Daniel M. Többens

A pressure-amplifying framework material with negative gas adsorption transitions

Adsorption-based phenomena are important in gas separations, such as the treatment of greenhouse-gas and toxic-gas pollutants, and in water-adsorption-based heat pumps for solar cooling systems. The ability to tune the pore size, shape and functionality of crystalline porous coordination polymers—or metal–organic frameworks (MOFs)—has made them attractive materials for such adsorption-based applications. The flexibility and guest-molecule-dependent response of MOFs give rise to unexpected and often desirable adsorption phenomena. Common to all isothermal gas adsorption phenomena, however, is increased gas uptake with increased pressure. Here we report adsorption transitions in the isotherms of a MOF (DUT-49) that exhibits a negative gas adsorption; that is, spontaneous desorption of gas (methane and n-butane) occurs during pressure increase in a defined temperature and pressure range. A combination of in situ powder X-ray diffraction, gas adsorption experiments and simulations shows that this adsorption behaviour is controlled by a sudden hysteretic structural deformation and pore contraction of the MOF, which releases guest molecules. These findings may enable technologies using frameworks capable of negative gas adsorption for pressure amplification in micro- and macroscopic system engineering. Negative gas adsorption extends the series of counterintuitive phenomena such as negative thermal expansion and negative refractive indices and may be interpreted as an adsorptive analogue of force-amplifying negative compressibility transitions proposed for metamaterials.

Charge-transfer crystallites as molecular electrical dopants

Ground-state integer charge transfer is commonly regarded as the basic mechanism of molecular electrical doping in both, conjugated polymers and oligomers. Here, we demonstrate that fundamentally different processes can occur in the two types of organic semiconductors instead. Using complementary experimental techniques supported by theory, we contrast a polythiophene, where molecular p-doping leads to integer charge transfer reportedly localized to one quaterthiophene backbone segment, to the quaterthiophene oligomer itself. Despite a comparable relative increase in conductivity, we observe only partial charge transfer for the latter. In contrast to the parent polymer, pronounced intermolecular frontier-orbital hybridization of oligomer and dopant in 1:1 mixed-stack co-crystallites leads to the emergence of empty electronic states within the energy gap of the surrounding quaterthiophene matrix. It is their Fermi–Dirac occupation that yields mobile charge carriers and, therefore, the co-crystallites—rather than individual acceptor molecules—should be regarded as the dopants in such systems.

In Situ Observation of Gating Phenomena in the Flexible Porous Coordination Polymer Zn2(BPnDC)2(bpy) (SNU-9) in a Combined Diffraction and Gas Adsorption Experiment

The intrinsic structural dynamic during the adsorption of CO2 at 195 K and N2 at 77 K on flexible porous coordination polymer Zn2(BPnDC)2(bpy) (SNU-9) was studied in situ by powder XRD. The crystal structures of as made and solvent free (activated) phases were determined by single crystal X-ray diffraction. During the structural transformation caused by activation, the rearrangement of Zn-O bonds occurs that leads to changes in coordination environment of Zn atoms. Such changes lead to the contraction of the unit cell and to decreasing unit cell volume of nearly 28% in comparison to the pristine as made structure. The solvent accessible volume of the unit cell decreases from 40.8% to 12.8%. The adsorption of CO2 and N2 on SNU-9 proceeds in a different way: the formation of intermediate phase during the CO2 adsorption could be postulated, while the transformation from narrow pore form to the open structure occurs in quasi-one-step in the case of N2 adsorption (the intermediate phase is formed only in very narrow pressure region). The transformation of the structure is guest dependent and the differences in the structures of CO2@SNU-9 at 195 K and N2@SNU-9 at 77 K were proven by Pawley and Rietveld refinements of powder XRD patterns. The structure of N2@SNU-9 is identical to this of as synthesized phase, while the structure of CO2@SNU-9 differs slightly.

A Tale of Two Polymorphic Pharmaceuticals: Pyrithyldione and Propyphenazone and their 1937 Co‐crystal Patent

A co-crystal of two polymorphic active pharmaceutical ingredients (APIs), first reported and patented in 1937, has been prepared and thoroughly characterised, including crystal structure analysis. The existence of four crystal forms of one of the APIs, the sedative and hypnotic active pharmaceutical ingredient 3,3-diethyl-2,4(1H,3H)-pyridinedione, pyrithyldione (PYR), and of three crystal forms of the co-crystal-forming second API, the non-steroidal anti-inflammatory drug 1,2-dihydro-1,5-dimethyl-4-(1-methylethyl)-2-phenyl-3H-pyrazol-3-one, propyphenazone (PROP), has been reported previously, but they have only been partly characterised. For both compounds, none of the metastable forms exist at room temperature. DSC, hot-stage microscopy, X-ray diffraction and powder synchrotron X-ray diffraction were employed to characterise the polymorphic forms and to determine the crystal structures of forms I-III of PYR and forms I and II of PROP.

Synthesis, Crystal Structure, and Vibrational Spectroscopy of K2Ca4Si8O21—An Unusual Single-Layer Silicate Containing Q2 and Q3 Units

Single crystals of the previously unknown potassium calcium silicate K2Ca4Si8O21 (1) have been grown from a nonstochiometric melt as well as using a KCl flux. The compound is triclinic with the following basic crystallographic data: space group P, a = 6.8052(3) A, b = 7.1049(3) A, c = 11.2132(5) A, alpha = 96.680(4)degrees, beta = 105.280(4) degrees, gamma = 109.259(4)degrees, Z = 1, V = 481.28(4) A3. The crystal structure was solved by direct methods based on a single-crystal diffraction data set collected at ambient conditions. From a structural point of view, K2Ca4Si8O21 belongs to the group of single-layer silicates. The layers parallel to (001) are characterized by a complex arrangement of 6-, 8-, 10-, and 12-membered tetrahedral rings. The sheets can be built from the condensation of loop-branched fnfer single chains running parallel to [100], i.e., the crystallochemical formula can be written as K2Ca4{lB,5, 1(infinity)2}[Si8O21]. Compound 1 is the first example of a loop-branched layer silicate containing secondary (Q2) as well as tertiary (Q3) tetrahedra. Linkage between the layers is provided by calcium and potassium cations, which are distributed among a total of three crystallographically independent nontetrahedral sites. Alternatively, the structure can be described as a heteropolyhedral framework, based on SiO4 tetrahedra and CaO6 octahedra. The irregularly coordinated K-cations in turn are incorporated in tunnels of the network running parallel to [110]. The structural investigations have been completed by Raman spectroscopy. The allocation of the bands to certain vibrational species has been aided by density functional theory (DFT) calculations.

Indium-Free Perovskite Solar Cells Enabled by Impermeable Tin-Oxide Electron Extraction Layers

Corrosive precursors used for the preparation of organic-inorganic hybrid perovskite photoactive layers prevent the application of ultrathin metal layers as semitransparent bottom electrodes in perovskite solar cells (PVSCs). This study introduces tin-oxide (SnOx ) grown by atomic layer deposition (ALD), whose outstanding permeation barrier properties enable the design of an indium-tin-oxide (ITO)-free semitransparent bottom electrode (SnOx /Ag or Cu/SnOx ), in which the metal is efficiently protected against corrosion. Simultaneously, SnOx functions as an electron extraction layer. We unravel the spontaneous formation of a PbI2 interfacial layer between SnOx and the CH3 NH3 PbI3 perovskite. An interface dipole between SnOx and this PbI2 layer is found, which depends on the oxidant (water, ozone, or oxygen plasma) used for the ALD growth of SnOx . An electron extraction barrier between perovskite and PbI2 is identified, which is the lowest in devices based on SnOx grown with ozone. The resulting PVSCs are hysteresis-free with a stable power conversion efficiency (PCE) of 15.3% and a remarkably high open circuit voltage of 1.17 V. The ITO-free analogues still achieve a high PCE of 11%.

Pavlovskyite Ca8(SiO4)2(Si3O10): A new mineral of altered silicate-carbonate xenoliths from the two Russian type localities, Birkhin massif, Baikal Lake area and Upper Chegem caldera, North Caucasus

Abstract The new mineral pavlovskyite Ca8(SiO4)2(Si3O10) forms rims together with dellaite Ca6(Si2O7)(SiO4)(OH)2 around galuskinite Ca7(SiO4)3CO3 veins cutting calcio-olivine skarns in the Birkhin gabbro massif. In addition, skeletal pavlovskyite occurs in cuspidine zones of altered carbonate xenoliths in the ignimbrites of the Upper Chegem caldera (North Caucasus). The synthetic analog of pavlovskyite has been synthesized before and is known from cement-like materials. Isotypic to pavlovskyite is the synthetic germanate analog Ca8(GeO4)2(Ge3O10). The crystal structure of pavlovskyite, space group Pbcn, a = 5.0851(1), b = 11.4165(3), c = 28.6408(8) Å, V = 1662.71(7) Å3, Z = 4, has been refined from X-ray single-crystal data to R1 = 3.87%. The new colorless mineral has a Mohs hardness of 6-6.5, biaxial (-), α = 1.656(2), β = 1.658(2), γ = 1.660(2) (589 nm), 2V (meas) = 80(5)°, 2V (calc) = 89.9°, medium dispersion: r > v, optical orientation: X = b, Y = c, Z = a. For comparison with pavlovskyite, the crystal structure of kilchoanite Ca6(SiO4)(Si3O10) from the Birkhin massif [space group I2cm, a = 11.4525(2), b = 5.0867(1), c = 21.996(3) Å, V = 1281.40(4) Å3, Z = 4] has been refined from single-crystal X-ray data to R1 = 2.00%. Pavlovskyite represents a 1:1 member of a polysomatic series with calcio-olivine γ-Ca2SiO4 and kilchoanite Ca6(SiO4)(Si3O10) as end-member modules. The structure is characterized by strongly folded trisilicate units (Si3O10) interwoven with a framework of CaO6 and CaO8 polyhedra. Olivine-like slices with orthosilicate groups are interstratified with the characteristic trisilicate module of Ca4(Si3O10) composition. Although the optical properties of pavlovskyite and kilchoanite are similar, both minerals can be distinguished by chemical analyses (different Ca/Si ratio), X-ray diffraction, and Raman spectroscopy. The new mineral is named after V.E. Pavlovsky (1901-1982), an outstanding geologist in the area of Eastern Siberia, in particular of the Baikal region.

Quantum-mechanical calculations of the Raman spectra of Mg- and Fe-cordierite

Abstract Quantum-mechanical calculations with a hybrid HF/DFT Hamiltonian (B3LYP) model yielded the Raman-active vibrational modes of the Mg- and Fe-cordierite structure. Maximum and mean deviation between experimentally derived bands and calculated modes of synthetic Mg- and natural Fe-rich cordierite are ±19 and 7 cm-1. Most of the observed bands could be related to specific vibrational modes of tetrahedral and octahedral sites of the cordierite structure, although the large number of Ramanactive modes (87) prevents a complete assignment. Atomic motions in cordierite are compared with those of the structurally similar mineral beryl. The calculations enable more accurate interpretation of the Raman spectra with respect to structural changes of cordierite, in particular Al-Si ordering and Mg-Fe exchange.

K2Ca3Si3O10, a novel trisilicate: high-pressure synthesis, structural, spectroscopic and computational studies

Single crystals of K 2 Ca 3 Si 3 O 10 have been obtained in a multi-anvil high-pressure synthesis experiment performed at 10 GPa and 1000 °C. The compound adopts the monoclinic space group C 2/ c with four formula units per cell and the following lattice parameters (at 25 °C): a = 10.3539(19) A, b = 10.6013(16) A, c = 9.8221(19) A, β = 118.079(13)°, V = 951.2(3) A 3 . The crystal structure was determined from single-crystal X-ray diffraction data using direct methods (Mo- K α radiation, 2𝛉 max = 59.53°, R int = 5.90 %) and refined to R (|F|) = 5.64 % using 1042 observed reflections with I > 2σ( I ). The structure belongs to the group of oligosilicates consisting of [Si 3 O 10 ] groups. The trimers of the anion complex are located in layers parallel to (001) at about z ≈ ¼ and ¾, respectively. Ca(1)-octahedra provide linkage between (1) the Si 3 O 10 groups of a single layer by corner sharing of the equatorial oxygen atoms of the terminal tetrahedra and (2) the trimers belonging to adjacent sheets by corner sharing of the apical O-atoms. To an upper limit of 3.3 A, the remaining two crystallographically independent non-tetrahedral cation sites Ca(2) and K(1) are coordinated by 8 and 10 oxygen atoms, respectively. From a topological point of view the crystal structure of K 2 Ca 3 Si 3 O 10 can be classified as a new type of mixed tetrahedral-octahedral framework. The thermal expansion of K 2 Ca 3 Si 3 O 10 has been determined in the temperature range between 25 and 750 °C. The temperature dependence of the cell volume can be described with a second-order polynomial: V ( T ) = 0.00002(3) T 2 + 0.026(2) T + 949.36(34). Structural investigations were completed by Raman spectroscopic studies. The assignment of the bands to certain vibrational species was aided by density functional theory (DFT) calculations.

Atomic and domain structure of the low-temperature phase of barium metagermanate (BaGeO3)

The crystal structure of the low-temperature form of barium metagermanate (BaGeO3) has been determined from laboratory X-ray powder diffraction data collected at 298.5 (5) K. The structure was found to consist of alternating layers of Ba cations and [GeO3]3 rings, and is closely related to pseudo-wollastonite. The rings show a twofold positional disorder owing to stacking faults. The stacking is not random, but can be rationalized by a twinning mechanism mapping the two non-congruent enantiomorphic polytypes of the structure onto each other. This model also explains the diffuse scattering and twinning observed in SAED and HRTEM, as well as the size and strain-like broadening effects found in the XRPD pattern.