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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.

In the dense metal-organic framework Na[Mn(HCOO)$_3$], Mn$^{2+}$ ions ($S=\frac{5}{2}$) occupy the nodes of a `trillium' hyperkagome net. We show that this material exhibits a variety of behaviour characteristic of geometric frustration: the N\'eel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the N\'eel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at $\frac{1}{3}$-saturation magnetisation. We demonstrate that a simple nearest-neighbour Heisenberg antiferromagnet model accounts quantitatively for each observation, and hence Na[Mn(HCOO)$_3$] is the first experimental realisation of this model on the trillium net. We develop a mapping between this trillium model and that on the two-dimensional Shastry-Sutherland lattice, and demonstrate how both link geometric frustration within the classical spin liquid regime to a strong magnetocaloric response at low fields.

Intercalation of alkali and alkaline earth metals into ZrSe3 via soft chemical routes injects electrons and has a significant effect on the selenide-selenide bonding. K, Rb and Cs intercalates of ZrSe3 prepared at low temperatures (-78 °C) from metal ammonia solutions contrast with related polymorphs obtained at high temperature (850 °C). KxZrSe3 synthesised at low temperatures crystallises in orthorhombic Cmc21, while the polymorph obtained at high temperatures crystallises in Immm. The two structures prepared under drastically different conditions differ by relative shifting of ZrSe3 layers. In contrast, CsxZrSe3 shows the Immm polymorph at low temperature and the Cmc21 polymorph at high temperatures, while a single RbxZrSe3 polymorph in Immm is formed at both temperatures. Intercalation of Ca from liquid ammonia facilitates the co-intercalation of the solvent because of the strong solvation of Ca2+ . This compound has severe faulting due to the flexibility in the relative shifts of adjacent ZrSe3 layers.

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., New lithium amide/ammonia intercalates of Bi2Se3 are revealed using in situ X-ray powder diffraction.

The development of sustainable and efficient cryogenic cooling materials is currently the subject of extensive research, with the aim of relieving the dependence of current low-temperature cooling methods on expensive and nonrenewable liquid helium. One potential method to achieve this is the use of materials demonstrating the magnetocaloric effect, where the cycling of an applied magnetic field leads to a net cooling effect due to changes in magnetic entropy upon application and removal of an external magnetic field. This study details the synthesis and characterization of a Ln

We report the synthesis, crystal structure, thermal response, and electrochemical behavior of the Prussian blue analogue (PBA) K

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., Oxidative deintercalation of Cu is used to control the magnetic ordering in a layered Mn oxide telluride, and the system is contrasted with the inert Co analogue and other related compounds.

Control over the spatial distribution of components in metal–organic frameworks has potential to unlock improved performance and new behaviour in separations, sensing and catalysis. We report an unprecedented single-step synthesis of multi-component metal–organic framework (MOF) nanoparticles based on the canonical ZIF-8 (Zn) system and its Cd analogue, which form with a core–shell structure whose internal interface can be systematically tuned. We use scanning transmission electron microscopy, X-ray energy dispersive spectroscopy and a new composition gradient model to fit high-resolution X-ray diffraction data to show how core–shell composition and interface characteristics are intricately controlled by synthesis temperature and reaction composition. Particle formation is investigated by in situ X-ray diffraction, which reveals that the spatial distribution of components evolves with time and is determined by the interplay of phase stability, crystallisation kinetics and diffusion. This work opens up new possibilities for the control and characterisation of functionality, component distribution and interfaces in MOF-based materials., Core–shell metal–organic framework nanoparticles have been synthesised in which the internal interface and distribution of components is found to be highly tunable using simple variations in reaction conditions.

For magnesium ion batteries (MIBs) to be used commercially, new cathodes must be developed that show stable reversible Mg intercalation. VS4 is one such promising material, with vanadium and disulfide anions [S2]2- forming one-dimensional linear chains, with a large interchain spacing (5.83 Å) enabling reversible Mg insertion. However, little is known about the details of the redox processes and structural transformations that occur upon Mg intercalation and deintercalation. Here, employing a suite of local structure characterization methods including X-ray photoelectron spectroscopy (XPS), V and S X-ray absorption near-edge spectroscopy (XANES), and 51V Hahn echo and magic-angle turning with phase-adjusted sideband separation (MATPASS) NMR, we show that the reaction proceeds via internal electron transfer from V4+ to [S2]2-, resulting in the simultaneous and coupled oxidation of V4+ to V5+ and reduction of [S2]2- to S2-. We report the formation of a previously unknown intermediate in the Mg-V-S compositional space, Mg3V2S8, comprising [VS4]3- tetrahedral units, identified by using density functional theory coupled with an evolutionary structure-predicting algorithm. The structure is verified experimentally via X-ray pair distribution function analysis. The voltage associated with the competing conversion reaction to form MgS plus V metal directly is similar to that of intermediate formation, resulting in two competing reaction pathways. Partial reversibility is seen to re-form the V5+ and S2- containing intermediate on charging instead of VS4. This work showcases the possibility of developing a family of transition metal polychalcogenides functioning via coupled cationic-anionic redox processes as a potential way of achieving higher capacities for MIBs.

Sr

Intercalation of lithium and ammonia into the layered semiconductor Bi2Se3 proceeds via ahyperextended (by > 60 %) ammonia-rich intercalate, to eventually produce a layered compoundwith lithium amide intercalated between the bismuth selenide layers which offer scope for furtherchemical manipulation.

Control over the spatial distribution of components in metal–organic frameworks has potential to unlock improved performance and new behaviour in separations, sensing and catalysis. We report an unprecedented single-step synthesis of multi-component metal–organic framework (MOF) nanoparticles, which form with a core–shell structure whose internal interface can be systematically tuned. We use scanning transmission electron microscopy, X-ray energy dispersive spectroscopy and a new composition gradient model to fit high-resolution X-ray diffraction data to show how core–shell composition and interface characteristics are intricately controlled by synthesis temperature and reaction composition. Particle formation is investigated by in situ X-ray diffraction, which reveals that the spatial distribution of components evolves with time and is determined by the interplay of phase stability, crystallisation kinetics and diffusion. This work opens up new possibilities for the control and characterisation of functionality, component distribution and interfaces in MOF-based materials.

For magnesium ion batteries (MIB) to be used commercially, new cathodes must be developed that show stable reversible Mg intercalation. VS4 is one such promising material, with vanadium and disulphide anions [S2]2- forming one dimensional linear chains, with a large interlayer spacing (5.83 Å) enabling Mg insertion. However, little is known about the details of the redox processes and structural transformations that occur upon Mg intercalation and deintercalation of VS4. Here we use a suite of local structure characterization methods including XPS, V and S X-ray Absorption Near Edge Spectroscopy and 51V Hahn-Echo and Magic Angle Turning with Phase Adjusted Sideband Separation NMR to elucidate the complex electrochemical reaction pathways. We show that the reaction proceeds via internal electron transfer from V4+ to [S2]2, resulting in the simultaneous and coupled oxidation of V4+ to V5+ and reduction of [S2]2- to S2-. We report the formation of a previously unknown intermediate in the Mg-V-S compositional space, Mg3V2S8, which is made of [VS4]3- tetrahedral units and identified using an evolutionary structure predicting algorithm and verified experimentally via X-ray Pair Distribution Function analysis. Subsequent magnesiation gives rise to the reduction of V5+ towards V4+. Further magnesiation sees conversion to MgS plus V metal; this reaction potential is close to the conversion potential of VS4 to Mg3V2S8, leading to competing reaction pathways. Demagnesiation results in the reformation of the V5+, S2- containing intermediate instead of VS4. This work showcases the possibility of developing a family of transition metal polychalcogenides functioning via anionic as well as combined cationic-anionic redox processes, as a potential way of achieving higher capacities for MIBs.

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li1-xFex(OH)Fe1-ySe (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 synthesised 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 emphasised by the demonstration that reductive post-synthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li1-xFex(OH) reservoir layer to fill vacancies in the selenide layer, 7 pages supplementary material available from Journal, Inorganic Chemistry 2015

The chemical accessibility of the Ce

The structures of two new oxide chalcogenide phases, Sr

Solid solutions between the known compounds Ca2FeO3CuCh and Sr2FeO3CuCh (Ch ​= ​S, Se) in which there are two fairly similar sites (8 and 9 coordinate) for the alkaline earth cations are not attainable under standard high temperature solid state syntheses under thermodynamic control. Instead compounds with greater condensation of FeO5 square pyramids form as these afford one 8-coordinate site and one 12-coordinate site for the alkaline earths which is better suited to the size-mismatched cations in the compounds Sr3-xCaxFe2O5Cu2Ch2 (Ch ​= ​S, Se; x ​= ​1, 2). Sr2CaFe2O5Cu2S2, SrCa2Fe2O5Cu2S2, Sr2CaFe2O5Cu2Se2 and SrCa2Fe2O5Cu2Se2 all crystallise in the tetragonal space group I4/mmm with two formula units in the unit cell with the crystal structure first described for Sr3Fe2O5Cu2S2. Oxide slabs composed of vertex-sharing FeO5 square pyramids are separated by Cu2Ch2 anti-fluorite-type layers. The larger Sr2+ ions have a strong preference for the 12-coordinate site in the oxide slabs, while Ca2+ cations dominate the 8-coordinate sites separating the oxide and chalcogenide slabs. Powder neutron diffraction reveals that all the compounds display antiferromagnetic long range ordering of the Fe3+ moments with ordering temperatures well above room temperature and exceeding 526 ​K in the case of Ca2SrFe2O5Cu2Se2.

The structural complexity of the antiferromagnetic oxide selenide CaFeSeO is described. The compound contains puckered FeSeO layers composed of FeSe2O2 tetrahedra sharing all their vertexes. Two polymorphs coexist that can be derived from an archetype BaZnSO structure by cooperative tilting of the FeSe2O2 tetrahedra. The polymorphs differ in the relative arrangement of the puckered layers of vertex-linked FeSe2O2 tetrahedra. In a noncentrosymmetric Cmc21 polymorph (a = 3.89684(2) Å, b = 13.22054(8) Å, c = 5.93625(2) Å) the layers are related by the C-centering translation, while in a centrosymmetric Pmcn polymorph, with a similar cell metric (a = 3.89557(6) Å, b = 13.2237(6) Å, c = 5.9363(3) Å), the layers are related by inversion. The compound shows long-range antiferromagnetic order below a Neél temperature of 159(1) K with both polymorphs showing antiferromagnetic coupling via FeOFe linkages and ferromagnetic coupling via FeSeFe linkages within the FeSeO layers. The magnetic susceptibility also shows evidence for weak ferromagnetism which is modeled in the refinements of the magnetic structure as arising from an uncompensated spin canting in the noncentrosymmetric polymorph. There is also a spin glass component to the magnetism which likely arises from the disordered regions of the structure evident in the transmission electron microscopy.

Polycrystalline samples of the monoclinic perovskites CaLa2Ni2-xCuxWO9 (x ​= ​0.25 and 0.5) have been prepared and characterised by neutron diffraction and magnetometry. The Ni2+, Cu2+ and W6+ cations are partially ordered over two crystallographically-distinct six-coordinate sites such that one is ~63% occupied by W6+ and both are occupied by Ni2+ and Cu2+ in a ratio of (2-x):x. The composition x ​= ​0.25 behaves as a spin glass below 35 ​K whereas x ​= ​0.5 is ferrimagnetic below 120 ​K with an ordered moment of 0.774(5) μB per magnetic cation. The atomic moments order in a G-type pattern and the imbalance in the number of magnetic cations on the two sites results in a net magnetisation.

CaKFe4As4 (1144) is a unique stoichiometric iron-based superconductor which harbours high upper critical fields and large critical current densities. In this work, we describe a study to optimize the synthesis conditions of stoichiometric polycrystalline samples of CaKFe4As4 and asses their structural, magnetic and transport properties. The samples were prepared over a wide temperature range (900-1100°C) and the pure phase formation is centered around 955°C. Outside this temperature region, impurity phases of KFe2As2 and CaFe2As2 can also form. Magnetic susceptibility and resistivity measurements establish that the critical temperature reaches ~34 K for the optimum synthesis conditions and the critical current reaches 2 × 104 A-cm−2. The post-annealing process demonstrates the stability of the 1144 phase up to 500°C, however, under higher temperature annealing, phase degradation occurs. Our study indicates that the formation of phase-pure 1144 occurs over a much narrower window and its highly prone to multi-phase formation as compared with the 122 family. As a result, the superconducting properties are enhanced for the pure 1144 phase but they are likely to be affected by the inter and intra-granular behaviour originating from the microstructural nature of polycrystalline CaKFe4As4, similar to other iron-based superconductors. Based on our study, we construct the phase diagram for polycrystalline 1144 and compared it with that reported for 1144 single crystal.

Sr

The effect of the isovalent substitution of Sr{sup 2+} by Ca{sup 2+} on the structure and superconducting properties of Sr{sub 2−x}Ca{sub x}VO{sub 3}FeAs is described in the compositional range 0≤x≤0.5. SQUID magnetometry measurements reveal that after an initial increase in T{sub c}, which is maximised at 29.5 K in Sr{sub 1.95}Ca{sub 0.05}VO{sub 3}FeAs, a rapid suppression of superconductivity is observed with increasing x. XANES spectra of Sr{sub 2−x}Ca{sub x}VO{sub 3}FeAs collected on the Fe and V absorption K-edges show that the position of both edges are invariant with composition within the experimental uncertainty. A combination of synchrotron X-ray powder diffraction and neutron powder diffraction techniques is used to rationalise the observed changes in T{sub c} with x, in terms of changes to the structure of the FeAs layer upon partial Ca substitution. These findings demonstrate that superconductivity in the Fe-based superconductors is extremely sensitive to the crystal structure with T{sub c} maximised in samples with regular FeAs{sub 4}-tetrahedra. - Graphical abstract: Superconducting transition temperature is controlled by structural parameters in Sr{sub 2−x}Ca{sub x}VO{sub 3}FeAs. - Highlights: • Substitution of Sr by Ca in the superconductor Sr{sub 2}VO{sub 3}FeAs is isovalent. • Relationship between superconducting T{sub c} and structural parameters ismore » demonstrated. • Linear dependence of T{sub c} on structural parameters rather than composition.« less

Lithiation of hydrothermally synthesized Li

The superconducting properties of a recently discovered high-Tc superconductor, Sr/ammonia-intercalated FeSe, have been measured using pulsed magnetic fields down to 4.2 K and muon spin spectroscopy down to 1.5 K. This compound exhibits intrinsic disorder resulting from random stacking of the FeSe layers along the c axis that is not present in other intercalates of the same family. This arises because the coordination requirements of the intercalated Sr and ammonia moieties imply that the interlayer stacking (along c) involves a translation of either a/2 or b/2 that locally breaks tetragonal symmetry. The result of this stacking arrangement is that the Fe ions in this compound describe a body-centered-tetragonal lattice in contrast to the primitive arrangement of Fe ions described in all other Fe-based superconductors. In pulsed magnetic fields, the upper critical field Hc2 was found to increase on cooling with an upward curvature that is commonly seen in type-II superconductors of a multiband nature. Fitting the data to a two-band model and extrapolation to absolute zero gave a maximum upper critical field μ0Hc2(0) of 33(2)T. A clear superconducting transition with a diamagnetic shift was also observed in transverse-field muon measurements at Tc≈36.3(2)K. These results demonstrate that robust superconductivity in these intercalated FeSe systems does not rely on perfect structural coherence along the c axis.

Intercalation of potassium into the molecular framework silver hexacyanoferrate occurs with remarkably small volume strain., Reductive intercalation of potassium within the molecular framework Ag3[Fe(CN)6] gives rise to a volume strain that is an order of magnitude smaller than is typical for common ion-storage materials. We suggest that framework flexibility might be exploited as a general strategy for reducing cycling strain in battery and ion-storage materials.

57Fe-Mossbauer spectra of superconducting beta-FeSe, the Li/NH3 intercalate product and a subsequent sample of this intercalate treated with moist He gas have been measured in temperature range 4.7 - 290 K. A correlation is established between hyperfine parameters and critical temperature Tc in these phases. A strong increase of isomer shift upon intercalation is explained by a charge transfer from the Li/NH3 intercalate to the FeSe layers resulting in an increase of Tc up to 42 K. A significant decrease of the quadrupole splitting above 240 K has been attributed to diffusive motion of Li+ ions within the interlamellar space., Comment: 6 pages, 5 figures, 1 table

The title compounds (0 ≤ x ≤ 0.5) are synthesized from stoichiometric mixtures of SrO, CaO, V, Fe2O3, Fe, and As (alumina crucibles, 1100 °C, 48 h).

The development of a technique for following in situ the reactions of solids with alkali metal/ammonia solutions, using time-resolved X-ray diffraction methods, reveals high-temperature superconducting ammonia-rich intercalates of iron selenide which reversibly absorb and desorb ammonia around ambient temperatures.

The recent discovery of high temperature superconductivity in a layered iron arsenide has led to an intensive search to optimize the superconducting properties of iron-based superconductors by changing the chemical composition of the spacer layer that is inserted between adjacent anionic iron arsenide layers. Until now, superconductivity has only been found in compounds with a cationic spacer layer consisting of metal ions: Li+, Na+, K+, Ba2+ or a PbO-type or perovskite-type oxide layer. Electronic doping is usually necessary to control the fine balance between antiferromagnetism and superconductivity. Superconductivity has also been reported in FeSe, which contains neutral layers similar in structure to those found in the iron arsenides but without the spacer layer. Here we demonstrate the synthesis of Lix(NH2)y(NH3)1-yFe2Se2 (x ~0.6 ; y ~ 0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far and four times higher at ambient pressure than the transition temperature, Tc, of the parent Fe1.01Se. We have determined the crystal structure using neutron powder diffraction and used magnetometry and muon-spin rotation data to determine the superconducting properties. This new synthetic route opens up the possibility of further exploitation of related molecular intercalations in this and other systems in order to greatly optimize the superconducting properties in this family., Comment: 9 pages, 4 figures; updated and with supplementary information

Magnetic fluctuations in the molecular-intercalated FeSe superconductor Li{x}(ND2){y}(ND3){1-y}Fe2Se2 (Tc = 43K) have been measured by inelastic neutron scattering from a powder sample. The strongest magnetic scattering is observed at a wave vector Q ~ 1.4 A^{-1}, which is not consistent with the (pi,0) nesting wave vector that characterizes magnetic fluctuations in several other iron-based superconductors, but is close to the (pi, pi/2) position found for A{x}Fe{2-y}Se2 systems. At the energies probed (~ 5kB Tc), the magnetic scattering increases in intensity with decreasing temperature below Tc, consistent with the superconductivity-induced magnetic resonance found in other iron-based superconductors., Comment: 5 pages, 4 figures