr2NiO2Cu2(Se1-x S x )2 shows direct evidence from the evolution of the crystal structure and the magnetic ordering for the transition from high-spin selenide-rich compounds to low-spin sulfide-rich compounds as a function of composition. Compression of Sr2NiO2Cu2Se2 up to 7.2 GPa does not show any structural signature of a change in the spin state. Consideration of the experimental and computed Ni2+ coordination environments and their subtle changes as a function of temperature, in addition to transitions evident in the transport properties and magnetic susceptibilities in the end members, Sr2NiO2Cu2Se2 and Sr2NiO2Cu2S2, suggest that simple high-spin and low-spin models for Ni2+ may not be entirely appropriate and point to further complexities in these compounds.

Structures and Magnetic Ordering in Layered Cr Oxide Arsenides Sr2CrO2Cr2OAs2 and Sr2CrO3CrAs.

Two novel chromium oxide arsenide materials have been synthesized, Sr2CrO2Cr2OAs2 (i.e., Sr2Cr3As2O3) and Sr2CrO3CrAs (i.e., Sr2Cr2AsO3), both of which contain chromium ions in two distinct layers. Sr2CrO2Cr2OAs2 was targeted following electron microscopy measurements on a related phase. It crystallizes in the space group P4/mmm and accommodates distorted CrO4As2 octahedra containing Cr2+ and distorted CrO2As4 octahedra containing Cr3+. In contrast, Sr2CrO3CrAs incorporates Cr3+ in CrO5 square-pyramidal coordination in [Sr2CrO3]+ layers and Cr2+ ions in CrAs4 tetrahedra in [CrAs]- layers and crystallizes in the space group P4/nmm. Powder neutron diffraction data reveal antiferromagnetic ordering in both compounds. In Sr2CrO3CrAs the Cr2+ moments in the [CrAs]- layers exhibit long-range ordering, while the Cr3+ moments in the [Sr2CrO3]+ layers only exhibit short-range ordering. However, in Sr2CrO2Cr2OAs2, both the Cr2+ moments in the CrO4As2 environments and the Cr3+ moments in the CrO2As4 polyhedra are long-range-ordered below 530(10) K. Above this temperature, only the Cr3+ moments are ordered with a Néel temperature slightly in excess of 600 K. A subtle structural change is evident in Sr2CrO2Cr2OAs2 below the magnetic ordering transitions.

Intercalates of Bi2Se3 studied in situ by time-resolved powder X-ray diffraction and neutron diffraction.

Intercalation of lithium and ammonia into the layered semiconductor Bi2Se3 proceeds via a hyperextended (by >60%) ammonia-rich intercalate, to eventually produce a layered compound with lithium amide intercalated between the bismuth selenide layers which offers scope for further chemical manipulation.

Structural Evolution of Layered Manganese Oxysulfides during Reversible Electrochemical Lithium Insertion and Copper Extrusion.

The electrochemical lithiation and delithiation of the layered oxysulfide Sr2MnO2Cu4-δS3 has been investigated by using a combination of in situ powder X-ray diffraction and ex situ neutron powder diffraction, X-ray absorption and 7Li NMR spectroscopy, together with a range of electrochemical experiments. Sr2MnO2Cu4-δS3 consists of [Sr2MnO2] perovskite-type cationic layers alternating with highly defective antifluorite-type [Cu4-δS3] (δ ≈ 0.5) anionic layers. It undergoes a combined displacement/intercalation (CDI) mechanism on reaction with Li, where the inserted Li replaces Cu, forming Li4S3 slabs and Cu+ is reduced and extruded as metallic particles. For the initial 2-3% of the first discharge process, the vacant sites in the sulfide layer are filled by Li; Cu extrusion then accompanies further insertion of Li. Mn2.5+ is reduced to Mn2+ during the first half of the discharge. The overall charging process involves the removal of Li and re-insertion of Cu into the sulfide layers with re-oxidation of Mn2+ to Mn2.5+. However, due to the different diffusivities of Li and Cu, the processes operating on charge are quite different from those operating during the first discharge: charging to 2.75 V results in the removal of most of the Li, little reinsertion of Cu, and good capacity retention. A charge to 3.75 V is required to fully reinsert Cu, which results in significant changes to the sulfide sublattice during the following discharge and poor capacity retention. This detailed structure-property investigation will promote the design of new functional electrodes with improved device performance.

Magnetic Ordering in the Layered Cr(II) Oxide Arsenides Sr2CrO2Cr2As2 and Ba2CrO2Cr2As2.

Sr2CrO2Cr2As2 and Ba2CrO2Cr2As2 with Cr2+ ions in CrO2 sheets and in CrAs layers crystallize with the Sr2Mn3Sb2O2 structure (space group I4/mmm, Z = 2) and lattice parameters a = 4.00800(2) Å, c = 18.8214(1) Å (Sr2CrO2Cr2As2) and a = 4.05506(2) Å, c = 20.5637(1) Å (Ba2CrO2Cr2As2) at room temperature. Powder neutron diffraction reveals checkerboard-type antiferromagnetic ordering of the Cr2+ ions in the arsenide layers below TN1_Sr, of 600(10) K (Sr2CrO2Cr2As2) and TN1_Ba 465(5) K (Ba2CrO2Cr2As2) with the moments initially directed perpendicular to the layers in both compounds. Checkerboard-type antiferromagnetic ordering of the Cr2+ ions in the oxide layer below 230(5) K for Ba2CrO2Cr2As2 occurs with these moments also perpendicular to the layers, consistent with the orientation preferences of d4 moments in the two layers. In contrast, below 330(5) K in Sr2CrO2Cr2As2, the oxide layer Cr2+ moments are initially oriented in the CrO2 plane; but on further cooling, these moments rotate to become perpendicular to the CrO2 planes, while the moments in the arsenide layers rotate by 90° with the moments on the two sublattices remaining orthogonal throughout [behavior recently reported independently by Liu et al. [Liu et al. Phys. Rev. B 2018, 98, 134416]]. In Sr2CrO2Cr2As2, electron diffraction and high resolution powder X-ray diffraction data show no evidence for a structural distortion that would allow the two Cr2+ sublattices to couple, but high resolution neutron powder diffraction data suggest a small incommensurability between the magnetic structure and the crystal structure, which may account for the coupling of the two sublattices and the observed spin reorientation. The saturation values of the Cr2+ moments in the CrO2 layers (3.34(1) µB (for Sr2CrO2Cr2As2) and 3.30(1) µB (for Ba2CrO2Cr2As2)) are larger than those in the CrAs layers (2.68(1) µB for Sr2CrO2Cr2As2 and 2.298(8) µB for Ba2CrO2Cr2As2) reflecting greater covalency in the arsenide layers.

Single phase charge ordered stoichiometric CaFe3O5 with commensurate and incommensurate trimeron ordering.

Mixed-valent transition metal compounds display complex structural, electronic and magnetic properties which can often be exquisitely tuned. Here the charge-ordered state of stoichiometric CaFe3O5 is probed using neutron powder diffraction, Monte Carlo simulation and symmetry analysis. Magnetic ordering is dominated by the formation of ferromagnetic Fe3+-Fe2+-Fe3+ trimers which are evident above the magnetic ordering transition. Between TN = 289 K and 281 K an incommensurate magnetically ordered phase develops due to magnetic frustration, but a spin Jahn-Teller distortion lifts the frustration and enables the magnetic ordering to lock in to a charge-ordered commensurate state at lower temperatures. Stoichiometric CaFe3O5 exhibits single phase behaviour throughout and avoids the phase separation into two distinct crystallographic phases with different magnetic structures and Fe valence distributions reported recently, which likely occurs due to partial Fe2+ for Ca2+ substitution. This underlines the sensitivity of the magnetism and chemistry of these mixed-valent systems to composition.

In-situ Electrochemical X-ray Diffraction: A Rigorous Method to Navigate within Phase Diagrams Reveals ß-Fe1+x Se as Superconductor for All x.

We report the precise postsynthetic control of the composition of ß-Fe1+x Se by electrochemistry with simultaneous tracking of the associated structural changes via in situ synchrotron X-ray diffraction. We access the full phase width of 0.01

Expression Patterns of Genes Encoding Sugar and Potassium Transport Proteins Are Simultaneously Upregulated or Downregulated When Carbon and Potassium Availability Is Modified in Shiraz (Vitis vinifera L.) Berries.

A link between the accumulation of sugar and potassium has previously been described for ripening grape berries. The functional basis of this link has, as of yet, not been elucidated but could potentially be associated with the integral role that potassium has in phloem transport. An experiment was conducted on Shiraz grapevines in a controlled environment. The accumulation of berry sugar was curtailed by reducing the leaf photoassimilation rate, and the availability of potassium was increased through soil fertilization. The study characterizes the relationship between the accumulation of sugar and potassium into the grape berry and describes how their accumulation patterns are related to the expression patterns of their transporter proteins. A strong connection was observed between the accumulation of sugar and potassium in the grape berry pericarp, irrespective of the treatment. The relative expression of proteins associated with sugar and potassium transport across the tonoplast and plasma membrane was closely correlated, suggesting transcriptional coregulation leading to the simultaneous translocation and storage of potassium and sugar in the grape berry cell.

Synthesis, Structure, and Compositional Tuning of the Layered Oxide Tellurides Sr2MnO2Cu2- xTe2 and Sr2CoO2Cu2Te2.

The synthesis and structure of two new transition metal oxide tellurides, Sr2MnO2Cu1.82(2)Te2 and Sr2CoO2Cu2Te2, are reported. Sr2CoO2Cu2Te2 with the purely divalent Co2+ ion in the oxide layers has magnetic ordering based on antiferromagnetic interactions between nearest neighbors and appears to be inert to attempted topotactic oxidation by partial removal of the Cu ions. In contrast, the Mn analogue with the more oxidizable transition metal ion has a 9(1)% Cu deficiency in the telluride layer when synthesized at high temperatures, corresponding to a Mn oxidation state of +2.18(2), and neutron powder diffraction revealed the presence of a sole highly asymmetric Warren-type magnetic peak, characteristic of magnetic ordering that is highly two-dimensional and not fully developed over a long range. Topotactic oxidation by the chemical deintercalation of further copper using a solution of I2 in acetonitrile offers control over the Mn oxidation state and, hence, the magnetic ordering: oxidation yielded Sr2MnO2Cu1.58(2)Te2 (Mn oxidation state of +2.42(2)) in which ferromagnetic interactions between Mn ions result from Mn2+/3+ mixed valence, resulting in a long-range-ordered A-type antiferromagnet with ferromagnetic MnO2 layers coupled antiferromagnetically.

ELDOR-detected NMR beyond hyperfine couplings: a case study with Cu(ii)-porphyrin dimers.

The pulse EPR method ELDOR-detected NMR (EDNMR) is applied to two Cu(ii)-porphyrin dimers that are suitable building blocks for molecular wires. One of the dimers is meso-meso singly linked, the other one is ß, meso, ß-fused. We show experimentally and theoretically that EDNMR spectra contain information about the electron-electron couplings. The spectra of the singly linked dimer are consistent with a perpendicular arrangement of the porphyrin planes and negligible exchange coupling. In addition, the resolution is good enough to distinguish 63Cu and 65Cu in frozen glassy solution and to resolve a metal-ion nuclear quadrupole coupling of 32 MHz. In the case of the fused dimer, we observe so far unreported signal enhancements, or anti-holes, in the EDNMR spectra. These are readily explained in a generalized framework based on [Cox et al., J. Magn. Reson., 2017, 280, 63-78], if an effective spin of S = 1 is assumed, in accordance with SQUID measurements. The positions of the anti-holes encode a zero-field splitting with |D| = 240 MHz, which is about twice as large as expected from the point-dipole approximation. These findings demonstrate the previously unrecognized applicability and versatility of the EDNMR technique in the quantitative study of complex paramagnetic compounds.

Layered CeSO and LiCeSO Oxide Chalcogenides Obtained via Topotactic Oxidative and Reductive Transformations.

The chemical accessibility of the CeIV oxidation state enables redox chemistry to be performed on the naturally coinage-metal-deficient phases CeM1- xSO (M = Cu, Ag). A metastable black compound with the PbFCl structure type (space group P4/ nmm: a = 3.8396(1) Å, c = 6.607(4) Å, V = 97.40(6) Å3) and a composition approaching CeSO is obtained by deintercalation of Ag from CeAg0.8SO. High-resolution transmission electron microscopy reveals the presence of large defect-free regions in CeSO, but stacking faults are also evident which can be incorporated into a quantitative model to account for the severe peak anisotropy evident in all the high-resolution X-ray and neutron diffractograms of bulk CeSO samples; these suggest that a few percent of residual Ag remains. A straw-colored compound with the filled PbFCl (i.e., ZrSiCuAs- or HfCuSi2-type) structure (space group P4/ nmm: a = 3.98171(1) Å, c = 8.70913(5) Å, V = 138.075(1) Å3) and a composition close to LiCeSO, but with small amounts of residual Ag, is obtained by direct reductive lithiation of CeAg0.8SO or by insertion of Li into CeSO using chemical or electrochemical means. Computation of the band structure of pure, stoichiometric CeSO predicts it to be a Ce4+ compound with the 4f-states lying approximately 1 eV above the sulfide-dominated valence band maximum. Accordingly, the effective magnetic moment per Ce ion measured in the CeSO samples is much reduced from the value found for the Ce3+-containing LiCeSO, and the residual paramagnetism corresponds to the Ce3+ ions remaining due to the presence of residual Ag, which presumably reflects the difficulty of stabilizing Ce4+ in the presence of sulfide (S2-). Comparison of the behavior of CeCu0.8SO with that of CeAg0.8SO reveals much slower reaction kinetics associated with the Cu1- xS layers, and this enables intermediate CeCu1- xLi xSO phases to be isolated.

The role of fruit exposure in the late season decline of grape berry mesocarp cell vitality.

Loss of cell vitality in the mesocarp of grape (Vitis vinifera L.) berries during late ripening is programmed and under genetic control. Environmental factors such as temperature and vine water status, however, have a strong influence on the onset and extent of cell death. Following preliminary observations made on field experiment fruit, the hypothesis that exposure (increased light interception and wind velocity) at the berry level is important to the progression of cell death was tested. Transpiration, mesocarp cell vitality and total soluble solids concentration were compared in exposed and sheltered berries within single Shiraz vines. At oenological maturity (24 °Brix), exposed berries exhibited the same extent of cell death as sheltered berries, and it was not until four weeks later that cell death was more extensive in the exposed treatment. Therefore, under the conditions of this study, increased exposure over the ripening period was not a strong predictor of the extent of cell vitality at maturity. However, the results are consistent with an increase in the importance of environmental effects (including rain and exposure) on mesocarp cell death over the course of berry development, particularly in overripe fruit.

Synthesis, Structure, and Properties of the Layered Oxide Chalcogenides Sr2CuO2Cu2S2 and Sr2CuO2Cu2Se2.

The structures of two new oxide chalcogenide phases, Sr2CuO2Cu2S2 and Sr2CuO2Cu2Se2, are reported, both of which contain infinite CuO2 planes containing Cu2+ and which have Cu+ ions in the sulfide or selenide layers. Powder neutron diffraction measurements show that Sr2CuO2Cu2Se2 exhibits long-range magnetic ordering with a magnetic structure based on antiferromagnetic interactions between nearest-neighbor Cu2+ ions, leading to a √2 a × âˆš2 a × 2 c expansion of the nuclear cell. The ordered moment of 0.39(6) µB on the Cu2+ ions at 1.7 K is consistent with the value predicted by density functional theory calculations. The compounds are structurally related to the cuprate superconductors and may also be considered as analogues of the parent phases of this class of superconductor such as Sr2CuO2Cl2 or La2CuO4. In the present case, however, the top of the chalcogenide-based valence band is very close to the vacant Cu2+ 3d states of the conduction band, leading to relatively high measured conductivity.

Complex Magnetic Ordering in the Oxide Selenide Sr2Fe3Se2O3.

Sr2Fe3Se2O3 is a localized-moment iron oxide selenide in which two unusual coordinations for Fe2+ ions form two sublattices in a 2:1 ratio. In the paramagnetic region at room temperature, the compound adopts the crystal structure first reported for Sr2Co3S2O3, crystallizing in space group Pbam with a = 7.8121 Å, b = 10.2375 Å, c = 3.9939 Å, and Z = 2. The sublattice occupied by two-thirds of the iron ions (Fe2 site) is formed by a network of distorted mer-[FeSe3O3] octahedra linked via shared Se2 edges and O vertices forming layers, which connect to other layers by shared Se vertices. As shown by magnetometry, neutron powder diffraction, and Mössbauer spectroscopy measurements, these moments undergo long-range magnetic ordering below TN1 = 118 K, initially adopting a magnetic structure with a propagation vector (1/2 - δ, 0, 1/2) (0 ≤ δ ≤ 0.1) which is incommensurate with the nuclear structure and described in the Pbam1 '( a01/2)000 s magnetic superspace group, until at 92 K ( TINC) there is a first order lock-in transition to a structure in which these Fe2 moments form a magnetic structure with a propagation vector (1/2, 0, 1/2) which may be modeled using a 2 a × b × 2 c expansion of the nuclear cell in space group 36.178 B a b21 m (BNS notation). Below TN2 = 52 K the remaining third of the Fe2+ moments (Fe1 site) which are in a compressed trans-[FeSe4O2] octahedral environment undergo long-range ordering, as is evident from the magnetometry, the Mössbauer spectra, and the appearance of new magnetic Bragg peaks in the neutron diffractograms. The ordering of the second set of moments on the Fe1 sites results in a slight reorientation of the majority moments on the Fe2 sites. The magnetic structure at 1.5 K is described by a 2 a × 2 b × 2 c expansion of the nuclear cell in space group 9.40 I a b (BNS notation).