Synthesis and characterisation of the quaternary cyanamide Li 2 MgSn 2 (NCN) 6

The Mixed Transition-Metal Cyanamide MnCr2(NCN)4

The ternary transition-metal cyanamide MnCr2(NCN)4 was synthesized by a solid-state metathesis reaction between MnCl2, CrCl3, and ZnNCN. Powder X-ray diffraction reveals that MnCr2(NCN)4 adopts an orthorhombic [NiAs]-derived structure with Pbcn symmetry, featuring a hexagonally close-packed array of NCN2- with metal cations in 3/4 of the octahedral interstitial holes. The question of cation order was addressed via the combinatorial use of X-ray powder diffraction, neutron powder diffraction, electron diffraction, and HAADF-STEM measurements. These studies support an average structure with complete ordering of Mn2+ and Cr3+ in single and double corrugated rows of like cations, respectively. HAADF images, however, suggest locally a degree of cation disorder, especially at the manganese site. UV-vis and XANES measurements confirm the assignment of divalent manganese and trivalent chromium, whereas IR spectroscopy reveals the presence of cyanamide-type NCN2- moieties with a degree of single and triple bond character due to the asymmetric coordination that is distorted away from the regular trigonal prismatic coordination encountered in the CoNCN aristotype due to vacancy and cation order. Finally, SQUID magnetometry unearths predominantly antiferromagnetic interactions with a transition to what appears to be a ferrimagnetically ordered state below 160 K.

Synthesis, characterization and chemical bonding analysis of the quaternary cyanamides Li2MnHf2(NCN)6 and Li2MnZr2(NCN)6

Li2MnHf2(NCN)6 and Li2MnZr2(NCN)6 were prepared via solid-state metathesis reactions either via a more exothermic direct reaction between Li2NCN, MnCl2 and HfCl4 or a milder two-step reaction in which ternary Li2Zr(NCN)3 was first prepared and subsequently reacted with MnF2. Their crystal structures were determined from powder X-ray diffraction data and found to crystallize isotypically in low-symmetry variants of the [NiAs]-type MNCN structure with P3̄1m symmetry and comprise corundum-like [T2(NCN)3]2+ layers (T = Hf4+, Zr4+) alternating with [Li2Mn(NCN)3]2- layers. In-depth chemical bonding analysis was undertaken using LOBSTER to calculate the Löwdin charges which reveal significant differences in covalency between the two metal layers that is also reflected in the crystal orbital bond indices (COBI) of the metal-nitrogen bonds as well as the carbon-nitrogen bonds that show distinct single and triple bond character, which is also evident from infrared spectroscopy measurements. A geometric analysis of all known quaternary cyanamides with the general formula A2MT2(NCN)6 (A: an alkali metal, M: a divalent metal and T: a tetravalent metal) demonstrates the adaptation of the NCN unit to cation size differences, expressed as total distortion δtotal, by tilting away from the stacking axis. This tilting impacts the octahedral environment of the three different metal sites causing a distortion, quantified by means of the quadratic elongation λoct, revealing that the divalent and alkali metal sites are strongly dependent on δtotal whilst the tetravalent site is less influenced by the total distortion. Electronic structure calculations reveal Li2MnHf2(NCN)6 to have an indirect band gap with a wide band gap of approximately 2.4 eV, in good agreement with the measured value of 2.1 eV. Furthermore, SQUID magnetometry measurements reveal predominantly antiferromagnetic interactions, but no transition to a long-range ordered state, presumably as a result of the magnetic dilution of the octahedral site, in which only 1/6 of the interstices are occupied by paramagnetic cations.

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.

NiAs-derived cyanamide (carbodiimide) structures – a group-theoretical view

The cyanamide and carbodiimide anions are complex nitrogen-derived one-dimensional species of the type NCN2− (hence, resembling O2− but more covalently bonding) that form a huge number of salt-like phases with a variety of metal cations stemming from the whole Periodic Table. Depending on the coloring (binary, ternary and quaternary salts are known), the cationic size and charge as well as covalent contributions, different distortion (tilting in particular) and/or vacancy ordering variants of cyanamides/carbodiimides occur. Herein we summarize those cyanamide/carbodiimide structures that derive from the aristotype NiAs. The crystal chemistry is discussed on the basis of group-subgroup schemes (Bärnighausen trees).

Ion transport mechanism in anhydrous lithium thiocyanate LiSCN Part I: ionic conductivity and defect chemistry

This work reports on the ion transport properties and defect chemistry in anhydrous lithium thiocyanate Li(SCN), which is a pseudo-halide Li⁺ cation conductor. An extensive doping study was conducted, employing magnesium, zinc and cobalt thiocyanate as donor dopants to systematically vary the conductivity and derive a defect model. The investigations are based on impedance measurements and supported by other analytical techniques such as X-ray powder diffraction (XRPD), infrared (IR) spectroscopy, and density functional theory (DFT) calculations. The material was identified as Schottky disordered with lithium vacancies being the majority mobile charge carriers. In the case of Mg²⁺ as dopant, defect association with lithium vacancies was observed at low temperatures. Despite a comparably low Schottky defect formation enthalpy of (0.6 ± 0.3) eV, the unexpectedly high lithium vacancy migration enthalpy of (0.89 ± 0.08) eV distinguishes Li(SCN) from the chemically related lithium halides. A detailed defect model of Li(SCN) is presented and respective thermodynamic and kinetic data are given. The thiocyanate anion (SCN)^- has a significant impact on ion mobility due to its anisotropic structure and bifunctionality in forming both Li-N and Li-S bonds. More details about the impact on ion dynamics at local and global scale, and on the defect chemical analysis of the premelting regime at high temperatures are given in separate publications (Part II and Part III).