Band 75, Heft 1-2 - Special Issue: Dedicated to Professor Arndt Simon of the Max-Planck-Institut für Festkörperforschung, Stuttgart, on the Occasion of his 80th Birthday

Superconductivity was discovered in 1911 by Kamerlingh Onnes and Holst in mercury at the temperature of liquid helium (4.2 K). It took almost 50 years until in 1957 a microscopic theory of superconductivity, the so-called BCS theory, was developed. Since the discovery a number of superconducting materials were found with transition temperatures up to 23 K. A breakthrough in the field happened in 1986 when Bednorz and Müller discovered a new class of superconductors, the so-called cuprate high-temperature superconductors with transition temperatures as high as 135 K. This surprising discovery initiated new efforts with respect to fundamental physics, material science, and technological applications. In this brief review the basic physics of the conventional low-temperature superconductors as well as of the high-temperature superconductors are presented with a brief introduction to applications exemplified from high-power to low-power electronic devices. Finally, a short outlook and future challenges are presented, finished with possible imaginations for applications of room-temperature superconductivity.

The present contribution defines rigorously terms such as chemical resistance, chemical capacitance and exchange reactivity. Several examples of interest are presented whose treatments show the relevance of these parameters for chemistry, in particular for solid state chemistry.

Silicon and indium co-doped ZnO thin films with both high optical and electrical performances have been successfully synthesised for the first time by the technique of spray pyrolysis. We find that this co-doping strategy can achieve comparable Figures-of-Merit performances to indium zinc oxide itself, but with, importantly, a significant saving in the indium content. The properties of the co-doped films are compared with those of either single indium or silicon doping.

The three nitrides ε-TaN, δ-NbN and γ′-Mo 2 N have been synthesized electrochemically from the elements at 450°C in a molten salt mixture LiCl/KCl:Li 3 N. For all compounds the working electrode consisting of a tantalum, niobium or molybdenum foil was anodically polarized and the system was fed with dry nitrogen. The applied constant voltage was 2.5 V (for ε-TaN), 2.2 V (for δ-NbN), and 2.8 V (for γ′-Mo 2 N). Chemical analysis on N and O resulted in compositions of TaN 0.81(1) O 0.13(2) , NbN 1.17(2) O 0.28(1) and MoN 0.88(1) O 0.11(1) , respectively. Lattice parameters of ε-TaN refined by the Rietveld method are a  =  519.537(4) and c  = 291.021(3) pm. The other two nitrides crystallize in the cubic system (rocksalt type) with a  = 436.98(2) pm for δ-NbN and with a  = 417.25(2) pm for γ′-Mo 2 N.

We have performed a detailed structural analysis of several crystals from the Sb 2 Te 3 − x Se x solid solution (with x  = 0–1.55) using a combination of single-crystal and powder X-ray diffraction and high-resolution scanning transmission electron microscope imaging combined with energy-dispersive X-ray spectroscopic mapping. The experimental study was supported by quantum-chemical calculations. All compounds crystallize in the rhombohedral tetradymite structure type in which the atoms occupy three symmetrically independent crystallographic sites; the two chalcogenide sites A1 and A2, and the antimony site Sb. They form quintuple A2–Sb–A1–Sb–A2 layers perpendicular to the [001] direction, held together via A2–A2 chalcogenide-chalcogenide interactions. The results of our experimental study show unambiguously, and in agreement with quantum-chemical calculations, a preferred incorporation of selenium into the A2 position inside the layers. An unexpected increase of the van-der-Waals inter-layer distances is observed for low Se contents ( x  < 1) and discussed with the help of DFT calculations.

The in vivo oxidation of heme yields bilirubin which is further degraded to the bilirubin oxidation end products (BOXes) that are biologically highly active. To study the mode of action and fate of ( Z )-2-(4-methyl-5-oxo-3-vinyl-1,5-dihydro-2 H -pyrrol-2-ylidene)acetamide (BOX A), the Suzuki-Miyaura cross-coupling reaction allows to introduce various alkenyl- and aryl-substituents in 3-position of the ( Z )-2-(4-methyl-5-oxo-1,5-dihydro-2 H -pyrrol-2-ylidene)acetamides (BOX A-type monopyrroles). The influence of these groups on structural and NMR-spectroscopic parameters of the central monopyrrolic system is negligible. Special focus has been given to derivatives with 3-positioned aryl substituents carrying trifluoromethyl groups for future in vivo 19 F NMR studies.

The active Lewis pairs (ALPs) 2,6-Me 2 H 8 C 5 N–C(H) = C(SiMe 3 )–AlR 2 ( 1a: R =  t Bu, 1b , R =  i Bu) have strained AlC 2 N heterocycles and relatively weak Al–N bonds. They react readily with a series of organic azides R′N 3 [R′ = Ph, CH 2 C 6 H 4 (4- t Bu), t Bu, SiMe 3 , CH 2 Ph] by cleavage of the heterocycles and addition of the azides with their α -N atoms to the Al atom. The Al–N interactions result in an activation of the azide groups which insert into the C–Si bonds of the vinyl groups with their terminal γ -N atoms. Compounds with approximately planar five-membered AlCN 3 heterocycles and intact N 3 groups are formed in highly selective reactions.

The gold-rich intermetallic compounds RE Au 4 Cd 2 ( RE  = La–Nd, Sm) were synthesized from the elements in sealed tantalum ampoules. Their characterization by X-ray powder and single crystal data confirmed the tetragonal YbAl 4 Mo 2 type, space group I 4/ mmm . The basic building units are Au 4 squares (278 pm Au–Au in CeAu 4 Cd 2 ) and infinite linear cadmium chains (275 pm Cd–Cd in CeAu 4 Cd 2 ). We exemplarily studied the solid solution CeAu 4+ x Cd 2− x for x  = 0–1 up to CeAu 5 Cd. Electron diffraction patterns on a CeAu 5 Cd sample confirm the single crystal data. They give no hint for complete gold-cadmium ordering. Temperature-dependent magnetic susceptibility measurements of CeAu 4 Cd 2 , CeAu 5 Cd, PrAu 4 Cd 2 and NdAu 4 Cd 2 show stable trivalent rare earth ions and give no hint for magnetic ordering above 3 K.

The potassium oxidofluoridostannate(IV) fluoride K 5 Sn 2 OF 11 was synthesized via a solid-state route at T  = 300°C in arc-weldeded copper ampoules. It crystallizes in the orthorhombic system in the acentric space group Ama 2 (no. 40) with a unit cell volume of V  = 2.5727(4) nm 3 . The cell parameters are a  = 1758.3(2), b  = 2452.3(2) and c  = 596.7(1) pm. As a main structural motif, K 5 Sn 2 OF 11 exhibits two different kinds of dinuclear [Sn 2 OF 11 ] 5‒ units, which are embedded into a matrix consisting of potassium cations and fluoride anions. The substance was characterized by single-crystal and powder X-ray diffraction, as well as by FT-IR spectroscopy.

Binary lithium silicides play a crucial role in high energy density anode materials for rechargeable batteries. During charging processes of Si anodes Li 15 Si 4 is formed as a metastable phase which has been stabilized through Li by Mg, Zn and Al substitution. Here we investigate Li by Zn substitution in the lithium-richest phase Li 17 Si 4 and report on the particular site preference of Zn atoms since Zn is substituting Li atoms only on one out of 13 possible lithium sites. This site preference shows an interesting relation to the closely related phase Li 21 Si 5 and thus Li 17− ε − δ Zn ε Si 4 with ε  = 0.025(1) and δ  = 0.033(1) can be considered as the missing link between the structures of Li 21 Si 5 (= Li 4.20 Si) and Li 17 Si 4 (= Li 4.25 Si).

The crystal structure of monoclinic Cs 2 Zn(CN) 4 ( C 2/ c , Z  = 4) was solved and refined from high-resolution synchrotron powder diffraction data (ESRF: Swiss-Norwegian beamline). In contrast to all other known cyanides of composition A 2 M(CN) 4 with A = alkali metal and M = group 12 metal, which crystallize in cubic or rhombohedrally distorted spinel variants and thus with A + in an octahedral coordination, the Cs + cation in Cs 2 Zn(CN) 4 shows an eight-fold coordination by CN − anions of the [Zn(CN) 4 ] 2− tetrahedra. Upon heating, no phase transition is observed. Instead, a reversible melting at approx. T  = 380°C occurs.

CaFeO 2 Cl is a unique exception in the family of the iron oxyhalides. Its crystal structure is not related to the Ruddlesden-Popper phases as known for the other members, but contains layers of edge-sharing FeO 2/2 O 3/3 pyramids without Fe–Cl contacts. The iron atoms form a distorted honeycomb substructure. Magnetization measurements on single crystals show an unexpected weak anisotropy and indicate antiferromagnetic ordering of the iron magnetic moments already at room temperature. 57 Fe Mössbauer spectra confirm the trivalent oxidation state of iron and the presence of magnetic order. DFT calculations using the LDA +  U approach support a Mott-insulating antiferromagnetic ground state and indicate a Néel-type ordered antiferromagnetic state in the honeycomb layer. The band gap from optical measurements is 1.3 eV and agrees with the red-brown colour as well as with the theoretical calculations.

The reaction of uranium tetrabromide with potassium cyanide in anhydrous liquid ammonia at room temperature leads to the formation of brown crystals of [U 2 (CN) 3 (NH 3 ) 14 ] 5+ [KBr 6 ] 5−  · NH 3 . We determined the crystal structure of the compound by single crystal X-ray diffraction. To the best of our knowledge it contains the unprecedented spatially separated [KBr 6 ] 5− anion and presents the first uranium(IV) cyanide compound which forms a layer structure. The compound crystallizes in the trigonal space group P 3̅ m 1 (No. 164) with a  = 10.3246(13), c  = 8.4255(17) Å, V  = 777.8(3) Å 3 , Z  = 1 at T  = 100 K and is well described with the Niggli formula [U(CN)32(NH3)71]∞22[KBr61].$\mathop {} \limits_{\infty}^{2}{\left[ {{\rm{U}}{{({\rm{CN}})}_{{3 \over 2}}}{{({\rm{N}}{{\rm{H}}_3})}_{{7 \over 1}}}} \right]_2}\left[ {{\rm{KB}}{{\rm{r}}_{{6 \over 1}}}} \right].$

The reactions of the binary metal chlorides AgCl, RbCl and PbCl 2 at T  = 180°C in the ionic liquid [BMIm]Cl · 4AlCl 3 (BMIm = 1- n -butyl-3-methylimidazolium) yielded air-sensitive, colorless, high-quality crystals of their tetrachloridoaluminate(III) salts. Thereby, the obstructions that hinder proper crystallization of these compounds from pristine AlCl 3 melts were circumvented. X-ray diffraction on single-crystals revealed a monoclinic structure (space group P 2 1 / c ) for Ag[AlCl 4 ], which unexpectedly has a closer relation to Cu[AlCl 4 ] 2 than to Cu[AlCl 4 ]. Rb[AlCl 4 ] crystallizes in the baryte structure type (orthorhombic, Pnma ), and Pb[AlCl 4 ] 2 is isotypic to α -Sr[GaCl 4 ] 2 (orthorhombic, Pbca ).

Due to their great importance in science, technology, and the life sciences, water and ice have been extensively investigated over many years. In particular, hexagonal ice Ih has been of great interest since it is the most common form of ice, and several modifications, Ih(a), Ih(b) and Ih(c) are known, whose structural details are still under discussion. In this study, we present an alternative theoretical model, called Ih(d), for the hexagonal ice modification in space group P 6 3 / mmc (no. 194), based on first-principles calculations that have been performed using DFT-LDA, GGA-PBE, and hybrid B3LYP and PBE0 functionals.

The compound Cs[OCN] has been synthesized and its crystal structure and Raman spectrum were determined on selected single crystals. As postulated in earlier work, the title compound crystallizes isopointal to KN 3 exhibiting the space group I 4/ mcm (no. 140, Z  = 4) with the lattice parameters a  = 653.79(2) and c  = 799.42(5) pm. The Raman spectrum verified the nature of the triatomic moiety and shows the frequencies typical for an [OCN] − anion with Fermi resonance between the 2 δ and the ν sym vibrations. The undisturbed frequencies and the resulting force constants have been calculated and compared to those of other alkali metal compounds containing comparable linear triatomic anions.

Two new quaternary selenides of the α -TlSe structure type have been synthesized and characterized. Single crystal X-ray diffraction analysis has revealed that Tl 2 Ga 2 SnSe 6 crystallizes with space group I 4 /mmc , a  = 8.095(1), c  = 6.402(1) Å, with a refined composition of Tl 1− x Ga 1− y Sn y Se 2 ( x  =  y  = 0.345(5)), Z  = 4, R 1 = 0.028; wR 2 = 0.066. The crystal structure of the isostructural compound Tl 2 Ga 2 GeSe 6 has been determined by means of powder X-ray diffraction: space group I 4 /mmc , Z =  4, a =  8.0770(4), c =  6.2572(5) Å, refined composition Tl 1− x Ga 1− y Ge y Se 2 , x =  0.343(5), y =  0.35(2), ( R B(I)  =  0.084; R P  =  0.041; R Pw  =  0.058). According to their optical absorption spectra all compounds are semiconductors with relatively narrow direct band gaps of 2.15(3) and 2.05(5) eV for the Ge and Sn phase, respectively.

The fluorooxoborates Sr[B 5 O 7 F 3 ] ( Ccm 2 1 (no. 36), Z  = 4, a  = 864.777(3) pm, b  = 1001.037(4) pm, c  = 810.110(5) pm, 630 refl., 68 param., R F  = 0.061, R Bragg  = 0.051) and Sr[B 5 O 7 F 3 ]:Eu 2+ were synthesized via solid-state reactions from Sr(BF 4 ) 2 and B 2 O 3 . Doping was achieved by adding EuF 3 , which was reduced to its divalent state during the synthesis. Sr[B 5 O 7 F 3 ] was investigated by infrared spectroscopy. Photoluminescence spectra of the doped compound revealed an Eu 2+ 5 d -4 f emission in the UV regime peaking at 371 nm with a full width at half maximum (FWHM) of 21 nm upon excitation at 239 nm, which proves a weak coordination of europium by the anionic fluorooxoborate network.

The quaternary rare earth nickel aluminum germanide series RE NiAl 4 Ge 2 ( RE  = Y, Sm, Gd–Tm, Lu) has been extended by several members. The compounds were synthesized from the elements by arc-melting, and single crystals of YNiAl 4 Ge 2 , GdNiAl 4 Ge 2 , and LuNiAl 4 Ge 2 were grown from an aluminum flux. All members crystallize isostructurally in the rhombohedral SmNiAl 4 Ge 2 -type structure ( R 3̅ m , Z  = 3). The compounds can be described as a stacking of RE δ+ and [NiAl 4 Ge 2 ] δ− slabs with an ABC stacking sequence, or alternatively as stacking of CsCl and CdI 2 building blocks. The results of the magnetic measurements indicate that all rare earth atoms are in a trivalent oxidation state. Of the RE NiAl 4 Ge 2 series, the members with RE  = Sm, Gd–Dy exhibit antiferromagnetic ordering with a maximum Néel temperature of T N  = 16.4(1) K observed for GdNiAl 4 Ge 2 . 27 Al NMR spectroscopic investigations yielded spectra with two distinct signals, in line with the crystal structure, however, significantly different resonance frequencies of δ iso ms (YNiAl 4 Ge 2 ) = 77(1) and 482(1) ppm as well as δ iso ms (LuNiAl 4 Ge 2 ) = 90(1) and 467(1) ppm were observed. These indicate significantly different s -electron densities at the two crystallographically different Al atoms, in line with the results from DFT calculations. The Bader charge analysis confirms that the present compounds must be considered as germanides, as expected from the relative electronegativities of the constituent elements, while the low charges on Al and Y indicate significant covalent bonding.

Five new compounds with the compositions [Cd(NCS) 2 (3-cyanopyridine) 2 ] n  · 3-cyanopyridine ( 1 ), [Cd(NCS) 2 (3-cyanopyridine) 2 ] n  · 1/3 3-cyanopyridine ( 2 ), [Cd(NCS) 2 (3-cyanopyridine) 2 ] n ( 3 ), {[Cd(NCS) 2 ] 2 (3-cyanopyridine) 3 } n ( 4 ), and {[Cd(NCS) 2 ] 3 (3-cyanopyridine) 4 } n ( 5 ) have been obtained by the reaction of Cd(NCS) 2 with 3-cyanopyridine in different solvents. While large amounts of compounds 1–4 could be prepared as powders, only a few single crystals of 5 were accidently obtained. Thermoanalytical investigations have shown that 4 could also be obtained by annealing of 1 or 2 and that under slightly different conditions 5 could be obtained as part of a mixture with 4 . The crystal structures of all compounds can be divided in two sets of compounds. Compounds 1 , 2 and 3 consist of chains in which the Cd cations show three different coordination environments and in which the coligands are only terminally bonded. In the structures of 4 and 5 similar chains are observed, which are connected into layers via some of the 3-cyanopyridine coligands.

Three new niobium cluster compounds with edge bridged, octahedral hexanuclear metal cores have been synthesised. They consist of cluster pairs with [Nb 6 Cl 12 ( R CN) 6 ] 2+ cations, [Nb 6 Cl 18 ] 2− anions, and co-crystallised nitrile molecules, with R  = C 2 H 5 (propionitrile), n C 3 H 7 (butyronitrile), i C 3 H 7 (isobutyronitrile). The synthesis is based on the dehydration of [Nb 6 Cl 14 (H 2 O) 4 ] · 4(H 2 O) with carboxylic acid anhydrides in the presences of an excess of the respective nitrile. An interesting aspect of these compounds is that the metal atoms of the cluster cation have an average oxidation state different from that of the cluster anion, the former being oxidized losing two electrons. In crystals of all three compounds layers of cluster cations are separated by layers of cluster anions. Perpendicular to these layers of the same cluster types, every cation is surrounded by four anions and vice versa. Between the cations and anions short distances are found between the halogenido ligands and the positively charged C atoms of the nitrile ligands (N–C · · · Cl angles of ~90°). These contacts indicate relatively strong dipole-dipole interactions, which presumably contribute to the arrangement of the cluster ions in the crystals.

The crystal structures of a new family of superconductors containing a Bi square net and their electronic structures around the Fermi level have been reviewed. The structures of these compounds can be viewed as stacked layers denoted by [Bi][( RE )( M 2 Bi 2 )( RE )] RE  = rare earth or alkaline earth metal, M  = transition metal. Flat/steep band features are shown to exist in all these new superconductors, though the pairing mechanisms may be very different. The Dirac Fermion behavior is reviewed and its implications are discussed.

Polycrystalline samples of La- and Lu-agardite with the composition RE Cu 6 (OH) 6 (AsO 4 ) 3  ·  n H 2 O ( RE  = La, Lu; n ≈3) have been prepared and the structure of the products was determined by X-ray powder diffraction studies. The characterization has been complemented by Raman and UV/Vis spectroscopic, magnetic and TGA investigations. DFT calculations support the conclusions drawn from the experiments. The arsenates RE Cu 6 (OH) 6 (AsO 4 ) 3  ·  n H 2 O ( RE  = La, Lu; n ≈3) are isostructural with the mineral mixite and crystallize with a hexagonal structure which contains ribbons of edge-sharing [CuO 5 ] square-pyramids extending along the hexagonal axis. They are interconnected via (AsO 4 ) 3− groups to form hexagonal tubes of about 10 Å inner diameter. Such zeolite-like tubes host water molecules, which can be reversibly removed at moderate temperatures ( T ≈100°C). Like in mixite and YCu 6 (OH) 6 (AsO 4 ) 3  · 3 H 2 O, the Cu 2+ cations in RE Cu 6 (OH) 6 (AsO 4 ) 3  ·  n H 2 O ( RE  = La, Lu; n ≈3) exhibit low-dimensional antiferromagnetic properties, which are subject to changes in the Cu–O–Cu bond lengths and angles due to the lanthanide contraction.

The new Ruddlesden-Popper-related phases A n +1 B n O 3 n +1 ( n  = 3) with the compositions Li 2 Eu 2 Nb 3 O 10 , Li 2 Eu 1.5 Ta 3 O 10 , Li 2 EuKNb 3 O 10 , and Li 2 EuKTa 3 O 10 were synthesized by solid-state reactions from Li 2 [CO 3 ] (+ K 2 [CO 3 ]) and the corresponding refractory metals along with their oxides in a high-frequency furnace at temperatures above T  = 1600°C. Their structures have been determined by single-crystal X-ray diffraction studies. Characteristic features are triple layers of corner-sharing [ M O 6 ] 7− octahedra ( M  = Nb and Ta), which are connected via [LiO 4 ] 7− tetrahedra. The Eu 2+ cations are cuboctahedrally surrounded by 12 oxygen atoms and according to the Eu–O distances of around 275 pm, they have the oxidation state +2, as confirmed by XPS measurements. In the potassium-containing samples they share their positions with K + cations. The black compounds are stable in air at room temperature. Measurements of the magnetic susceptibilities in the range of T  = 5–300 K revealed Li 2 Eu 2 Nb 3 O 10 , Li 2 Eu 1.5 Ta 3 O 10 and Li 2 EuKTa 3 O 10 to be paramagnetic without any ordering.

The binary strontium germanide SrGe 6 was synthesized at high-pressure high-temperature conditions of approximately 10 GPa and typically 1400 K before quenching to ambient conditions. At ambient pressure, SrGe 6 decomposes in a monotropic fashion at T  = 680(10) K into SrGe 2 and Ge, indicating its metastable character. Single-crystal X-ray diffraction data indicate that the compound SrGe 6 adopts a new monoclinic structure type comprising a unique three-dimensional framework of germanium atoms with unusual cages hosting the strontium cations. Quantum chemical analysis of the chemical bonding shows that the framework consists of three- and four- bonded germanium atoms yielding the precise electron count Sr[(4bGe 0 ] 4 [(3b)Ge − ] 2 in accordance with the 8 −  N rule and the Zintl concept. Conflicting with that, a pseudo-gap in the electronic density of states appears clearly below the Fermi level, and elaborate bonding analysis reveals additional Sr–Ge interactions in the concave coordination polyhedron of the strontium atoms.

The title compound was prepared from the elements by arc-melting. The crystal structure was investigated by means of single-crystal X-ray diffraction. It crystallizes in the TbFeSi 2 structure type, orthorhombic space group Cmmm , a  = 4.0496(8), b  = 16.416(2), c  = 3.9527(6) Å, Z  = 4, R 1 = 0.041, wR 2 = 0.11 for 207 unique reflections with I o  > 2 σ ( I o ) and 19 refined parameters. The Fe position is not fully occupied and the refinement results in a composition GdFe 0.68 Si 2 in agreement with a chemical analysis. The structure consists of zig-zag chains of Si(1) atoms which are terminally bound to additional Si(2) atoms. For an ordered variant GdFe 0.5 Si 2 the Zintl concept can be applied which results in formal oxidation states Gd 3+ (Fe 2+ ) 0.5 Si(1) 1− Si(2) 3− . The electronic structure of this variant GdFe 0.5 Si 2 was analyzed using the tight-binding LMTO method and the results confirm the simple bonding picture.

The crystal structures of the lanthanide orthothiophosphates Ln PS 4 ( Ln =lanthanide) have been extensively investigated in the past. Up to now, however, single crystals of two members of this series – TmPS 4 and YbPS 4 – have not been available. Here, we report a modified synthesis protocol for TmPS 4 and YbPS 4 yielding single crystals suitable for X-ray diffraction. Both compounds crystallize in the tetragonal space group I 4 1 / acd (no. 142) with 16 formula units per unit cell and adopt the SmPS 4 parent structure, like most reported lanthanide orthothiophosphates. The structures contain isolated [PS 4 ] 3− tetrahedra and two crystallographically independent Ln 3+ cations, which form trigonal-dodecahedral [ Ln S 8 ] 13− polyhedra. The lattice parameters for TmPS 4 are a  = 10.598(2), c  = 18.877(4) Å with V  = 2120.2(6) Å 3 , and for YbPS 4 a  = 10.577(2), c  = 18.827(4) Å with V  = 2106.2(7) Å 3 . The DFT-calculated electronic band structures indicate semiconducting behavior and reveal indirect band gaps of 2.1–2.2 eV, consistent with the reddish brown color of YbPS 4 , but underestimating the band gap of pale-yellow TmPS 4 . The Raman spectra are dominated by [PS 4 ] 3− vibrations as confirmed by DFT-calculated phonon spectra. DTA measurements reveal remarkably high thermal stability compared to other known orthothiophosphate compounds.

The room temperature reaction of Hg(NO 3 ) 2  · H 2 O, cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) and K 8 {Nb 6 O 19 } · 16 H 2 O in a mixture of H 2 O and DMSO led to crystallization of the novel compound {[Hg(cyclam)] 6 Nb 6 O 19 }(NO 3 ) 4  · 14 H 2 O, which is the first mercury containing polyoxoniobate. The structure consists of a {Nb 6 O 19 } 8− cluster core which is expanded by six [Hg(cyclam)] 2+ complexes via Hg– μ 2 -O–Nb bond formation. The title compound contains a positively charged polyoxoniobate cluster. The crystal water molecules form small aggregates by O–H · · · O hydrogen bonding which are joined into larger aggregates by N–O · · · H–O hydrogen bonding integrating the nitrate anions.