Synthesis, crystal structures and semiconducting properties of new hexacyanidometallates.

We describe the synthesis, crystal structure and semiconducting properties of a number of hexacyanidometallates with the formula A2[MFe(CN)6]·xH2O (A = Na, K; M = Mg, Ca, Sr and Ba). All crystal structures were studied via single-crystal or powder X-ray diffraction. The unexpectedly low-symmetric structures in these ferrocyanides are described and contrasted with analogous transition-metal compounds which have been reported to be strictly or nearly cubic. The amount of crystal water in the structure for powder samples was determined by the thermogravimetric analysis (TGA), supported by IR and Raman spectroscopy. Electronic-structure calculations of K2[MgFe(CN)6] and K2[CaFe(CN)6] are compared with experimental UV-Vis measurements. The large band gaps by advanced theory indicate that the smaller experimental band gaps are due to surface effects of impurity states. Mott-Schottky curves of K2[MgFe(CN)6], K2[CaFe(CN)6] and K2[BaFe(CN)6]·3H2O exhibit positive slopes, which characterizes these compounds as n-type semiconductors.

Synthetic Engineering in Na2MSn2(NCN)6 (M = Mn, Fe, Co, and Ni) Based on Electronic Structure Theory.

Quaternary transition metal cyanamides Na2MSn2(NCN)6 with M = Mn, Fe, Co, and Ni were prepared via solid-state metathesis reactions between Na2Sn(NCN)3 and binary transition metal fluorides MF2 in a 2:1 molar ratio. All phases crystallize isotypically in [NiAs]-derived structures (P3̅1m) with inter- and intra-layer cation ordering over the octahedral sites. This leads to a highly asymmetric coordination of the NCN2- anion, resulting in a strong degree of cyanamide character, which is confirmed via IR measurements. Intriguingly, the optical properties of Na2MSn2(NCN)6 phases change markedly as the nature of the divalent transition metal is varied, and UV-vis measurements evidence a band gap reduction from Mn (3.43 eV) via Fe (1.90 eV) to Co (1.75 eV), which broadly mirrors the DFT+U calculated energetic interval from the Fermi level to the unoccupied 3d states. Mott-Schottky analysis then goes on to characterize Na2FeSn2(NCN)6 and Na2CoSn2(NCN)6 as n-type semiconductors with flat-band potentials of 0.46 and -0.24 eV, respectively, vs RHE. This study demonstrates the utility of transition metal substitutions, within a flexible cyanamide framework, to electronically tune this growing family of pseudo-oxides.

Synthesis, Crystal Structure, Symmetry Relationships, and Electronic Structure of Bismuth Carbodiimide Bi2(NCN)3 and Its Ammonia Adduct Bi2(NCN)3·NH3.

Bi2(NCN)3, the first binary pnictogen carbodiimide, and its ammonia derivative Bi2(NCN)3·NH3 have been prepared via nonaqueous liquid-state low-temperature ammonolysis. The crystal structure of Bi2(NCN)3·NH3 in space group Cc solved via single-crystal X-ray diffraction corresponds to a two-dimensional-like motif with layers of NCN2- alternating with honeycomb-like layers of edge-sharing distorted BiN6 octahedra, half of which are also coordinated by molecular ammonia occupying the octahedral holes. By contrast, Bi2(NCN)3 adopts a higher-symmetric C2/c structure with a single Bi position and stronger distortion but empty octahedral voids. In both cases, Bi3+ and its 6s2 lone pair are well mirrored by antibonding Bi-N interactions below the Fermi level. Density functional theory calculations reveal an exothermic reaction for the intercalation of NH3 into Bi2(NCN)3, consistent with the preferential formation of Bi2(NCN)3·NH3 in the presence of ammonia. A Bärnighausen tree shows both compounds to be hettotypic derivatives of the R3̅c M2(NCN)3 corundum structure that express highly distorted hexagonal-close-packed layers of NCN2- in order to accommodate the aspherical Bi3+ cations. Although Bi2(NCN)3 does not resemble the isovalent Bi2Se3 in forming two-dimensional layers and a topological insulator, theory suggests a driving force for the spontaneous formation of Bi2Se3/Bi2(NCN)3 sandwiches and a conducting surface state arising within the uppermost Bi2(NCN)3 layer.

Metathetic synthesis of lead cyanamide as a p-type semiconductor.

Lead cyanamide PbNCN was synthesized by solid-state metathesis between PbCl2 and Na2NCN in a 1 : 1 molar ratio, and its structure was confirmed from Rietveld refinement of X-ray data. Electronic-structure calculations of HSE06 density-functional type reveal PbNCN to be an indirect semiconductor with a band gap of 2.4 eV, in remarkable quantitative agreement with the measured value. Mott-Schottky experiments demonstrate PbNCN to be a p-type semiconductor with a flat-band potential of 2.3 eV vs. the reversible hydrogen electrode (RHE) which is commonly used to estimate the value of the valence band edge position. Moreover, thin films of powderous PbNCN were assembled into a photoelectrode for photoelectrochemical water splitting. On the example of p-type PbNCN, this study provides the first experimental evidence that MNCN compounds can be applied as photocathodes for reductive reactions in photoelectrochemical cells.

Tailoring the Surface Properties of Bi2O2NCN by in Situ Activation for Augmented Photoelectrochemical Water Oxidation on WO3 and CuWO4 Heterojunction Photoanodes.

Bismuth(III) oxide-carbodiimide (Bi2O2NCN) has been recently discovered as a novel mixed-anion semiconductor, which is structurally related to bismuth oxides and oxysulfides. Given the structural versatility of these layered structures, we investigated the unexplored photochemical properties of the target compound for photoelectrochemical (PEC) water oxidation. Although Bi2O2NCN does not generate a noticeable photocurrent as a single photoabsorber, the fabrication of heterojunctions with the WO3 thin film electrode shows an upsurge of current density from 0.9 to 1.1 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE) under 1 sun (AM 1.5G) illumination in phosphate electrolyte (pH 7.0). Mechanistic analysis and structural analysis using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM EDX) indicate that Bi2O2NCN transforms during operating conditions in situ to a core-shell structure Bi2O2NCN/BiPO4. When compared to WO3/BiPO4, the in situ electrolyte-activated WO3/Bi2O2NCN photoanode shows a higher photocurrent density due to superior charge separation across the oxide/oxide-carbodiimide interface layer. Changing the electrolyte from phosphate to sulfate results in a lower photocurrent and shows that the electrolyte determines the surface chemistry and mediates the PEC activity of the metal oxide-carbodiimide. A similar trend could be observed for CuWO4 thin film photoanodes. These results show the potential of metal oxide-carbodiimides as relatively novel representatives of mixed-anion compounds and shed light on the importance of the control over the surface chemistry to enable the in situ activation.

A new tilt and an old twist on the nickel arsenide structure-type: synthesis and characterisation of the quaternary transition-metal cyanamides A2MnSn2(NCN)6 (A = Li and Na).

In this work, we describe the synthesis and structure of the quaternary transition-metal cyanamides Na2MnSn2(NCN)6 and Li2MnSn2(NCN)6. These phases crystallise isotypically in layered structures, with P3[combining macron]1m symmetry, that comprise hexagonal close-packed arrays of NCN2- anions with metal cations in 5/6 of the octahedral holes, thereby reflecting low-symmetry modifications of the hierarchical [NiAs]-type MNCN structure. The distinct coordination requirements of the metal cations template an ordered decoration across the octahedral sites with corundum-like [Sn2(NCN)3]2+ layers alternating with [A2Mn(NCN)3]2- layers which resemble a portion of the Li2Zr(NCN)3 structure. This motif is also mirrored in the form of the NCN2- anions which adopt N-C[triple bond, length as m-dash]N2- cyanamide shapes with clear single- and triple-bond character. Distortion-mode analysis reveals the importance of K1 octahedral twist and K2 cyanamide tilt displacements in stabilising these phases, the latter of which is only accessible because of the extended nature of the NCN2- anion. These are the first examples of non-binary transition-metal cyanamides to be discovered and this study highlights how the additional flexibility of the NCN2- anion affords a novel structure-type not observed in oxide chemistry.

Band Gap Tuning in Bismuth Oxide Carbodiimide Bi2O2NCN.

Layered bismuth oxides exhibit a broad range of tunable physical properties as a result of their excellent structural versatility which facilitates compositional substitutions at both cationic and anionic positions. Here we expand this family in a new direction through the preparation of the first example of a bismuth-containing oxide carbodiimide, Bi2O2NCN, which assumes an extended variant of the anti-ThCr2Si2 structure-type adopted by Bi2O2 Ch ( Ch = Se or Te) oxide chalcogenides. Electronic structure calculations reveal the title compound to be an indirect band gap semiconductor with a band gap of approximately 1.4 eV, in good agreement with the measured value of 1.8 eV, and intermediate between that of structurally related Bi2O2S (1.12 eV) and ß-Bi2O3 (2.48 eV). Mott-Schottky experiments demonstrate Bi2O2NCN to be an n-type semiconductor with a conduction band edge position of -0.37 V vs reversible hydrogen electrode. This study highlights the pseudochalcogenide nature of the -N═C═N- carbodiimide anion, which may be substituted in place of oxide or chalcogenide anions in this and potentially other structural classes as an effective means of electronic tuning.

Room Temperature Magnetically Ordered Polar Corundum GaFeO3 Displaying Magnetoelectric Coupling.

The polar corundum structure type offers a route to new room temperature multiferroic materials, as the partial LiNbO3-type cation ordering that breaks inversion symmetry may be combined with long-range magnetic ordering of high spin d5 cations above room temperature in the AFeO3 system. We report the synthesis of a polar corundum GaFeO3 by a high-pressure, high-temperature route and demonstrate that its polarity arises from partial LiNbO3-type cation ordering by complementary use of neutron, X-ray, and electron diffraction methods. In situ neutron diffraction shows that the polar corundum forms directly from AlFeO3-type GaFeO3 under the synthesis conditions. The A3+/Fe3+ cations are shown to be more ordered in polar corundum GaFeO3 than in isostructural ScFeO3. This is explained by DFT calculations which indicate that the extent of ordering is dependent on the configurational entropy available to each system at the very different synthesis temperatures required to form their corundum structures. Polar corundum GaFeO3 exhibits weak ferromagnetism at room temperature that arises from its Fe2O3-like magnetic ordering, which persists to a temperature of 408 K. We demonstrate that the polarity and magnetization are coupled in this system with a measured linear magnetoelectric coupling coefficient of 0.057 ps/m. Such coupling is a prerequisite for potential applications of polar corundum materials in multiferroic/magnetoelectric devices.

Spin-reorientation transition in CeMnAsO.

High-resolution X-ray and neutron powder diffraction are used to reveal details of the spin-reorientation transition in the layered oxide pnictide CeMnAsO. Above 38 K, the localized moments on Mn(2+) are antiferromagnetically ordered in a checkerboard fashion within the antifluorite-type MnAs planes and are oriented perpendicular to the planes. Below 38 K, reorientation of these moments into the planes commences. This is complete by 34 K and is coincident with long-range ordering of the Ce(3+) moments. The Ce(3+) and Mn(2+) moments have an arrangement that is different in detail from that in the isostructural NdMnAsO and PrMnSbO. There is no evidence for structural distortion, as found for PrMnSbO and related Pr(3+)-containing compounds, although there is evidence for a very slight (0.025%) misfit between the magnetic and structural cells below the spin-reorientation transition. It is clarified that neutron powder diffraction methods are unable to distinguish between collinear and noncollinear arrangements of manganese and lanthanide moments when the moments have a component parallel to the MnAs planes. A proposal from computational analysis that NdMnAsO and CeMnAsO should adopt different magnetic structures on the basis of the different balances between biquadratic and antisymmetric exchange interactions should be tested using alternative methods.

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.