K-Ion Slides in Prussian Blue Analogues

We study the phenomenology of cooperative off-centering of K+ ions in potassiated Prussian blue analogues (PBAs). The principal distortion mechanism by which this off-centering occurs is termed a "K-ion slide", and its origin is shown to lie in the interaction between local electrostatic dipoles that couple through a combination of electrostatics and elastic strain. Using synchrotron powder X-ray diffraction measurements, we determine the crystal structures of a range of low-vacancy K2M[Fe(CN)6] PBAs (M = Ni, Co, Fe, Mn, Cd) and establish an empirical link between composition, temperature, and slide-distortion magnitude. Our results reflect the common underlying physics responsible for K-ion slides and their evolution with temperature and composition. Monte Carlo simulations driven by a simple model of dipolar interactions and strain coupling reproduce the general features of the experimental phase behavior. We discuss the implications of our study for optimizing the performance of PBA K-ion battery cathode materials and also its relevance to distortions in other, conceptually related, hybrid perovskites.

Lithium Intercalation into the Excitonic Insulator Candidate Ta2NiSe5

A new reduced phase derived from the excitonic insulator candidate Ta₂NiSe₅ has been synthesized via the intercalation of lithium. LiTa₂NiSe₅ crystallizes in the orthorhombic space group Pmnb (no. 62) with lattice parameters a = 3.50247(3) Å, b = 13.4053(4) Å, c = 15.7396(2) Å, and Z = 4, with an increase of the unit cell volume by 5.44(1)% compared with Ta₂NiSe₅. Significant rearrangement of the Ta-Ni-Se layers is observed, in particular a very significant relative displacement of the layers compared to the parent phase, similar to that which occurs under hydrostatic pressure. Neutron powder diffraction experiments and computational analysis confirm that Li occupies a distorted triangular prismatic site formed by Se atoms of adjacent Ta₂NiSe₅ layers with an average Li-Se bond length of 2.724(2) Å. Li-NMR experiments show a single Li environment at ambient temperature. Intercalation suppresses the distortion to monoclinic symmetry that occurs in Ta₂NiSe₅ at 328 K and that is believed to be driven by the formation of an excitonic insulating state. Magnetometry data show that the reduced phase has a smaller net diamagnetic susceptibility than Ta₂NiSe₅ due to the enhancement of the temperature-independent Pauli paramagnetism caused by the increased density of states at the Fermi level evident also from the calculations, consistent with the injection of electrons during intercalation and formation of a metallic phase.

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 Sr₂MnO₂Cu_1.5Ch₂ (Ch = S, Se) into Cu-deintercalated phases where antifluorite type [Cu_1.5Ch₂]^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.

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 D_4h symmetry, adopt a high-spin configuration and carry localized magnetic moments which order antiferromagnetically below ∼160 K in a √2a × √2a × 2c expansion of the nuclear cell with an ordered moment of 1.31(2) μ_B 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(Se_1-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.

Uncovering the Interplay of Competing Distortions in the Prussian Blue Analogue K2Cu[Fe(CN)6]

We report the synthesis, crystal structure, thermal response, and electrochemical behavior of the Prussian blue analogue (PBA) K₂Cu[Fe(CN)₆]. From a structural perspective, this is the most complex PBA yet characterized: its triclinic crystal structure results from an interplay of cooperative Jahn-Teller order, octahedral tilts, and a collective "slide" distortion involving K-ion displacements. These different distortions give rise to two crystallographically distinct K-ion channels with different mobilities. Variable-temperature X-ray powder diffraction measurements show that K-ion slides are the lowest-energy distortion mechanism at play, as they are the only distortion to be switched off with increasing temperature. Electrochemically, the material operates as a K-ion cathode with a high operating voltage and an improved initial capacity relative to higher-vacancy PBA alternatives. On charging, K⁺ ions are selectively removed from a single K-ion channel type, and the slide distortions are again switched on and off accordingly. We discuss the functional importance of various aspects of structural complexity in this system, placing our discussion in the context of other related PBAs.

Geometric Frustration on the Trillium Lattice in a Magnetic Metal-Organic Framework

In the dense metal-organic framework Na[Mn(HCOO)_{3}], Mn^{2+} ions (S=5/2) occupy the nodes of a "trillium" net. We show that the system is strongly magnetically frustrated: the Néel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the Néel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at 1/3-saturation magnetization. A simple model of nearest-neighbor Heisenberg antiferromagnetic and dipolar interactions accounts quantitatively for all observations, including an unusual 2-k magnetic ground state. We show that the relative strength of dipolar interactions is crucial to selecting this particular ground state. Geometric frustration within the classical spin liquid regime gives rise to a large magnetocaloric response at low applied fields that is degraded in powder samples as a consequence of the anisotropy of dipolar interactions.

Investigations of the Magnetocaloric and Thermal Expansion Properties of the Ln3(adipate)4.5(DMF)2 (Ln = Gd-Er) Framework Series

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 Ln₃(adipate)_4.5(DMF)₂ series (where Ln = Gd–Er) of metal–organic framework (MOF) materials incorporating a flexible adipate ligand and their associated magnetocaloric and thermal expansion properties. The magnetocaloric performance of the Gd₃(adipate)_4.5(DMF)₂ material was found to exhibit the highest magnetic entropy changes of the series, with a peak entropy change of 36.4 J kg^–1 K^–1 for a 5-0 T field change at a temperature of 2 K, which is suited for ultra-low-temperature cooling applications. Thermal expansion properties were also investigated within these materials, demonstrating modest negative and large positive thermal expansion identified along the different crystallographic axes within the MOF structures over a 100–300 K temperature range that demonstrated the novel mechanical properties of these adipate framework structures.

The magnetocaloric and thermal expansion properties of the Ln₃(adipate)_4.5(DMF)₂ metal−organic framework series (where Ln = Gd−Er) are examined using a combination of magnetometry and crystallographic techniques. The 1D Ln chains within these materials demonstrate magnetocaloric effects, with the magnetic entropy changes maximized in the Gd phase, while the flexible adipate linkers within the structure lead to anisotropic thermal expansion properties that are correlated with Ln cation size.

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.

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

Single-step synthesis and interface tuning of core-shell metal-organic framework nanoparticles

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.

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 T_{N1_Sr}, of 600(10) K (Sr₂CrO₂Cr₂As₂) and T_{N1_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) μ_B (for Sr₂CrO₂Cr₂As₂) and 3.30(1) μ_B (for Ba₂CrO₂Cr₂As₂)) are larger than those in the CrAs layers (2.68(1) μ_B for Sr₂CrO₂Cr₂As₂ and 2.298(8) μ_B 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 T_N= 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.

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, Sr₂MnO₂Cu_1.82(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 Sr₂MnO₂Cu_1.58(2)Te₂ (Mn oxidation state of +2.42(2)) in which ferromagnetic interactions between Mn ions result from Mn^2+/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.

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 × √2 a × 2 c expansion of the nuclear cell. The ordered moment of 0.39(6) μ_B 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 T_N1 = 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 ( T_INC) 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 T_N2 = 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).

Complex Microstructure and Magnetism in Polymorphic CaFeSeO

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 Fe-O-Fe linkages and ferromagnetic coupling via Fe-Se-Fe 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.

Zero-strain reductive intercalation in a molecular framework

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.

Enhancement of the superconducting transition temperature of FeSe by intercalation of a molecular spacer layer

The 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 between adjacent anionic iron arsenide layers. Superconductivity has been found in iron arsenides with cationic spacer layers consisting of metal ions (for example, Li(+), Na(+), K(+), Ba(2+)) or PbO- or perovskite-type oxide layers, and also in Fe(1.01)Se (ref. 8) with neutral layers similar in structure to those found in the iron arsenides and no spacer layer. Here we demonstrate the synthesis of Li(x)(NH(2))(y)(NH(3))(1-y)Fe(2)Se(2) (x~0.6; y~0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer between FeSe layers, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far. 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 to greatly optimize the superconducting properties in this family.